DETERMINING ANCHOR HOLDING POWER FROM MODEL TESTS

DETERMINING ANCHOR HOLDING POWER FROM MODEL TESTS
BY LIEUTENANT
AND L I E U T E N A N T
(JG)
(JG)
~ T I L L I A S I ~-~. L E A t I Y , ( C C ) ,
U.S.N.,
~[ESIBER,
JAZZES M . F A R R I N , J R . , ( C C ) , U . S . N . , VISITOR
Heretofore, the selection of anchors for ships
has been largely a trial and error process. I t
was thought that the evolution of a method by
which the exact holding power of an anchor
could be determined would be valuable. I t was
also felt that this could best be done by using
small models and stepping the results up to full
scale.
By combining the results of these tests with
those obtained for anchor chain in a previous
investigation ~ it was expected that a more rational basis for selecting anchors and anchor
chain for ships would be reached.
Because of the large number of variables involved, it was not expected that a general law
could be found which would a p p l y to all types
of anchors. However, the determination of a
law governing geometrically similar anchors
seemed possible and the research was carried out
with this in mind.
In order to reproduce as nearly as possible
actual conditions to which an anchor is subjected, the a p p a r a t u s was so a r r a n g e d that the
pull on the anchor shank would be horizontal.
A t a n k f 30 inches deep, 21 inches wide and 15
feet long was filled with fine sand, average grain
diameter 0.88 millimeters, to a depth of 16 inches. H o w a r d and J a m e s found that this sand
was satisfactory and that no correction need
be made for grain size when testing anchors of
five pounds and above. The sand was then covered with nine inches of water and the model
anchor buried in the sand. The model was so
buried that the shank lay horizontally along the
surface of the sand. A wire, led horizontally
from the anchor shackle, passed over a sheave
and vertically to a spring balance. The balance,
in turn, was connected to a chain fall which
was fixed to an overhead ring. The pull on
the chain fall was t r a n s m i t t e d through the
* I n v e s t i g a t i ' o n of A n c h o r C h a r a c t e r i s t i c s by M e a n s of
Models. by W. E. H o w a r d a n d R. K . . l a m e s , M a s s a c h u s e t t s
I n s t i t u t e of Technology, 1933.
t L o c a t e d in t h e R i v e r H y d r a u l i c s L a b o r a t o r y a t t h e
M a s s a c h u s e t t s I n s t i t u t e of T e c l . l o l o g y .
spring balance to the anchor and its magnitude
recorded on the balance. A sketch of the apparatus is shown in Fig. 1.
During the first p a r t of the pull the anchor
buries itself, building up a mound of sand ahead
of it. D u r i n g this time the reading increases
<1,1>
Sand
F I G . I . - - S K E T C I [ OF A P P A R A T U S
gradually to a m a x i m u m value, which is held
during the remainder of the pull. This value
is the actual holding power of the anchor in
sand. A f t e r these tests were completed in sand,
similar tests were made using a blue clay bottom
which is one of exceptionally good holding
quality.
The weight of all the model anchors tested
was determined to within one-tenth of a pound
and the fluke areas obtained by measuring their
shadows projected by a point source of light,
the measurement being made by a planimeter.
The area of this shadow was corrected by multiplying it by. the square of the ratio of the distance of the anchor from the point source to the
distance of the shadow from the point source.
The fluke area in the case of stockless anchors
105
106
ANCHOR
HOLDING
POWER
FR05[
MODEL
TESTS
<2,1>
Fro.
2.---TYP]7 " W "
was taken to be the area of one fluke exchsive of
the crown, while for stock anchors it was taken
to be the area of one palm. I f any other part
of the anchor were included in the fluke area,
it would nlerely change the constants in the
holding power equations involving fluke area.
Three spring balances were used in these tests,
one reading from zero, to 100 pounds, one from
zero to 200 pounds and one from zero to 60=0
pounds. The gages were calibrated by means
of known weights and the friction in the sheave
was measured and found to be negligible (on
the order of one per cent). The values of holding power given are the average of ten separate
runs and no difficulty was experienced in obtaining' successive values differing by not more
than five per cent.
TESTS
ON VARIOUS
TYPES
OF ANCHORS
Type " W " Stockless Anchors. I n order to
determine a law governing the holding power of
anchors, it was decided to test a series of
geometrically similar models. A series of eight
model anchors was therefore obtained ranging
in weight from five to 50 pounds (see Fig. 2).
These anchors were of the stockless type, which
is standard in the United States Navy, and are
hereafter referred to as type " W "
stockless
anchors. A f t e r weighing and measuring fluke
areas, the angle between shank and flukes was
set at 50 degrees, for reasons which will be discussed later.
t~ESULTS OF TESTS
W e i g h t , lb. . .
5.7 10.2 15.5 20.9 29.0 34.9 38.1 50.6
Area, sq. i n . . .
8.3 13.9 16.0 20.2 26.1 30.8 34.0 40.0
P (sand), lb.. 30.4 64.0 84.0
123 175 225 255 344
P (clay), lb...
53 110 145 210 300 380 440 580
w h e r e a r e a - - f l u k e a r e a a n d P - - h o l d i n g power.
I t was decided to plot the holding power
against fluke area rather than weight. Obviously, the holding power is proportional to the
STOCKT, ESS ANCIIORS
volunle of the bottom material disturbed and
this volmne depends upon the sizc of the object
pulled through it rather than the objeet's
weight. I f the models had been made so that
their weights were exactly proportional to the
three-halves power of their fluke areas, a plot
of holding power against weight would follow
definite law, but due to weight tolerance this
is not true. It should be realized at this point
that fluke area and not weight is the determining factor. In this connection, an aluminum
anchor of tile same size as the 29-pound model,
weighing nine pounds, was tested. While it
was found that its weight was not sufficient to
bury it fully, when it was artificially buried
its holding power was that corresponding to its
fluke area, and in either ease was much greater
than that corresponding to its weight. The function of the weight is to give the anchor sufficient strength and enable it to overcome the
u p w a r d component of the resistance of the
crown.
A very large logarithmic plot was there2ore
made using holding power and fluke area as coordinates and a plot of residuals made to locate
the line accurately. As a result of this plot the
following' equation for holding power was
obtained :
P ~
K
(sand)
~
1.20
K A TM
<2,2>
K
(day)
~
2.04
where P is holding power in pounds and A is
fluke area in square inches.
I t was noted when testing the 29-pound anchor of the above series that the holding power
was f a r less than expected and that the anchor
failed to bury in the sand when pulled. When
the angle between the flukes and shank was
measured, it was found to be 65 degrees rather
than the specified 55 degrees. I t then occurred
ANCHOR
HOLDING
POWER
FR05~[ 5 i 0 D E L
107
TESTS
20o
19o
[8o
-~ I'/0
\
y_ iso
i
l4-0
<3,1>
o.. 150
~
\
[Zo
~110
<3,2>
\
100
\
FIG. 4 . - - ~ T A V ¥ STOCKLESS ANCHORS ( N E W STA~'DARD)
9O
80
4O
¢5
50
55
Angle i n D e g r e e s
¢aO
r~
FIG. 3 . - - T E S T RESULTS OF 2 9 - P o u N D T Y P E " W " STOCKLESS ANCHOR AT VARIOUS F L U K E A.NGLES
<3,3>
to us that there must be some optimum angle.
In an effort to find this, this anchor was tested
in sand at ten different angles with the following results :
Angle ..
P
.....
