Whoops! Red face. I foolishly relied on memory to do the calculations that I posted on 27/06/2010. They are incorrect.
The Ampair 100 manual says that the drag force is 22kg in 50kts of wind.
The drag force increases as the square of the wind velocity, so the drag force in 35kts of wind is 22x35^2/50^2 = 10.8kg.
For my 10m yacht, I estimate that the wind generator
is 6m aft of the Centre of Lateral Resistance (CLR) and that the centre of force on the rudder is 5.5m aft of the CLR.
If the yacht yaws Q degrees from the direct downwind course, the rudder force (R) necessary to stop the yacht yawing further off course is related by,
I estimate that the centre of force on the (unbalanced) rudder is 0.2m aft of the rudder shaft and the grip of the tiller is 1.2m from the line of the rudder shaft.
The force on the tiller grip (T), is related by Tx1.2=Rx0.2, T=Rx0.2/1.2
For 20 degrees yaw, the force on the tiller necessary to prevent further yaw is 0.7kg
For 45 degrees yaw, 1.4kg
For 90 degrees yaw, 2.0kg
These are not large forces, so why was it so difficult to steer with the wind generator
1. My assumptions are wrong.
2. My calculations are wrong, and the forces are higher.
3. It was not the magnitude but the erratic rapid fluctuations of the force that made the steering
A friend with a PhD in mechanical engineering has checked that the above assumptions and calculations are correct. He agreed with “3” above.
I contacted Ampair, who said they have never heard of the generator affecting
. The newer Ampair 100s have a considerably greater tail “volume” (area x distance of centroid from pivot) than my old one. The newer Ampair 100’s may be considerably more stable in direction than mine.