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Old 29-06-2010, 22:06   #16
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Boat: Brolga 33, Osprey-A
Posts: 6
I am not assuming a static hull. The rudder cannot put a force on the water if the boat is not moving through the water.

When the boat yaws off course, the moment created by the force of the wind on the wind generator about the CLR must be countered by the moment induced by a force on the rudder for the boat to return to its original downwind course. More generally, if the moment of the windage aft of the CLR exceeds the moment of the windage forward of the CLR, the rudder must be used to counter the difference.

The wind forces increase as the square of the apparent wind velocity. The force that the rudder can exert on the water is limited by how fast the boat can go. If a displacement monohull is pushed too fast downwind it becomes uncontrollable ie the wind forces exceed the force that the rudder can exert on the water.

Big light boats can go fast because their displacement speed potential is higher and because the can plane. They can remain in control with relatively small rudders because the water moves over the rudders faster and, down wind, the apparent wind is significantly reduced. Just because they can be controlled by tillers does not mean that there is only a small force on the rudders All of those boats have high efficiency foil rudders which are balanced i.e. about 25% of the rudder area is forward of the rudder shaft. This design allows the rudders to put large forces on the water with very small forces on the tiller.

I am sorry if the above sounds a bit pedantic, but I cannot understand what you wrote.


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Old 30-06-2010, 17:16   #17
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Boat: Brolga 33, Osprey-A
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Wind Generator Affects Steering

I am not assuming a static hull. The rudder cannot put a force on the water if the boat is not moving through the water.

When the boat yaws off course, the moment created by the force of the wind on the wind generator about the CLR must be countered by the moment induced by a force on the rudder for the boat to return to its original downwind course. More generally, if the moment of the windage aft of the CLR exceeds the moment of the windage forward of the CLR, the rudder must be used to counter the difference.

The wind forces increases as the square of the apparent wind velocity. The force that the rudder can exert on the water is limited by how fast the boat can go. If a displacement monohull is pushed too fast downwind it becomes uncontrollable ie the wind forces exceed the force that the rudder can exert on the water.

Big light boats can go fast because their displacement speed potential is higher and because the can plane. They can remain in control with relatively small rudders because the water moves over the rudders faster and, down wind, the apparent wind is significantly reduced. Just because they can be controlled by tillers does not mean that there is only a small force on the rudders All of those boats have high efficiency foil rudders which are balanced i.e. about 25% of the rudder area is forward of the rudder shaft. This design allows the rudders to put large forces on the water with very small forces on the tiller.

I am sorry if the above sounds a bit pedantic, but I cannot understand what you wrote.

Ian
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Old 09-07-2010, 23:18   #18
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Boat: Brolga 33, Osprey-A
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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,
10.8x6sinQ=5.5xR, R=10.8x6sinQ/5.5.
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
T=(10.8x6sinQ/5.5)x(0.2/1.2)
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 spinning?
Possible answers:
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 difficult.

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
steering. 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.


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