65 62.5
60 57.5
9 9 103 130 140
55 52.5
170 172
50 47.5
175 172
45
170
40
124
A plot of this was made (Fig. 3) and from
it it was seen that the maximum holding power
lay between 45 and 55 degrees and that between
these limits there was little variance. F o r this
reason it was decided to test all future anchors
of this type at 50 degrees, the middle of this
range. It was noted that 55 degrees was specified .on the blue print for these anchors and it
is felt that this is too near the upper limit and
also that too large a tolerance is allowed. This
does not apply to anchors made to United States
Navy specifications, as the specified angle is 45
degrees and the tolerance very small.
Navy Stockless Anchor (Old Standard). The
old standard stockless anchor of the United
States Navy has the sanle form but is of heavier
construction than the type " W " stockless and
it was thought that models of tiffs type would
throw f u r t h e r light on the holding power equation. Two models of an 11,000-ponnd old standard Navy stockless anchor w e r e accordingly
tested. The following results were obtained:
Weight
12.3
29.5
Fluke area
13.0
22.2
Holding power Holding power
( sand )
53.0 ~:
120
(clay )
95.0
200
These results were plotted in the same manner
as for the type " W " stockless anchors and it
was found that a straight line through the two
points was parallel to the line obtained in the
FIG. 5 . - - T Y P E
~
STOCKLESS -5,-NCHORS
plot of the type " W " stockless results. Using
the equation P - - K A 15~, the following constants were obtained: K ( s a n d ) ~ l . 0 7 , K ( c l a y )
~--1.82.
Navy Stockless
Anchors (New Standard).
The new standard stockless anchor of the United
States Navy was developed by the Navy Department in an effort to obtain greater holding
power without an increase in weight (see Fig.
4). A limitation was placed on the design by
the requirement that this anchor fit the same
hawse pipe as the old standard stockless anchor.
By lightening the crown it was possible to get
longer flukes and, as will be shown later, this
leads to increased holding power.
t~ESULTS OF TESTS
W e i g h t , lb . . . . . . . . . . . . . . . . . . .
5.6
F l u k e a r e a , sq. i n . . . . . . . . . . . . .
8.8
H o l d i n g p o w e r ( s a n d ) , lb . . . . .
35
H o l d i n g p o w e r ( c l a y ) , lb . . . . . .
47
16.7
18.3
110
145
30.7
25.9
183
240
A plot of these values using logarithmic coordinates gave a straight line, the equation for
which is :
P~-- K A1.~
K(sand) ~-- 1.25 <3,4>
K(clay) ~2.22
Type " A " Stockless Anchors. The type " A "
stoekless anchor gains greater holding power
108
ANCHOR
HOLDING
POWER
FROM
MODEL
TESTS
tested at 15 degrees and practically no change
was found in the holding power. A p p a r e n t l y
for this type of anchor the effective range extends lower, because the absence of a crown
enables it to bury deeply even at small angles.
A n anchor of this type goes to the extreme
in that ruggedness is sacrificed in order to obtain as large a fluke area as possible. The test
results are interesting in that they clearly show
the importance of fluke area to the holding characteristie~s of an anchor, but it is doubtful if an
anchor of this type would be practicable for any
but the very lightest duties.
<4,1>
]~ESULTS OF TEST
W e i g h t , lb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F l u k e a r e a , sq. ill . . . . . . . . . . . . . . . . . . . . . . . . . . .
H o l d i n g p o w e r ( s a n d ) , lb . . . . . . . . . . . . . . . . . .
12.2
25.5
218
This model was not tested in clay.
Stock Anchor (Admiralty Type).
FIG. 6 . - - T Y P E " H " STOCKLESS ANCHOR
than the old standard Navy stockle~ type
through longer flukes and greater fluke area
(see Fig. 5). Because of the increased holding
power this type has been used by the Navy on
the Yangtze River where anchoring conditions
are particularly poor.
I~ESUI.TS OF TESTS
W e i g h t , lb . . . . . . . . . . . . . . . . . . .
F l u k e a r e a , sq. in . . . . . . . . . . . . .
4.8
9.8
H o l d i n g p o w e r ( s a n d ) , lb . . . . . .
H o l d i n g p o w e r ( c l a y ) , lb . . . . . .
29
47
13.9
19.8
29.1
32.4
82
139
183
290
These values give the following equation:
P ~
K(sand)
--
0.83
K A ~-55
K(elay)
~
I n order
to get a contrast with the stoekless types, we
secured a set of geometrically similar stock anchors, which were of the same general type as
the old standard Navy stock anchors. These are
shown in Fig. 7 and it may be noted that,
although their area is smaller, the arms are long,
allowing the fluke to attain a greater depth than
that of the stoekless type. Also, the stock placed
at right angles to the shank enables the anchor
to dig in more readily. It should be noted from
the fignre that for these anchors the angle between the flukes and shank is fixed.
RESULTS OF TESTS
W e i g h t , lb . . . . . . . . . . . . .
5.5
10.1
11.5
15.3
26.1
F l u k e a r e a , sq. in . . . . . . .
7.0
10.2
8.9
13.7
14.9
H o l d i n g p o w e r ( s a n d ) , lb.40
77
57
112
135
H o l d i n g p o w e r ( c l a y ) , lb.80
150
11~
220
270
It may be noted from the above results that
the ll.5-pound anehor, through error, had less
fluke area than that corresponding to its weight
and that its holding power was correspondingly
less. This further substantiates the theor.v ~/d-
1.41
Type " H " Stockless Anchor. The type " H "
stoekless anchor (Fig. 6) has much greater
holding power, on the basis of weight, than any
of the other anchors tested. The one model of
this type tested has a fluke area approximately
the same as that of a 29-pound new standard
Navy stoekless anchor, whereas its weight is
only 12.2 pounds. This saving in weight is accomplished by practically eliminating the crown
and making the flukes relatively thin. The angle
specified, between shank and flukes, is 35 degrees
for this anchor. As 45 to 55 degrees had been
found to be the best range of angle for the usual
stockless anchors with crowns, this anchor was
<4,2>
FIG. 7 . - - S T O C K ANCHORS (ADMIRALTY T Y P E )
ANCHOR, HO[.DING
POWER
vanc~d previously that the holding power depends on the fluke area.
These results were plotted on large-scale logarithmic coordinates in the same manner as for
the stockless anchors and the following equation was arrived a t :
FROM
MODEL
TESTS
109
1,000
750
NeWSfamdard-
Type"~/"3J~ocklless]~
P ~ K A~.~
K(sand) ~ ].95. <5,2>
K(clay) ~ 3.31
DISCUSSION OF CLAY RESULTS
I t may be noted from the above results that
the curves of holding power in clay differ from
those in sand only by a constant. The ratio of
the holding powers in sand to clay should be the
same for the various types, but from the results
it m a y be seen that it varies slightly. This may
be explained by the variation in saturation of
the clay due to dragging the anchors through it
and its necessarily non-uniform character. The
average of the ratios obtained in our tests is
1.7 and this ratio is used in calculating the constant for clay in the various holding power equations. Ordinarily the types of bottom encount e r e d in anchoring will vary widely in holding
quality. For this reason the value of the constant K used in the various holding power equations will also vary. I t is felt that the type of
sand bottom used in these tests gives approximately the minimum K, while the K values for
the clay lie near the upper limit.
.9
300
/
0
0-
'
./
ZOO
<5,1>
tc
150
"rio 0
30.3 / ~
l
I0
I5
7-0 25 30
40
Fluke Arec~ in S~uare Inch~s
50
];IO. 8.--~EPRODUCTION OF LARGE-SCALE LOGARITHMIC
PLOT (SAND)
SUMMARY OF EQUATIONS
Anchor Typv
" W " stockless . . . . . .
