Prop placement

I am a neophyte regarding boat design. I'm looking for comments regarding shaft/prop placement on a 12'-14' (6' beam) HPV/electric catamaran:

1) what effects on performance/steering are there if the shaft is placed 12" off-center (either to the starboard or port)?

2) what effects on performance/steering are there in the case of the above AND the prop is closer to mid-boat than fully in the stern?

Thanks.

 

I'm no architech, but this one is pretty easy- If you put the prop off center, it will push that side of the boat over, just like in a twin engine aircraft- for example, if you lose the #1 engine (port) it's gonna swing over to the left hard. On a boat you'd have to have a constant rudder correction that is variable for the speed you're going. That means as you speed up, the angle of deflection will have to increase proportionately to your thrust.
If the prop was placed mid hull, vice in the stern area, I would imagine that it wouldn't be as efficient because the boat would never plane to full advantage. If such a designed boat where to try to plane out, the hull would try to, but once you got air in the prop you'd start to porpoise.

 

Thanks, Danny. FYI, it is not a planing hull, and we are talking about speeds of under 10 mph.

Carlazzomark

 

Assuming the Drive shaft is placed 12" off center, there will be torque steer toward the opposite direction. Since power levels are minimum (less than 10 mph) this can be offset somewhat by placing the rudder, on the same side as the power, as far as possible from CL, thus creating an offsetting drag.

Jango

 

At low thrust level on an easily driven hull an offset prop will not make much difference.

It is quite easy to work out but I doubt that it is worth the effort. The rudder will have a much greater moment arm than 12" (lets say 10ft so 10X arm) and the lift to drag on the rudder could be easily 10 or more so the extra drag to offset the offline thrust would be:
Drag = Thrust/10/10 = 1% of the total thrust in this example. Unlikely to cause serious concern.

You also have to remember that there is a tendency for a prop to walk sideways due to the rotation and difference in flow/pressure across the height of the prop so it could be argued that the ideal would be a small offset anyhow.

With my pedal powered boats I rotate a starboard mounted prop anti-clockwise and a port mounted prop clockwise. This means the torque on the prop shaft can be set up to counter the roll caused by the offset weight of the shaft and drive system.

Yellow boat in this video has port mounted prop and black one has starboard mounted prop:
tp://www.youtube.com/watch?v=ckWqIgmVM4Y
The yellow one will steer slowly to starboard if you go flat out. The black one has less shaft offset and tracks true through out the speed range.

Rick W.