RE: Electric vs. gas motor question

Gordon (hlg@pacbell.net)
Sun, 6 Jun 1999 19:18:43 -0700


- - - - - - - - - - - - - - - - - - - - - - - - - - -
West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
- - - - - - - - - - - - - - - - - - - - - - - - - - -
> Ohms Law and Circuit Fundamentals says, to wit
> !hp=746 watts expended assuming 100% efficieny
> Amps= watts divided by the Voltage....I=P/E
> Watts=E times I....So...the motor is an inductive
> device and limits it`s wattage/horse power by
> a phenomenom known as reactance or XL
> So, double it`s design voltage and in most
> cases it shortly over heat and burn up.
> Some motors are designed for variable
> voltage (sewing machine motor) they can do
> this because the design allows the motor to
> change speed with a voltage change and the
> power utilization rate remains the same
> (or close enough) The laws that govern this
> are seldom what we want or need and cannot
> be changed. (The above is peculiar to DC
> circuits)
> Regards....Bill p15 #2001
> "Small Wonder"

I hope nobody thought I was recommending using 24 V on a 12 V motor. My
motor is designed for 24 V so it is not inclined to overheat and burn up at
that voltage. Others are designed for 36 or 48 V. I think the advantage of
the higher voltage is to get more power without a proportionate increase in
current, which would require heavier wiring.

I just checked the MinnKota specs. My 24 volt RT70/S (70 lbf thrust) uses
45 A at full power, not 50 A as I think I said.

The next size smaller in the RipTide series is the 12 volt RT55/S (55 lbf
thrust), which uses 55 A at full power.

According to MinnKota, the RT70/S is supposed to be adequate for a boat up
to 25 ft weighing 3500 lb. The RT55/S is rated for a boat up to 22 ft
weighing 2500 lb. That seems very optimistic to me. They are probably
assuming a bass boat trolling slowly in quiet water and no wind.

Today, in my heavy first-generation P14 with the RT70/S, I used 100% power
to motor, sails down, at 3 plus knots, head-on against a 15-20 knot wind.
The power was quite adequate to get me where I was going in a reasonable
time, but was less than most gas outboards, including Dave Kautz's Mercury
(2 hp?) or my own 3 hp Seagull.

The clean, quiet performance and the positive control of the electric motor
are wonderful. I only take the Seagull if I need its greater range. On a
windless day (or downwind) I use 50-75% power to extend the range of the
electric. I recharge the batteries as soon as the boat is back in my
driveway, and I top off the charge the day before I plan to use it again.

MotorGuide has some motors that use either 12 or 24 V.

Harry Gordon
P14 #234, Manatee
Mountain View, CA