Type
Navy stockless (old std.)
Navy stockless {new std.)
Type " A " stockless . . . . . .
Stock (Admiralty) . . . . .
Equation
P
P
P
1'
P
~
~
~
~
~
K
K
K
K
K
K(sand) K(clay)
A~.~3
A=~
A~'5~
:1~'~
A=~
1.20 2.04
1 . 0 7 1.82
].20 2.]2
0.83
1.41
1.95 3.31
The large-scale logarithmic plot from which
the above equations were obtained is reproduced
in Fig. 8. The clay curves differ only by a
constant and hence were omitted.
I t is interesting to note that the coefficient
K not only varies with the type of bottom but
also reflects the form of the anchors. Since K
takes care of variations in form, the exponent
of A, which allows for a change in size only,
should be the same for all types. Also, since
holding power is a function of the volume of
bottom material disturbed, the right-hand side
of the holding power equations should dimensionally be a volume or linear dimension cubed.
I n this connection, the mass of bottom material
can be considered as being moved along a plane
surface and developing a frictional force equal
to the holding power of the a n c h o r . Thus, for
the equations to be dimensionally correct, since
A is the square of a linear dimension, it must
be raised to the three-halves power. I t can be
seen from the equations that the exponents of
A are as close to 1.5 as would be expected for
experimental results. Therefore, in obtaining
the weight equation and constants which follow, an exponent of 1.5 was used for A.
WEIGHT EQUATION
I n discussing the holding characteristics of
the various anchors and deriving the equations,
the fluke area has been used as a basis for obtaining holding power, because, as has been
shown, this is the determining factor. In view
of the fact that the weight of an anchor is
always known, while its fluke area is not, it is
more convenient to determine the holding power
from the weight. Therefore, in the holding
power equations weight to the two-thirds power
has been substituted for fluke area and the constants recomputed, giving the following results:
110
ANCHOR
150
I
HOLDING
i
POWER
I
.~lZ0
I
x )I"
.1
/;,;if'!
c
t,,g l l O
"~ i oo
I:
g 9o
D
0
Z///
.1:: 8o
I--
<6,1>
t_
/,,/
70
o ~0
¢ 50
/
5O
zo
]
/-
//
/
// , /
i
'2/"
,o
oF
0
FIG.
9.. 4
¢~
8
I0
IZ
14lO
18
Anchor Weight- in Thousands of Pounc~
9.--HOLDING
POW~
Equation:
OF FuLL-SIZE
A~CHOI~S
P ---- K IV
Anchor Type
Type " W " s t o c k l e s s . . . . . . . . . . . . . . .
N a v y s t o c k l e s s (old std.) . . . . . . . . .
N a v y s t o c k l e s s (new std.) . . . . . . . .
Type " A " s t o c k l e s s . . . . . . . . . . . . . . . .
Type " I t " s t o c k l e s s . . . . . . . . . . . . . . . .
Stock ( A d m i r a l t y ) . . . . . . . . . . . . . . .
K(sand)
6.20
4.70
7.10
7.20
17. l0
7.30
MODEL
TESTS
apply corrections in order to obtain the holding
power for a 29-pound anchor of each type. It
was decided to base the comparison on the sand
results because of their uniform character. The
comparison follows :
140
150
FROM
K(clay)
10.55
8.00
12.08
12.22
29.07
12.40
The above equations will not give as accurate
results as would those for fluke area, for reasons previously explained. Because of the consistency of the results within the test limits,
we feel justified in extending the law to include
full-sized anchors. Hence the holding power of
full-sized anchors of various types is shown
graphically in Fig. 9.
COMPARISON OF VARIOUS T Y P E S OF A N C H O R S
It was felt that a comparison of the different
types of anchors tested would be of interest and
would show what effect shape has on holding
power. Since we had a model of four stockless
types weighing about 29 pounds, it was decided
to make the comparison at this weight. Due to
slight variations in weight, it was necessary to
Type
Navy
Navy
Type
Type
" W " stockless . . . . .
s t o c k l e s s (old) . . . .
stockless ( n e w ) . . .
"A" stockless . . . . . .
P
175
116
183
183
A
26.1
21.9
25.9
32.4
M M(rel.) P(rel.)
124 1.51
1.51
82 1.00
1.00
127 1.55 1.58
130 1.58 1.58
W h e r e A ~ fluke a r e a in s q u a r e inches, P ---- h o l d i n g
pow e r in pounds, a n d M ~ m o m e n t ,
(see Fig. 12).
The r e l a t i v e v a l u e s of M a n d P a r e b a s e d on t h e N a v y
s t o c k l e s s (old s t a n d a r d ) .
W o r k i n g on the theory that holding power
depends on fluke area and that the effectiveness
of this area depends on its depth below the surface of the bottom, it was decided to calculate
the moment of these areas below this surface
and see if they gave an indication of the relative holding powers. Mr. Herreshoff, an anchor
designer, in a letter to the New York Yacht
Club in 1907 a~sumed holding power to be a
function of fluke area times the square of the
distance below the surface. Dimensionally, it
would appear that holding power should depend
on a linear dimension cubed (a volume) which
is reflected in its moment. The results of these
investigations using the moment are given in
the table above. It can be seen that the holding
power is related to the moment of the area
rather than the area alone. This reflects the
fact that the effectiveness of a unit area depends
on its depth below the bottom surface.
F r o m the table it may be noted that the Navy
stockless anchor (old standard) has less fluke
area than the new standard Navy stockless and
its moment is even less in proportion. It may
be seen that its holding power corresponds to
its moment. The increased moment in the case
of the new standard Navy stockless is obtained
by lengthening the flukes, thereby making their
area more effective.
I n the case of the type " A " stockless anchor,
very good holding power is again obtained by
long flukes. It may be seen from Fig. 5 that
this anchor has a fin which occupies the area
between the flukes for a short distance from the
crown. I t has been found from tests in both
air and soil that when relatively close objects
are moved through such mediums they act in
the same manner as a solid barrier filling in the
space between ttlem. Thus it would appear that
this fin does not add to the effective area of the
flukes. Therefore, while it was included in the
fluke area, it was not used in calculating the
moment. The results indicate the correctness
ANCHOR
HOLDING
POWER
FROM
MODEL
TESTS
111
<7,1>
FIG. ]0.--TXVENTY-NINE POUND ANCI[OttS: (LEFT TO R[GIIT) -~IUSttRoo~r. NAVY STOCKLESS (NE\V ~TANDARD).
NAVY STOCKLESS (OLD STANDARD), TYPE "_Aft' STOCKLESS, TYPE " W " STOCI-~LESS, STOCK (AD_~IIRALTY TYPE I
of this assumption. It would seem that the
weight devoted to the fin could be more advantageously employed in the flukes.
The fluke area of the type " W " stockless
anchor is greater than that of the new standard
Navy stoekless but its flukes are slightly shorter
and hence the moments are nearly the same. It
may be noted that the holding powers are practically the same.
It should be noted that while the type " H "
stockless anchor tested has slightly less fluke
area than the new standard Navy stockless anchor weighing 30.7 pounds, its holding power
is 19 per cent greater. This can be best explained by pointing out the fact that the type
" H " stockless, because of the omission of the
crown, buries more deeply than the new standard Navy stockless and other similar types and
hence has a greater moment about the bottom
surface.
