Cal,
If direct drive. Not sure what your Kv is but a low loaded rpm/volt and therefore higher torque/amp works best, keeping shaft rpm down, providing you have the voltage you need for the max power you need. You may find a 24 or 36 V battery might be enough for your boat with increased Ah, if you happy about series/parallel, for better peukert. As I'm sure you are aware due to copper losses you will likely have to go smaller on prop diameter and/or less pitch to keep those Amps down. Typically as you may recall I use 1,000 Watts for cruise at 4 knots and up to 1600 'continuous'. 1600 for me is about 20V/80 Amps at motor/controller and so around 65 at battery, which is fine with my 200 Ah Lithiums. Peak shorter term power for my 3.5 ton/26ft boat is around 2.3kW at the battery. Being as I chose to minimise batteries, my Lithiums at 25.6V nominal and 200 Ah give me 5.22kWh stored x 80% so 4kWh useable and therefore 16 miles range or a little more if say 3.5 knots.
I know there are those on the list that advocate more power for 'situations' but my view is minimising batteries keeps costs down, and it's a sailing boat with an auxiliary motor and I'm happy to sail that way.
John
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