Autogiro Boats - Simplified Theory Part 2

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What I would like to be able to afford to do would be to put something like this trimaran into a wind tunnel with a datalogger and measure all the forces on it. Any offers of funding please contact me.

Of course I can calculate them using CFD, but how can you trust the answers when the CFD programs were certainly not intended for this particular use ?

Centre of gravity

One obvious disadvantage shared by all the wind turbine boats that we have been looking at is that they all have a very high centre of gravity - roughly up at hub height. This is made worse if you are using the wind turbine to drive a propeller, since you not only have more heavy machinery up in the hub, but you also have to gear the drive through several angles to get it to the propeller. It is not just the centre of gravity which is high, but also the centre of rotation and the centres of the gyroscopic forces, giving stability problems. Not to mention the problems of having to climb up the mast for maintenance - suppose you lost a blade in mid ocean ?

Those few built so far have all been seriously underpowered, but I think that any yacht built for high speeds with a large high-hub turbine could hardly help capsizing itself. My one, on the other hand, would be rather less likely to capsize than an ordinary boat.

I have been considering a higher tech version of the Dumpleton idea. As you can see, the centre of gravity is very low, and if driving a propeller is necessary, it would be practically a direct drive. The hub could be positioned above head height, because of course you don't want the blades catching the wave crests. The layout of the boat can be arranged so that there is no need to walk in the path of the blades while they are turning - in fact it could be controlled entirely from the inside - they can be stopped and lashed to some convenient part of the boat when the ship isn't moving, so that it would be perfectly safe to walk across the front of the boat while mooring or berthing it.

Efficiency

This type of turbine has less theoretical efficiency than the straight flat type, but I think that it would beat it for a number of other reasons, because the theoretical efficiency is not the only thing that should be taken into account.

The important thing to remember is that maximising the forward thrust and lift-to-drag ratio are not everything. They matter, but not as much as some other points - maximising one thing generally involves reducing another, and the gain in one can be smaller than the loss in the other.

Too many researchers have concentrated on maximising the theoretical efficiency, and then have not been able to understand why much less efficient boats are overtaking them - but the reasons are obvious if you think about them. Apart from everything else, yachts with greater stability which require much less work from the crew during a long race do frequently beat theoretically more efficient yachts whose crews become too tired to operate their ship efficiently long before they reach the finishing line.

Effects of yaw angle



Turning the turbine at an angle to the wind does not reduce the thrust by all that much. The power drops off, but there is not much drop in the thrust. Of course, you also get a side force - but then you do with a sail as well. Thrust increases as pitch becomes finer, side force reduces. Power is maximum at medium pitch.



These are the loads, forces and bending moments along any blade, in-plane (left) and out-of plane (right). As you can see, the yaw angle has very little effect on these, as the loads are due largely to the high rotation and not so much to the oncoming wind. Turning your turbine at an angle to the wind does not put nearly so much stress on it as you might suppose.

Possible improvements to the basic turbine

There are many flash-looking devices which people come up with which are supposed to increase the efficiency of a wind turbine, but when these devices are installed it is usually found that the turbine worked better without them. I could give a long list of examples from the Wind Energy field of devices on which very large amounts of development money have been spent, and whose publicity was hyped up to make them sound like an enormous benefit, but which in practice actually reduced the performance of the turbine. One reason for this was that the devices frequently went wrong, and sometimes stopped the whole turbine every time that they did so. Another reason was that some basic principle had been ignored in the design of the device - such as its own drag. But frequently the problem was simply that the person designing the device had been concentrating solely on the effect that the device was intended to have, and not even noticing the consequent effects caused by it. For example, turning a turbine blade to a different angle may give it a theoretically higher lift force - but will the mechanism needed to do the turning cause a lot of drag, or need power to run it ? Will running with the blade in that position cause structural problems ? All too often, these devices have turned out to be one step forwards and two steps back, and the theoretically less efficient turbines actually worked better.

As a general principle, the more moving parts you have, the less well the boat is likely to work. I do not mean moving as in adjustable - I mean moving as in relative motion while the boat is sprinting down the measured track. For the rotor to spin as a rigid body would be essential for any speed attempt - although even that is unlikely to beat a boat with no moving parts. Any added mechanisms like cyclic pitch variation will only slow the whole thing down, because they will absorb energy.

In fact, you do not actually need cyclic pitch variation on a boat, unlike on a helicopter. There, you have one blade going into the wind and another retreating out of it, with significantly different lift forces, so that you have to have either cyclic pitch or flapping hinges. That is because the helicopter rotor is moving sideways-on to the flow, whereas the wind turbine rotor is moving almost head-on into the flow.

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