I t was thought that stock and mushroom anchors of 29 pounds would throw further light on
the relation between moment of fluke area and
holding power because of their unusual forms,
differing so markedly from the stoekless anchors
(see Fig. 10).
RESULTS OF TESTS
Type
Stock (Admiralty type) . . . . . . . . . . . . . .
Mushroom . . . . . . . . . . . . . . . . . . . . . . . . . .
P
A
195
63
19.0
20.3
Although exact values of moment were not
obtained for these anchors, it can be said that,
comparing them with the stockless types, the
stock anchor has a much larger moment arm
while the mushroom anchor has a very much
shorter one. Thus these anchors bear out, at
least qualitatively, the theory that anchor holding power depends on the moment of the fluke
area about the bottom.
DISCUSSION OF RESULTS
Suggested Method o/Anchor Selection. Theoretically, it would be desirable to select an anchor for a ship on the basis of the holding power
required. However, the required holding power
is not a fixed quantity, as it depends on wind
and current conditions and the kinetic energy
developed in the ship by variation in them.
Also, the holding power which may be obtained
from a given anchor depends on the type of bottom encountered. Thus it may be seen that an
exact solution is not possible. F o r this reason
the following method is proposed.
Since holding power has been shown to be proportional to anchor weight, anchor weight is
proportional to the drag on the ship due to wind
and current. Therefore, knowing a ship whose
anchor is satisfactory, a satisfactory anchor for
any other ship can be determined by multiplying the anchor weight by the ratio of the drag
of the new ship to that of the original.
The drag on a ship due to wind can be calculated by a formula proposed by Captain E. F.
Eggert, (CC), U.S.N.
D a ~-
0.0044 A Va2
<7,2>
where A z s a i l area of ship in square feet (i.e.
cross sectional area which ship presents to the
(beam) -~
wind). For ships of ordinary form
<7,3>
2
A_NCHOR HOLDING POWER FROM MODEL TESTS
112
3~ooo
Comparison of Results with American Bureau
Rules. The American Bureau of Shipping rules
2~000
ZO,O00
J
,,~ 15,ooo
/
American
"o
Bureau
;:)
0
~ I0,000
o~
,4--
~Tes?
2
Resulf5
<8,1>
?~ •
5,ooo
o
<
2,000
3,00(1
FIG.
20; )00
5,000 7,000 10,000 15,000
American Bureau Number
ll.--COMPARISON
OF T E S T R E S U L T S
ICAN BUREAU RULES
"WITH _~_MER-
may be substituted for A. V a ~ v e l o c i t y of wind
in knots.
The drag due to current can be calculated
from the fornmla:
and S ~
D c ~ f S V ~~
<8,2>
C ~/ d i s p l a c e m e n t X l e n g t h
where f =
coefficient of f r i c t i o n (see T a y l o r ' s "Speed
a n d Power of S h i p s " ) .
S = wetted s u r f a c e in s q u a r e feet.
V ~ c u r r e n t in knots.
C - - - a coefficient (see T a y l o r ) .
The sum of Da and De gives the total drags
on the ships at anchor. In finding their ratio
any wind and current velocity can be assumed.
A sufficiently accurate approximation for this
ratio is the ratio of the sail areas of the two
ships. It follows that for ships of similar form
the two-thirds power of the displacement may
be substituted for sail area.
As an example, assume that the 11,000-pound
anchor of the U.S.S. Trenton (displacement 9000
tons) is satisfactory. From it can be calculated
the correct anchor size for the U.S.S. San Francisco (11,500 tons) and the U.S S. Farragut
(1726 tons) as follows:
San Francisco:
( 11,500 "~ -"/~
\ 9000 ]
X ll.000:13.000
ponnds.
<8,3>
Farragut : ~
( . ~1726
- ] "~ ~/~ X 11,000 ~ - 3,630 i)ounds.
These ships are of similar form and they use
13,00.0 and 3000-pound anchors, respectively.
for anchor selection are based on experience in
service over a great many years and it was felt
that it would be of interest to see if this practice followed the law indicated by our results.
I n order to make this comparison, a number
of different U. S. Navy ships were chosen and
their anchor weights selected by the American
Bureau rules. Then, following the method outlined in the preceding section, the drags on these
vessels were calculated, and, using the Trenton
as a standard, their anchor weights were determined. Since the American Bureau uses a number which is based on the ship's form, called
" E q u i p m e n t T o n n a g e , " as a basis for selecting
anchors, it was decided to plot the two sets of
data against this number and to make the plot
on logarithmic coordinates in order to compare
their slopes. Tile results of this plot are shown
in Fig. 11.
It may be seeu that the two curves are parallel, which shows that the American Bureau
practice conforms to the theoretical law which
tile results of our experiments indicate. AIthough the slope of our experimental curve is
fixed, the magnitude of its ordinates depends
entirely on the condition of wind and sea which
one desires the anchor to withstand. Thus our
curve may be moved up or down parallel to
itself to conform to any given practice. The
American Bureau curve represents merchant
ship practice, while for warships there is a tendeney to reduce anchor size in order to save
weight.
Comparison of Results with Full-Scale Tests.
In February, 1934, a series of anchor and chain
tests wm~ conducted at Coronado Roads, California, by the U.S.S. Trenton. I n these tests
the following anchors were used: ll,000-pound
and 14,000-pound old standard Navy stockless,
and ll,000-pound new standard Navy stockless.
The procedure was to drop the anchor, lay out
the desired scope of chain, and build up the
strain slowly by backing the main engines until
dragging was noticed. The strain throughout
each run was recorded graphically by an electrical instrument connected to a special strain
Hnk located just outside the hawse pipe. Unfortunately, in most of the tests, there was not
sufficient scope out to prevent lifting the anchor
shank and hence maximum holding power of the
anchors was not developed. Also an analysis of
the resul~ shows a number of inconsistencies.
However, by using all the values obtained and
correcting them for lift of the shank (see Fig.
ANCHOR HOLDING POWER FROM MODEL TESTS
113
'~/a'fe r S u r f a c e
B o'H'om Sur~oce
~
A
Momeni" M'A'xL:AxSIn ~xL
Where A isfheflukec~rect
AI is ~he projecledfluke orec~
L is ÷hedlstancefrorn C.G.offluke
areaiolhe boll'ore surface
is'~he on~e belween shcmk
(~nclflukes
<9,1>
I~leval-ion "A-A"
FIG. 1 2 . - - ~ I O M E N T
(M)
OF F L U K E AREA BELOW BOTTOm[ SURFACE
13 reproduced from Massachusetts Institute of
Technology thesis by Howard and James for
correction to allow for lift of shank) it was possible to obtain average holding powers for each
anchor. These values are shown below compared
with calculated values based on our experimental
results.
Average
Trenton t e s t s
59,000
76,000
70,000
Anchor
ll,000-1b, old standard ........
ll,000-1b, new standard ........
14,000-lb. old standard ........
I00
go
\
\
L
~J 80
\
g
\
\
.-r-
<9,2>
g
\
Computed
(sand)
51,600
78,100
65,800
\
13
30
ZO
10
0
0
5
10
15
l0
25
30
Oegrees
55
4-0
4-5
50
FIG 13.--PERCENTAGE OF MAXIMU~f HOLOING POWER AT
VARIOUS ANGLES Vv'IT}I THE I-IORIZONTAL
The bottom for the T r e n t o n tests was fine g r a y
sand, which is not very unlike the sand bottom
used in our tests, and it can be seen that the
results agree within the limits of accuracy to be
expected in full-scale tests of this sort.
