Autogiro Boats - Advantages

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WHY have wind turbine boats at all - what are the advantages:

Sail area rules:

One obvious advantage was noticed by Lord Brabazon in 1933 - if the rules allow a certain sail area, and you are allowed to count this as the blade area rather than the swept area, it is then possible to race your boat against sail boats with a smaller sail power than yours.

A wind turbine has a much higher drag coefficient than a sail, in the usual way of calculating turbine forces, and also a much lower thrust coefficient - when calculated using the swept area. So a wind turbine boat would stand no chance against a sail with area equal to its swept area. Thrust depends on area times thrust coefficient. However, if you are allowed to count only the blade area, the apparent thrust coefficient calculated using the blade area is much higher - you could win all the limited area classes, if your boat was allowed to enter them at all.

Of course, it would stand no chance in any unlimited class speed trial - it could not compete with a solid wingsail in a short sprint. Its advantages lie in the longer-distance races and voyages.

Ease of reefing:

Another big advantage is that reefing the turbine to reduce the forces in high winds does not involve having to reduce the blade area. There is none of the usual bother of having to change to a smaller sail involved. The thrust force depends on area times thrust coefficient.

With a sail, the thrust coefficient can't be varied except by turning the sail to a different angle - and even then the wind direction keeps changing, so reducing the thrust safely really means reducing the sail area.

With a wind turbine, you can alter the thrust coefficient by altering the pitch of the blades. (If you can arrange to be able to do at the turn of a switch from the cabin, you won't even get wet). This reduces the force coefficients, while keeping the area the same - therefore reducing the forces.

What do you mean, 'takes all the fun out of it'! Who said that ?

Response to controls:

Similarly, turbines can also be stopped quickly. Changing the pitch of the blades is much quicker than taking down the mainsail, and IF the turbine is fitted with an adequate brake, the boat can be stopped very quickly.

Another big advantage for single-handed sailing is that the turbine could be controlled entirely from inside the cabin. Steering, as well as reefing. With an automatic control device rigged to an anemometer, it could be arranged for the reefing to be adjusted while you slept, so that the turbine would put out a constant force along a constant path - like a self-steering device that also does all your sail changes for you.

In a long-distance race, the boat should be able to carry more sail power than if it was using a sail, since the full sail area will be always present and the sail power can be altered at the turn of a switch, the sails can be left on full power for more of the time than if a lengthy sail change was required every time the wind changed.

Upwind sailing:

One frequently quoted advantage, which might not be as advantageous as all that so far as speed goes, is that the boat can be driven directly upwind. Unfortunately, they do not seem to go upstream as fast as the velocity made good to windward on a tacking sailboat, so maybe all the emphasis which other people have put on this particular point is a pity. They could have spent their effort on some of the other advantages of wind turbine boats instead.

Additional advantages are that the wind turbine puts less stress on the boat than an equivalent sail would do, and that the high drag of the turbine gives good dynamic stability in roll. It should have a steadier trip, which should make it faster in waves.

Fringe Benefits:

There are also fringe benefits - like you no longer need space inside the boat to store sails - let alone a whole lot of wet sails. If you want to take a spare blade, you can strap this on to the outside, since it will be a streamlined shape - along the top of a trimaran float, for example. You do not get wet doing sail changes, and therefore do not have the problems of storing or drying wet clothes. You can manage with a smaller crew - not just fewer in number, but also smaller, weaker people - ideal for a retirement boat or a pleasure cruiser, not just for shorthanded racing. Also, you can take off some of the power for charging batteries - especially if you are not in a race at the time.

Noise:

And then there is the question of noise. With no shrouds and stays continually jangling against the mast whenever the wind blows, it will always be much quieter than a sail boat when it is at rest. While sailing, a properly designed turbine with solid blades makes much less noise than flapping sails - just a gentle swishing sound. If anyone is thinking that helicopters make a hell of a row - that is mostly the noise of the engine rather than the blades. I know that some types of wind turbines do make a lot of noise, but these are usually badly designed two-bladers with hollow blades. Anything thin-skinned and hollow will always be noisy - you can get around that by filling hollow blades with foam. The two bladed turbine is dynamically unstable (see under "Theory"), hence the noise.

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