Thought I'd look at the power requirement for my own boat - 27' 9000lb (4000kg). Seems a bit low, can anyone see a mistake?
Power required to push boat at certain speed + prop losses + motor losses = power required from battery.
Force to push boat at 5 knts (2.5m/s) = 85 lb (40kg or 400N - from computer).
Sorry, I'll have to do this in metric.
Power = force x speed = 400 x 2.5 = 1000W or 1KW
Lets say prop efficiency = 66%
Power at shaft = 1/0.66 = 1.5 KW
Lets say 90% motor efficiency, which I believe is now attainable
Power from battery = 1.5/0.9 = 1.7 KW
So at 12v current I = Power/Volts = 1700/12 = 142 Amps!!
Can I travel at 5knots for an hour with 2 (allowing for 50% discharge) 12v 150 Ahr batteries? That sounds fine for my requirements.