CONCLUSIONS
(1) The angle between the shank and flukes
should lie between 45 and 55 degrees for stockless anchors.
(2) F o r all types of anchors the holding
power is proportional to fluke area to the threehalves power, where the constant of proportionality is determined by the type of anchor and
the bottom. Thus, the fluke area, should be as
large as practicable.
(3) Since weight is proportional to fluke area
to the three-ha]yes power, holding power is proportional to weight to the first power. Thus it
is possible to compare holding powers on the
basis of weight, if the constants are known.
(4) I n comparing various types of anchors,
the moments of the fluke areas about the bottom
surface indicate their relative holding abilities.
Thus fluke area should be so distributed that
its moment is a maximum, consistent with
strength requirements.
(5) A correct size anchor for a new ship m a y
be determined from that of a known ship by
using the ratio, of their wind and current drags.
(6) American Bureau rules for anchor selection show that merchant ship practice is in accordance with the results of our tests.
114
ANCHOR
HOLDING
POWER
(7) The determination of anchor holding
power f r o m model tests appears to be a practical and accurate method. In using this method
FROM
5IODEL
TESTS
it should be remembered that model results
should be stepped up to full scale on the basis
of fluke area r a t h e r than weight.
DISCUSS[ON
THE PRESIDENT: Lieutenants Leahy and F a r rin have given us a concrete example of the
value of a thesis p r e p a r e d as a p a r t of the course
in naval architecture as students at M.I.T., their
p a p e r being a rewrite of their thesis prepared a
year ago. It has been clearly presented, and
covers a practical subject in which they have
certainly shown how real improvements can be
made in several features of anchor design.
~{R. E. H. RIGG, Vice-President: This p a p e r
forms a useful sequel to that given us by Admiral Land last year. Both papers give light in
dark places and are cheerful reading for the proponents of weight-saving chains.
The lighter chain has involved revised anchor
design to get equal overall anchoring characteristics, which is a check on the ideas that m a n y
people held as to the amount of holding power
contributed by the heavier chains, but not known
exactly until checked up by suitable experiments
and records thereof on a logical basis.
I t is not correct to attribute holding power
to the chain; on the other hand, scope and
weight of chain do contribute to anchoring characteristics. These experiments, therefore, m a r k
a definite gain in the art, by pointing the way
to e q u a l overall characteristics on less total
weight to be carried by the ship. This change
in anchor design is as pertinent to merchant
shipping as to naval.
DR. WILLIAM HOVGAARD, Life Member: The
action of an anchor falls in two stages: First, it
bites in the ground by the ploughing effect of the
flukes; second, it buries the flukes more or less
deeply in the ground and builds up a mound in
front of it. The tendency or faculty of an
anchor to bite in the ground, which is the prim a r y action, may be called the biting capacity;
the resistance to dragging after it has got hold
of the ground might be called the holding capacity. I t is the latter which forms the subject of
this p a p e r and which is called by the authors
the holding power.
The authors compared the more important
types of anchor by varying the linear dimensions
within each type. They eliminated as f a r as
possible all other variables by placing model anchors artificially ill the most favorable position
iu a bottom of homogeneous consistency and subjeeted thenl iu all eases to a horizontal pull.
I t was found that the resistance to draggino'
of any given type of anchor was depemlent ou
the fluke area, being proportional to A a/~. This
seems reasonable, since in similar anchors the
projected area of the entire ancllor must be proportional to that of the flukes, and the mass of
nlatter put in motion nmst be proportional to
that quantity. Since, moreover, the weight is
proportional to A3/°-, we have, as pointed out by
the autllors, the holding capacity proportional to
the weight, or P = K W , for similar anchors.
Thus the coefficient K characterizes the form
or design of the whole anchor and varies greatly
for different types. Tile information obtained
from the experiments as to tile vahie of K is
very interesting.
I t must not be overlooked, however, that in
these tests, as I understand it. tile anchors were
initially placed in the best possible position
for holding, and that therewith was eliminated
their biting eapaeity. In the absence of this
faculty, that is their tendency to take hold and
bite in the bottom, the anchor would simply
slide along the surface of the bottom and would
not possess any holding power. This point is
of the utmost importance when the bottom is
hard and stony.
A glance at the table on page 110, left-hand
column, shows that K is much greater for the
" H " type of anchor than for the A d m i r a l t y
type, while the service vahle of the latter must
be far superior when the bottom is hard and
whenever biting is difficult or doubtful.
In other words, while these experiments are
very valuable as f a r as they go, it is desirable
to have them supplemented by tests on the
biting capacity. Probably it will be found that
on soft bottoms, mud and sand, the biting
capacity is 100 per cent f o r all anchors, but
for clay and harder bottoms such as clay and
ANCHOR
HOLDING
POWER
gravel, clay and stones, etc., which may yet be
characterized as " g o o d " or " f a i r l y g o o d "
anchorages, common in regions of glacial origin,
the A d m i r a l t y anchor will probably be found
vastly superior to all the stockless types. Among
the stockless, the design of the tripping-pahns
will l~e pre~mnably the determinative factor,
which m a y establish a new order of sequence
in point of merit.
I realize the difficulties in making such tests,
but with the assistance of the H y d r o g r a p h i c
Office it should be possible to prepare model
bottoms of certain standard consistencies. I n
this way I believe that some broad conclusions
could be drawn as to what I would call the
"service v a l u e " of different types of anchors,
comprising both the biting and the holding
capacity. Thus the value of the experimental
work, so well begun by the authors of this
paper, would be much increased.
I n conclusion, I would like to add a few
words about the A d m i r a l t y anchor.
Fig. 8
brings out its superiority, but, if I u n d e r s t a n d
the p a p e r correctly, the curves are plotted on
the area of only one fluke. I f we take account
of the fact that on bottoms of medium hardness
both flukes of the stockless anchors may be active, while only one is active in the A d m i r a l t y
anchor, it appears that the abcissae for the
former should be multiplied by two, and in that
case the superiority of the A d m i r a l t y anchor
would become even more evident. There are
several good reasons why stockless anchors are
now generally adopted, but the inherent superiority of the A d m i r a l t y anchor with stock, as
established by these experiments, should be kept
in mind when it comes to the selection of anchors
for smaller vessels required to work under
strenuous service conditions all over the world.
REAR ADMIRAL E. S. LAND, (CC), U. S. N.,
Vice-President: I n this interesting p a p e r the
authors include a definite formula for estimating the holding power of various types of
anchors and suggest a method of anchor
selection.
While the value of the holding power shown
in Fig. 9 for the new Navy standard stockless
anchors has been obtained a number of times
in service tests, it must be borne in mind that
the holding power of any stockless anchor
varies considerably in the same type of bottom.
The stockless anchor consists essentially of a
crown with two flukes on opposite sides and
equidistant from the shank.
I n service one
of these flukes often penetrates more deeply
FROM
MODEL
TESTS
115
than the other. As the pull on the anchor is
increased, this difference in the depth at which
the two flukes are buried increases until finally
the u p p e r fluke comes out of the ground and
the lower one continues to rotate and comes out
on the opposite side of the shank from which
it entered. This is an inherent defect of the
stockless anchor. In comparison with the stock
less anchor the old stock anchor is a stable device and when the load upon it is in excess of its
holding value it drags through the bottom like a
plow and continues to offer its maximum resistance, thus setting up a steady drag. The
p r i m a r y reason that the stockless anchor is
used on all modern war vessels is the relative
ease with which it can be handled and housed
as compared with the stock type anchor.
