Builders' Forum Archives
Re: electric kayak
Posted by Laszlo on Mar 17, 2007
It sounds like it's the off-center thrust causing the problem. So mount your motor so that the prop is as close to the surface as possible, while still staying completely submerged. Then, as you speed up and the bow comes up, tilt the motor backwards until the bow comes down to where it should be. That'll fix your problem.
The perfect solution would be to raise the prop to the point where it's on the vertical center of gravity. Unfortunately, it would cavitate and you'd be wasting power just whipping up the water. So the next best solution is the Space Shuttle take-off approach - tilt the motor backwards until the center of thrust is aligned with the center of gravity. Then the upward thrust component will cancel the downward torque component. (If you watch a shuttle launch very carefully, you can actually see this happen. The shuttle stops moving forward as it climbs up away from the tower when the engines get properly aligned with the center of gravity.)
Tilting the motor backwards is the most efficient way of handling this. Adding weight forward will slow you down and stres the hull. Moving existing weight forward will stress the hull. Adding beam in the stern will add weight & mess up the hull shape. Tilting the motor will lose you a bit of the thrust, but only proportional to the cosine of the tilt angle. At a 10 degree angle you'd only be losing 2%, or about a lb of thrust to the tilt. A real engineer would probably point out that the angle of attack on the propellor has changed and will affect the thrust, but it's too early in the morning for that kind of calculation :-)
Have fun with your electric yak and don't run over your extension cord,
In Response to: electric kayak by steve evans on Mar 16, 2007