A Flettner-Driven Catamaran (draft 23 February 2007)

A Flettner-Driven Catamaran
(draft 23 February 2007)
S H Salter, School of Engineering and Electronics, University of Edinburgh. S.Salter@ed.ac.uk
Background
As part of a plan to use could albedo control for global warming we need to build remotely-controlled
wind-driven vessels dispersing a fine spray of sea water from remote offshore locations anywhere in
the world. These will make the clouds over the sea reflect more solar energy back out to space. The
main reason for Flettner drive is that it particularly convenient for automated navigation because the
direction and magnitude of the thrust are controlled directly by the rotation speed up to a point where
the force self limits in high winds. A computer can change motor speeds much more easily than it can
change sails. The power to drive the spin will be between 5 and 10% of the power of a conventional
engine driving the vessel at the same speed. This will come from turbines being dragged through the
water as in the Solomon Technologies electric-boat system. Their motor could also be used for the
rotor drive. The lowest power rating on offer is 6 hp or 4.47 kW and would be suitable for the rotor
drive.
There are several other advantages. Forces go with the first power of wind speed not the square. With
the addition of Thom fences, lift coefficients of 20 and lift drag ratios of 35 can be achieved so that
vessels can sail much closer to the wind, within 20 degrees according to Joe Norwood writing in 21st
Century Multihulls. Vessels can apply brakes and go rapidly into reverse. With two rotors or more they
can turn about their own axis. If, in a head wind, the spin direction of one rotor is reversed there is a
powerful yawing torque that avoids all possibility of being stuck in irons. The only weak point of
sailing is directly down wind!
The vessel can have controls just like those of a car: a steering wheel, a foot accelerator, a foot-brake
and a forward/reverse selector. We can have what the machine tool people call ‘potted cycles’ for
going about and very easy connections for the most sophisticated cruise-control and auto-pilots linked
into collision avoidance radar and global positioning systems.
The final vessels will have water line lengths of 45 metres and a displacement of about 200 tonnes.
But before we finish the design we want user experience of the smallest Flettner vessel which can
safely be used at sea. The Discovery Channel has offered £100,000 towards the project. They want an
exciting visual image of a fast, very agile and very easily controlled vessel but it will not be possible to
include any cloud spraying.
Specifications
The present plans are for the final vessel to be a trimaran but we should consider catamarans and
mono-hulls. Catamarans are said to be cheaper than trimarans and may be better with regard to
heeling moment. The only reason for the trimaran was the space for spray equipment and this might
not be a very strong argument.
The case for two rotors is quite strong because of the extra agility and very tight turns. However the
high pressure field round one rotor may interfere with the low pressure field of the other and so they
must be as far apart as possible. A lot of the cost is the tooling and so it may well be cheaper to get the
same rotor area with two small ones as one big one.
Most sailing vessels pass forces as direct tensions through stays and shrouds as well as a direct force at
the mast foot. The mast is often pin-jointed to avoid transferring any bending moments and allow
unshipping for transport or even going under bridges. But shrouds and stays are less convenient for a
Flettner drive because things are rotating where the mast head should be. There is plenty of room
inside a rotor for a very strong, large diameter mast but it will have large bending moment at the base.
These might well be equivalent to the forward thrust applied at, say, 5 metres above deck level, larger
than allowed for by the hull designer who was expecting direct tensions and thrusts.
We have to find a way to pass the bending moment into the hull at the places where it is strong,
probably where the stays would have been attached. We need information about safe loads at these
points and may also need an increase of local strength.
A good way to fit rotors to a catamaran would be to make a triangulated framework with either three
or four arms and a central socket. The plan view of a typical catamaran is a capital letter H with a very
wide cross bar which forms the cabin. The bows (top verticals of the H) are usually linked by a
second cross bar which supports a net. The two forestays are connected close to the ends of the bow
cross-bar. A four-bar frame would replace the bow cross-bar with the upright of a capital letter K and
bring the sloped arms of the K to the junction of the forward line of the cabin and the hulls. A three
bar one would be in the form of a capital T with the ends of the top of the T at the bows and the bottom
of the upright of the T at the centre of the cabin.
In order to resist the bending moments for the rotor the truss should go as far below deck level as
possible but could be faired in regions likely to be hit by waves.
The stern attachment would be more difficult. Most designs have fittings or clamp plates for outboard
motors which deliver thrust with a much smaller lever arm that a Flettner rotor It should be possible
the make an attachment to these fittings and then add a central, triangular fore-and-aft skeg below the
hull under the cockpit to withstand the bending moment. This will need a pattern of drilled holes
through the underside of the cabin which may not be at all welcome and might also reduce future
resale value.
A convenient interface to the mast for both fore and aft trusses would be a vertical conical socket
forming part of the truss.
Option one would be to estimate the bending-moment resistance of existing hulls and choose rotor
dimensions and spin speeds which would come just up to that level. This would allow the use of quite
an old catamaran with scruffy insides.
Option two would be to estimate the cost and weight of extra local strength added to an existing
catamaran. This would allow more rotor thrust and a more exciting vessel.
Option three would be to make a new hull in an existing mould with extra material thickness at the
critical points. This would avoid anxiety about bonding on to old, dirty glass-reinforced plastic. The
vessel could have minimal internal fixtures.
The water line length should be small enough for safe operations with two, six hp Solomon drives but
be able to take two crew, one camera-man and one producer.
I suggest that we send this note to a selection of catamaran builders and ask if they are interested. If
they are, I would need numbers for applied loads fore and aft and enough shape and dimension
information to allow me to design the trusses.
We should also think about the future ownership of what could be a very attractive, safe and high
performance sailing vessel and of the intellectual property which will arise. If we have a prospective
purchaser the very tight finance a limit can be loosened.
Future owners will be interested in the following pictures and graphs taken from Flettner’s paper and
Joe Norwoods book 21st Century Multihulls.
Figure 1 shows the size of Fletter rotors compared with the sails used on the ship prior to conversion.
Buckau won a race from the Baltic to Leith against her unconverted sister ship.
Figure2 shows Flettner data for the lift and drag coefficients of rotor and sails.
Figure 3 shows lift and drag coefficients of Flettner rotors with aspect ratios of 6.7 and 12 along with
one having additional Thom fences at intervals of 0.75 of the core diameter.
From Norwood 21st Century Multi-hulls, published by AYRS.
Figure 4 shows the self-reefing effect of a spinning rotor at very high winds speeds. The heavy curve
with the lower case v is force at a peripheral speed of the rotor of 24 metres per second for both
Buckau’s rotors. The dashed curve is masts shrouds and stays but no sail. This should make rotors
much safer from capsize than conventional sails. Sorry about the units and tilted scan.
Figure 5 is Joe Norwood’s predictions of the speed of a man-powered vessel at various course angles
to the true wind.