The method suggested for anchor selection
by the authors appears sound provided chain
for use with the anchors is selected in a corresponding manner.
However, if relatively
lighter and stronger chain is used, as is the
tendency in the N a v y and the merchant marine,
when die-lock and cast-steel chain are specified,
some increase in the proportional weight of the
anchor used m a y be necessary. Service tests
are now being conducted by the Navy with the
view to ascertaining the increase necessary in
the weight and holding power of stockless anchors, as f u r t h e r decreases are made in the size
of the chain. This is being done in the effort
to decrease the aggregate weight of the ground
tackle without decreasing the safety of the ship
when at anchor.
Anyone who has ever tried to weigh anchor
on any kind of aviation craft on water, or has
tried to anchor, realizes that it is a rather more
complicated problem than weighing anchor or
dropping anchor on board ship. The efforts
that are made in this country and abroad will
undoubtedly be coordinated in the course of
the next few years and there may be some rather
interesting developments from the experimental
work that has been done in both countries.
COMMANDER H. E. ROSSELL, (CC), U. S. N.,
Member: The authors of this p a p e r have thrown
light upon several important features of anchor
design. I t appears from Fig. 9 that, by virtue
of comparatively minor changes in the shape
and size of the flukes of the old type N a v y
stockless anchor, the same holding power can
be retained with a reduction in weight of about
one-third. This is indeed a remarkable improvement and one that apparently can be obtained
witho~ut sacrifice.
116
ANCHOR
HOLDING
POWER
The authors are to be congratulated upon
the ingenuity of their experimental methods
and upon the consistency of their results.
There are two minor points which I should
like them to explain further. One is the portion of each anchor which was counted as p a r t
of the fluke; the other is the method of determining the moment arm of fluke area.
MR. G. I. TAYLOR,* Visitor: The results obtained by Lieutenants Leahy and F a r r i n agree
v e r y well with those obtained some years ago
by L. P. Coombes, t who showed that for geometrically similar a n c h o r s o f full-scale, halfscale and 1/5 linear scale the ratio of holding
power to weight is constant when tested in sand.
I n using the data obtained f r o m models for
designing full-scale anchors, however, care must
be taken that the strength of the full-scale
anchor is sufficiently great to withstand the
load which the design is capable of exerting.
F o r geometrically similar anchors the holding
power is proportional to the cube of the linear
dimensions.
This means that the stress per
square inch developed in the material is proportional to the linear dimensions. F o r this
reason, if an anchor is designed with a definite
factor of safety on the niaximmn load that the
design is capable of developing, the ratios cross
section to (length)-" of the members must increase as the linear dimensions increase. F o r
this reason the ratio holding power to weight
decreases as the size increases when the anchor
is designed with a definite factor of safety. The
diagram Fig. 9, which shows the relationship
between holding power and weight, does not
therefore represent the best that can be done,
but only the relationship for a given design
which must be unnecessarily heavy in the small
sizes, if it is strong enough in the biggest sizes,
or too weak in the biggest sizes, if it is the best
that can be done in the small sizes.
That this has not been understood in the past
is clear from the British " A n c h o r s and Chain
Cables Act 1899," which contains a schedule of
proof loads which must be applied to all anchors
from 126 pounds to 10 tons weight in British
ships. The proof loads v a r y from 65 times the
weight for the smallest to only 9.7 times the
weight for the largest anchor. The way in which
this table has been compiled is not stated in the
act, but by examining the figures I find that,
if a given design of anchor, of a given material,
* Cambridge. Eng.
+ " A n c h o r s f o r use on Flying" B o a t s . " by L. P. Coombes.
Reports and Memoranda, Aeronautical Research Committee.
May. 1931.
FROM
MODEL
TESTS
will just stand the proof load, then all anchors
of similar design and material, but of v a r y i n g
weights, will also just stand the proof loads
given in the table.
I t is clear from the figures given by Lieutenants Leahy and Farrin, that anchors of the
type " A " and " n e w s t a n d a r d , " which give a
pull of 12 times their weight in clay would be
fully strong enough if they pass Lloyd's proof
load test, provided they are small; but very
large anchors of about 10 tons might actually
fail when dragged through clay even though
they passed Lloyd's proof load test.
In discussing the increase in holding power
which can be obtained by increasing the blade
area Lieutenants Leahy and F a r r i n describe the
stockless anchor of type " H " in which the
crown is practically eliminated, the blades thus
being able to penetrate deeply into the ground.
They mention as an objection to its use that
robustness is sacrificed to efficiency. There is,
however, a f a r more serious objection, namely,
that anchors of this type arc unstable in the
ground. I f such an anchor is carefully placed
symmetrically on the ground and is dragged
exactly along the line of its shank, it will go
deep into the ground, but if the pull is exerted
off the center line, so that one blade gets a little
deeper than the other, the deeper blade will
exert a greater couple round the shank than the
less deep blade and the anchor will roll round
its shank till the lower blade comes out of the
ground on the opposite side to that on which it
entered.
The short extensions of the crown
which are perpendicular to the blades in all
stockless anchors get over this effect by preventing the blades from penetrating completely
into the ground, but in making the anchor
stable they r e d u c e its holding power to weight
ratio.
A new stockless anchor known as the C.Q.R.
has recently been designed which can b u r y itself and yet remain stable in the ground. F o r
this reason its holding power to weight ratio
is far greater than that of any other known
anchor. A 52-pound model dragged in m u d d y
sand gave holding powers varying from 3300
to 4000 pounds while in mud it gave a pull of
2500 pounds.
Anchors of this type have so far been used
only on yachts and seaplanes but, since riley can
be stowed in a suitably designed hawse-hole in
the same way that ordinary stockless anchors
are stowed, it seems that they might be used for
shipping generally.
The C.Q.R. anchor was designed entirely by
ANCHOR
HOLDING
POWER
means of models weighing 11A pounds. A f t e r
the design had been perfected on the model
scale, the first anchor weighing 521/.2 pounds
was made. Its performance was exactly in accordance with the predictions made from the
tests on the l~/~-pound model, using the law
P - K A 1"~ for the holding power. A 370-pound
anchor of the same design was tested officially
in the presence of Lloyd's technical representatives from a tug in the lower Thames. I t behaved according to prediction, but it was not
possible to make an exhaustive set of tests on so
large an anchor in view of the fact that its
holding power in clay bottom was found to be
over 10 tons which was the m a x i m u m force that
the tug could exert.
A model weighing 43~ ounces worked in fine
sand and gave slightly less pull than that calculated on the law P - K A , ~'5 but this model
would not bite into clay unless it were assisted
at first by pressing down the point.
FRO~.f MODEL
TESTS
117
day. We could not get it up. We got tugs and
tried to tow in the ship, but the cable parted.
I thought that might interest you as a practical experiment.
MR. RALPH A. MILLER, M e m b e r : Lieutenants
F a r r i n and Leahy have contributed a worthy
companion p a p e r to that of Admiral Land, read
before this Society last year. These papers show
that there is hardly a line of endeavor that can
not be benefitted by scientific research. Anchors
and chain are expensive, and uselessly large
ground tackle increases the investulent charges
a n d . decreases deadweight c a p a c i t y - - b o t h of
which militate against economical operation.
