Re: Islander 28 propeller location from shaft support?
Electric motors are are a really different beast when compared to diesel engines. Unlike diesels, you can't damage your electric motor by under loading it (e.g. under propping it) but you sure can by overloading it.
For the rest of this post, I will assume your boat has an induction motor. Wikipedia has a really nice write-up for induction motor basics.
Take a look at the section on torque. Those graphs shown are for polyphase motors rather than single phase, but the graphs for your motor will look similar. As you can see from the graphs, torque is determined by the difference in power supply frequency and motor RPM. That difference is determined by load on the motor. When load goes up, current goes up as speed slows down, to a point.
However, the "to a point" never comes into play in properly designed applications. In practice, you don't ever want slipping to be more than 5% of frequency or so. Otherwise, current goes up far enough that your motor overheats if you leave it that way very long. That means it really doesn't matter whether your induction motor is type A, B, C, or D (although I am betting on single phase type B). You will always operate it in the steep part of the torque curve to the far right of those graphs where slippage is small.
T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama
Last edited by dacap06; 04-25-2013 at 08:37 AM.