The conversion factors I've seen for converting thrust to horsepower have
appeared very optimistic to me, probably because thrust is a measure of
force, not power. Instead, if you multiply the voltage times the maximum
rated current for the motor, that will give you the maximum power INPUT to
the motor in watts. To convert watts to horsepower you divide by 746.
For example, a MinnKota 42 lbf thrust motor uses 34 amperes at 12 volts,
which equals 408 watts, approximately 1/2 hp.
On my P14, I'm using a 70 lbf thrust MinnKota Riptide RT70/S that draws a
maximum 45 A at 24 V, which equals 1080 W, or about 1.45 hp. It pushes my
P14 at better than 3 knots against a 15-20 knot wind on smooth water, but
might not be much good in rough water and strong currents. It has
substantially less power than my 3-3.5 hp Seagull, so the estimate of 1.45
hp is probably pretty close.
MinnKota's Motor Selection Guide says the max boat length/weight for a 42
lbf thrust motor is 18 ft/2500 lb. That is very optimistic, and I would
assume that is a bass boat with no wind, waves, or current.
MinnKota offers a 96 lbf thrust Neptune, which is actually two 48 lbf
motors mounted together with a single control. A better arrangement for a
sailboat might be two separate electric motors so you could keep one
switched off and retracted until you needed some extra push.
MinnKota markets the Neptune line as primary propulsion motors, not
trolling motors, but I can't see from their literature how they differ in
any significant way from the other MinnKotas.
Regards,
Harry Gordon
P14 #234, Manatee
Mountain View, CA