I t is interesting to notice that, for a given
weight, the new Navy anchor is, for all p r a c tical purposes, equal in holding ability to any
other anchor tested, excluding the extremely
light type " H " anchor, which is not considered
a service anchor. I n designing the new Navy
anchor, its holding power per pound of weight
COMMANDER A. L. P. I~{ARK-WARDI~AXV,
~:' was increased 50 per cent over the old Navy
Visitor: I notice in this p a p e r with interest, at
standard.
the end of p a r a g r a p h 7, page 114, that it says,
The authors have limited themselves to test" I n using this method it should be remembered ing available anchors, and deducing therefrom
that model results should be stepped up to full general laws applying to a series of sizes from
scale on the basis of fluke area rather than
a given design. I t is hoped that the success of
weight."
the Navy in increasing the holding power of
I thought it might be of interest to the mem- their anchors by 50 per cent on a weight basis
bers for me to recount briefly an experience will encourage the making of systematic investiI had when I was serving in Greece in the Naval gations to determine the effect of varying the
Mission thereabout six years ago. I was on board ratios of the various parts of anchors.
An examination of the four stockless anchors
the Kilkis, one of your interesting old ships sold
to the Greek Navy. One day, a young Greek in Fig. 10 shows that considerable difference
inventor brought us an idea he had, which was exists .in tlae shape of the crown, and the size
a patent anchor about which some of you no and shape of the " f i n s " that cause the flukes.
doubt already know. I t was stockless and had to dig in. The spacing of flukes, and their
t h e flukes laid in opposite ways, the idea being length-width ratios also vary. I t is suggested
that the influence of these features on holding
t h a t it would screw into the ground.
We encouraged him, as we thought it was a power should be investigated by a systematic
good idea. W e took it up and we made some series of models, as it is only in this manner
tests. The model tests were quite satisfactory. that the optimum design can be de,termined.
.Regarding the advantage of large .fluke area,
The anchor dug into the ground and, as far as
it may be interesting to know that anchors usedwe could tell on the model, it held well so that
in sand in the Mississippi River are provided
it was decided we would make a full-scale one.
Anchors with.
The full-scale model was lighter than an ordi- with unusually large flukes.
n a r y anchor, of the same material. We com- standard flukes have been enlarged by rivetin~:
missioned one of their old store vessels and we thick steel plates to .the flukes. Both A d m i r a l t y
got a strong cable. We laid this anchor in the and stockless types have had their fluke areas
bay. I n the early stages, where we got a sand enlarged in this manner. I t may b e said that
bottom, the results were v e r y promising, until none of these-anchors was housed in hawse pipes.
• F i g . 3 indicates t h a t the.angle between flukes
we got t0 clay bottom. Then its holding power
and. stock should be between 45 and 55 degrees:
w a s p e r f e c t , and it is probably still there to this
Since. there is a possibility of a short chain~. Ol:
a heavy surge raising the shank, it.appears that
A~sistant Naval Attache, British Embassy..
118
ANCHOR
HOLDING
POWER
the designed angle should be near the maximum.
say 521/.2 degrees. This would permit the shank
lifting 71/e degrees before the holding power
would be appreciably reduced.
I t is probable that the high holding power of
the A d m i r a l t y type anchor is largely due to the
long length of its shank, and the eoncentration
of the weight of the stock dose to the chain
shackle.
This weight distribution cannot be
other than highly effective in retarding the
lifting of the shank (and flukes) and, therefore, the " b r e a k i n g - o u t " of the anchor. The
manner of conducting the tests described in
the p a p e r was such that this feature of the
A d m i r a l t y anchor was not operative. At a certain load, a given shank will lift, and the
anchor's holding power will shortly s t a r t to
diminish. I t is evident that the greater the resisting nmment of the shank against lifting, the
greater will be the holding power of the anchor,
within the limits of fluke area and kind of
bottom. It, therefore, appears that stoekless
anchors should be designed with the shanks as
long as practicable, and with the largest practicable weight eonsentration near the chain
shackle. The data given in Tables I and 2 of
Admiral L a n d ' s p a p e r of last year appear to
indicate the correctness of the above views regarding shanks for stockless anchors.
It is interesting to compare the weight distributions of the A d m i r a l t y and stoekless
anchors. The weight of the A d m i r a l t y anchor is
distributed about as follows:
Stock, 20 per
cent; shank, 30 per cent; head, 50 per cent.
The weight of the stoekless anchor is about 40
per cent for the shank, and 60 per cent for the
head. I t is possible that a stockless anchor with
a weight distribution approaching that of the
A d m i r a l t y anchor would show greater holding
ability on a weight basis than the A d m i r a l t y
anchor, as the stoekless anchor has two flukes
imbedded, whereas the A d m i r a l t y anchor has
but one fluke imbedded.
The lengthening and weighting of the stockless anchor shank appears necessary in order to
obtain the maximum advantage of the highstrength anchor chains now available. The important consideration is not the cost and weight
of the anchor alone, but the cost aud weight of
the anchor and chain. While the data presented by Admiral L a n d last year are not
wholely eonsistent, the general indication is
that the new Navy anchor with 1.~/s-ineh chain
has better holding ability per pound of weight
of anchor and chain than any other combination
tested, although there is little difference be-
FROM
MODEL
TESTS
tween the holding ability per pound of the
ground tackle used in Series 3 tests and Series
4 tests.
However, assuming that a heavier
anchor will be needed to compensate somewhat
for the lighter chain, it is important to realize
that two 14,000-pound anehors, and 330 fathoms
of l');-ineh die-lock chain weigh a total of 83.400
pounds, whereas 330 fathoms of 21/2-inch ehain
and two ll,000-pound anchors weigh a total of
142,450 pounds. The adoption of the lighter
ground tackle would result in a saving of
59,050 pounds, or 26.4 t o n s - - a saving well worth
while.
This reduction of weight would also
reduce the size, power and weight of the anchorhandling gear. Usual anchor-hoisting tests are
conducted with 60 fathoms of chain overboard.
Sixty fathonls of 2V2-ineh chain and one 11,000pound anchor weigh 32,900 pounds, whereas 60
fathoms of l'~/;-inch chain and one 14,000-pound
anchor weigh but 22,940 pounds, thus redueing
the load one-third.
In view of the above, it is evident that the
next need is two comprehensive series of tests:
One to determine the true function of the ctlain
(in addition to tllat of securing the anchor to
the ship) to the end that the lightest chain,
compatible with safety, m a y be determined;
and the other, to determine the most efficient
form of anchor.
LIEUTENANT W. S. KURTZ, (CC), U. S. N.,
Mcmbcr: I t was very pleasing to read the p a p e r
by Lieutenant (jg) Leahy and Lieutenant (jg)
F a r r i n because their laboratory studies brought
out conclusions which were similar to what I
learned from larger scale experinlents made at
the Norfolk Navy Yard in 1932. The various
discussions of Admiral L a n d ' s p a p e r last year,
plus the interest shown this year on a subject
about which so little has been written, have
prompted me to add a few considerations that
were included in the design of the Navy stockless anchor which is now being used, in the hope
that it may guide or assist future investigators.
When faced with the problem ]laving to deal
with the design of an anchor, it was impossible
for us to find any information which guided
past developments and designers in reaching
their conclusions. The actual reasons for the
shape of flukes, for the weight of crown, for
the fluke length, etc., on the old Navy anchor,
the Eels anchor (referred to in this p a p e r as
type " A " ) , etc., could not be found anywhere.
It is hoped that the discussion given below, in
addition to the methods of testing and results
of same plus the practical results already given
ANCHOR
HOLDING
POWER
in Rear Admiral L a n d ' s paper last year, will
furnish those concerned in future development
with the reasons and thoughts centered in the
present Navy type stockless anchor.
