Re: Electric Motor for WWP 15

Gordon (
Thu, 11 Feb 1999 11:13:45 -0800

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West Wight Potter Website at URL
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A number of Potter sailors are using electric outboards. Most of them also
have gas outboards. Some have reported that electrics with thrust ratings
up to around 50 lbf have not been able to cope with much current, wind, or
chop, and, of course, range is limited by the number and size of batteries.

I recently bought a 70 lbf thrust Minn Kota, the Riptide RT-70S model for
saltwater use, with the idea that it would give me more reserve thrust for
brief encounters with current and wind. It requires 24 volts, and I have
two 12-volt, group-24, deep-cycle batteries, one on each side of the
centerboard trunk (an extra 90 lb of ballast), connected in series. (My
first-generation Potter 14 does not have an internal liner so the batteries
are essentially sitting on the bottom of the hull.) The motor has a 36-inch
shaft, which is maybe longer than necessary but was the shortest available
on this model. Both the angle and depth are easily adjusted.

I've only had it out a couple of times so have more testing to do. Look on
the mail list archives for my post entitled, "Manatee Rams a Destroyer,"
which describes in detail my first venture onto San Francisco Bay (on a
quiet day) with the electric outboard. If you can't find the post I'll be
glad to e-mail it to you. I have no experience with MotorGuide. MinnKota
claims their plastic shaft is superior to the MotorGuide's stainless shaft
because the plastic shaft flexes instead of bending or breaking. I've
already put that feature to the test a couple of times without damage.

I love the quiet and the positive control the electric outboard provides.
Mine has the Maximizer pulse-width modulation that should reduce battery
drain and gives infinite speed control from about 10 or 20% to 100% power.
The motor uses 50 A at 100%, so I don't expect to go far at full speed. Use
large cable from the battery to the motor to minimize the voltage drop. I'm
using #4. Most of my connections are both crimped and soldered for the same
reason. I have a 60 A fuse in the circuit.

The conversion factors I've seen to translate thrust ratings to horsepower
seem extremely optimistic. My 70 lbf thrust motor uses 50 A at 24 V, which
is 1200 watts, which is 1.6 hp. That is the INPUT to the motor. Obviously
the output will be even less.

My Minn Kota will be great for getting in and out of crowded marinas,
exploring coves and sloughs, nature watching, etc. Local reservoirs that
don't allow gasoline engines do permit electric motors.

Many Potterers rely on their outboard to bring them home when the wind
exceeds their comfort level. The electric outboard may not be adequate for
that purpose because of the limited range and power. If I'm on big water
and the wind comes up too much I would expect to just reef or drop the
sails and run for shelter under bare poles or jib. Oars would be a good
backup also although I haven't gotten around to installing oarlocks.

The propellers used on trolling motors are optimized for fishing and are
probably not optimum for use on a sailboat auxiliary motor. A larger
diameter (and lower pitch?) propeller would be more efficient, but I don't
know if anyone makes such a thing.

For recharging, I am pleased with the Sears DieHard charger I bought. It
has both 12 and 24 volt outputs so I can charge my two batteries in series.
It's a "smart" charger with a microprocessor and has modes for different
types of batteries including deep cycle and gel cel so I don't have to
worry about overcharging. The charger cost about $90.

I bought the batteries from Costco (their Kirkland brand) for about $40 a
piece. They are rated at 90 ampere-hours, more than most group 24
batteries. Gel cels would be neater but would be larger, heavier, and much
more expensive for the same capacity. If you are going to go with one
battery I'd recommend a group 27 or larger.

I was aware that I could have bought a pretty good gas outboard for what I
have invested in my electric setup, but the idea of clean, quiet electric
power was so appealing I wanted to experiment with it. I still have my old
Seagull gas outboard but intend to use it as seldom as possible.

For more info on electric boat power, check the links on the Potter
Yachters web page <>.

Harry Gordon
P14 #234, Manatee
Mountain View, CA

>Dear Members,
>I recently purchased a WWP 15, new from the factory (the boat still has not
>left my garage). I am considering the purchase of an elctric motor as
>auxiliary power for the boat. I narrowed down my search to two companies,
>Motorguide and Minnkota.
>I need suggestions and advice on a few topics. Here they are:
>1) Is there any other company that I should consider
>2) Any opinion on pros and cons of the two manufacturers
>3) What shaft length should I consider, 30" or 36"
>4) I think I need 30 to 40 lbs of thrust. Is this correct?
>5) I anyone familiar with the advantages of "Pulse Width Modulation" for speed
>control (available on some models for a cost)
>6) Any other comment of the merits and demerits of elctric motors for the
>I would appreciate any information that anyone could provide. Thanks.