Well, as I understand it, the rule of thumb is electric has to be half the power of the diesel it is replacing. Don't ask me why, something to do with torque curves
I'm shooting for 8.5Kw (~11hp), that should beat the crap out of 8hp Honda outboard and the broken 12hp Farymann and be more than enough for the boat. The big question is how long will the battery hold out at that power level if I need it. Probably not long, but I'll have to figure out how much power I would really need to get out of a tight spot. At some point any engine would be overpowered.