Our problem was the development of a stockless anchor of greater holding power than that
then in use, and which would fit the same hawse
pipe used for the old standard stockless anchor.
Because of this latter limitation, and also that
of time for desired completion, the complete
study was confined solely to that involving increased fluke area and increased moment of
this area effected by the depth of fluke area
buried below the bottom surface. Also, since
lhe final design was to be embodied into an
ll,000-pound anchor, a 500-pound model was
used because it was not only readily available in
store, but because more reliable and comparative data could be obtained from a model of
about 1/20 scale than from a 5-pound, or even
30-pound model.
Maintaining the old type anchor crown, the
fluke area, volume or weight and length were
determined from the point to a section at the
throat of the fluke where greatest stress occurs
and the fluke fairs into the crown. Minor
modifications on the crown were made to permit
housing conditions because of the increased
length of flukes. A final design resulted in the
triangular sectional shape of fluke now adopted
on Navy anchors. This section o'ave desired
factors without increasing the weight of the replaced part of the fluke, and. in addition, its
stress diagram followed very closely that of an
anchor which Navy experience has thus far
dictated.
Preliminary tests with a 5-pound model in
shallow water at the end of building ways at
the Norfolk Yard showed that the holding power
of an anchor increases as the bottom material
piles up in front of the crown, and as the fluke
digs in at maximum angle until the shank is
parallel to the bottom. Also, that it is the
eccentric application of bottom resistance, as
load is continually applied, which finally causes
one side of the anchor to lift an d eventually
cause the anchor to flop over, and to drag. Any
looseness in the fitting of the shank in the crown
will also set up a couple which adds to this
action. With these tJaoughts in mind the base
of the triangular section was placed as far as
possible from the neutral axis, contrary to the
method adopted on the Eels type anchor. The
shape of the fluke points and their sharpness
were adopted to decrease resistance to digging
into hard bottom.
The sharpness permitted
FROM
MODEL
TESTS
119
better holding in r o c k y or coral bottom. It way
a compronlise. The adjacent sections to the
sharp points were designed to permit bending
thereof, and the dragging of the anchor on a
rocky bottom whenever a sudden or instantaneous severe load approaching the breaking point
of the chain nlight occur.
The longer flukes required the use of a longer
shank, which is limited by the length of the
hawse pipe. It was hollowed or fluted into an
" X " shape to compensate for the additional
weight of the longer shank, the stress equalling
that of the old rectangular type. I n determining this length, as well as the angle of pivot, it
must be remembered that it is desirable to have
a small vertical component on the pull of the
chain which will trip the anchor before stressing the chain beyond its safe working load. To
remain within weight limits it was necessary to
lighten the crown. This was done by wedgeshaped, cored-out sections in the bottom of the
crown, which aid as well in obtaining a better
crown cast section.
Results so far obtained have proved to a limited extent the greater holding power possible
with the new Navy stockless anchor. As data
accumulate from service experience with tim
new type Navy stockless anchor, it may be possible to make' still further improvement in the
holding power of anchors.
LIEUTENANT LEAIIY: First, I wish to express
my thanks to all of those who have interested
themselves in the paper and discussed it and
have, by their comments, greatly increased its
value.
Mr. Rigg and Admiral Land mentioned something which seems to me important and wbich
should be further stressed. This paper has dealt
only with the holding power of the anchor itself.
We all know that the chain plays quite a large
part in the holding ability of ground tackle. It
seems to me that it would be of great value, if
experiments, in addition to the service tests mentioned by Admiral Laud, were carried o11~to find
out just how light we can make this new,
stronger chain and whether it would not be better perhaps to increase the size of the present
anchors and by going to a lighter chain save
weight on the ground tackle as a whole.
Referring to Professor H o v g a a r d ' s discussion,
I heartily agree that it would be very desirable
to have the biting capacity of the various anchors
investigated. In the tests we made, we did not
touch on that at all. It certainly is a very large
part of the service value of an anchor.
120
ANCHOR
HOLDING
POWER
I am very glad that both Admiral Land and
Mr. Taylor have pointed out a feature of a stockless anchor which is objectionable; namely, that
it is unstable in the ground. This condition is
aggravated by reducing the weight of the crown
and especially that of its extensions, sometimes
called tripping-pahns.
Mr. Taylor states that this is a very serious
objection to the type " H " anchor, where the
crown has been practically eliminated.
Evidently, the designer thought of this because he
has provided a wide shank which resists rolling
about the axis. It is well to realize that in considering fluke area and its distribution, we must
not lose sight of stability.
I n answer to Commander Rossell's questions,
first, what portion of each anchor was counted as
part of the fluke? The flukes are those parts
of the anchor proper which are designed to dig
into the bottom. In measuring the fluke area of
the stockless anchors, we included as flukes all
that portion from the fluke tip to the crown which
lies approximately in one plane; thus all of the
anchor proper, exc'ept the crown and its exten,
sions, which are sometimes called tripping-palms,
was included in the flukes.
I n the determination of the fluke area of the
stock anchors, the fluke was considered to be one
of the approximately triangular portions, often
referred to as pahns, at the end of the arms.
I n case of the mushroom anchor, the whole bowlshaped head was included in the fluke.
Second, what method was used in determining
the moment arm of the fluke area? At a point
during the pull of e ~ch anchor when maxinmm
holding power had been developed, the distance
of each end of the shank below the surface of
the water was measured and the height of the
water above the bottom surface was noted. Having these measurements and the location of the
center of gravity of the fluke area, which was
calculated from the projected shadow with the
aid of a planimeter having a moment wheel, it
was possible to determine the position of the
anchor in the sand, and hence the moment arm
of the fluke area. Because of slight variations in
the sand level, it was deemed preferable to
measure down in each case from the surface of
FROM
MODEL
TESTS
the water and to make measurements on ten separate runs to obtain an average value.
Commander Mark-Wardlaw's practical experiment is very interesting and points out possibly
that we have to be careful not to make the
anchor so efficient that when it once buries itself
we are unable to get it up. On the old stock type
anchors, as we all know, sometimes it was necessary to use a tripping line. Unfortunately, the
average stockless type does not lend itself readily to such gear.
Referring to Mr. Miller's discussion, I do not
feel that the high holding power of the Admiralty type anchor is due so much to the length
of its shank and concentration of stock weigh~
close to the chain shackle, as it is to the large
lnoment of the fluke area about the bottom surface. I doubt if long heavy shanks would add
appreciably to the holding power of stockless
anchors. The weight so used could be placed in
the flukes to lnuch greater advantage. This is
demonstrated by the new standard Navy stockless anchor which was developed from the old
standard by lightening the crown and shank and
putting the weight so saved into the flukes, the
result, as has been pointed out, was greatly increased holding power.
I want to thank Lieutenant K u r t z for his interesting exposition of the problems encountered
in designing the new standard Navy stockless
anchor; it should be of interest to anyone concerned with anchor design.
THE PRESIDENT: The paper is such an interesting one, on such an interesting subject, one
which we have been so glad to have in the
Transactions, that I am sure I express the
thanks of the members of the Society to Lieutenant Leahy and Lieutenant F a r r i n for presenting it and to those who have discussed it.
I feel so strongly the necessity, or great desirability, of having operating officers here to help
in the discussion, that I will repeat my thanks
to the young officer from the British Embassy
who has joined in the discussion. I think that
those little incidents of actual experiences afloat
cover problems of design and are very helpful. They always add zest to the subject.