Surface drive side thrust

[BarendGrobler]

Hey all
I was wondering, how do/can you counter or compensate for the side thrust from surface props on single drive installations? Surely if just ignored, handeling will be hampered and the boat may even become unstable?

[Rik]

It would be great if you gave more details on your boat, and the drive. A picture would go a long ways also.

[BarendGrobler]

I don't have such a boat, I'm asking out of a design point. I've been involved in the design of a couple of high speed patrol boats with twin surface drives; and in model testing we found the upward thrust from the surface props to have a significant effect on the trim of the boat, which then can be compensated for by moving the LCG further aft.
I was however now considering a surface drive for a smaller boat (7m) on which there is only room for one propulsion system, and I am concerned about the unsymmetrical nature of the drive; the upward prop force I mentioned above, now only being present on the one side of the prop (the side "slapping" into the water).
Surely this force will affect the roll-stability of the boat and have a large affect on the boat when banking into turns?

[mydauphin]

Originally Posted by BarendGrobler

Hey all
I was wondering, how do/can you counter or compensate for the side thrust from surface props on single drive installations? Surely if just ignored, handeling will be hampered and the boat may even become unstable?

Yes, your right. That is why many surface drives specially single engine small units like Arneson and Levi have side rudders to curtail that. On larger boats with two units it does not matter. Someone told me the run better with a central fin separating propellers. I have to try that one day.