Hull speed for your boat is about 5.3 knots, so 4 knots is about 75% of that. I don't think a 50lb trolling motor will give the performance you want. If you change the prop, you could make it more efficient and get closer to your desired speed. A 100lb thrust motor would probably be appropriate.
One rule of thumb is 1A for each pound of thrust at 12V. I'd suggest going with a higher voltage system, like 24V or even 48V, since that will divide the current draw by 2 or 4, so you don't need to use really thick wires and the associated connections. Also, your Peukert losses will be divided as well, so less wasted energy.
It sounds like you want to use your batteries as ballast, which works in your favour. So you can use reasonably priced heavy batteries i.e. lead-acid.
Power required vs. speed is quite exponential, meaning it takes very little energy to make the boat start moving, but each increase in speed (say 1/2 knot) takes more and more energy than the last increase. So if you want to go 4 knots, and you're operating at 75% of hull speed, the last knot to get there will require a fair bit of power. I've seen graphs of power vs displacement speed on this forum.
Let us know how you progress with your plans.
Posted by: cpcanoesailor@yahoo.ca
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