Max:
Here's one more report for you.
I really like the idea of electric power, but I kept seeing reports that
the small trolling motors were easily overpowered by any substantial
current or wind. So I decided to make a serious investment and bought a 72
lbf thrust, saltwater model MinnKota, the RT72S. I think the price was
about $480, ordered through West Marine. It is a 24-volt motor so it
requires two 12 V batteries in series. It has the Maximizer feature, which
means the speed is controlled by pulse-width modulation. That conserves
power and gives infinite speed adjustment, instead of the five stepped
settings on the cheaper motors.
Despite the higher thrust rating, I didn't expect it to be equivalent to a
gasoline motor, and it isn't, but it does much better than the $100
electrics. Dave Kautz said he was impressed to see how well Manatee was
motoring against a stiff breeze when we were sailing together recently at
Redwood City. Dave said his small trolling motor wouldn't have been any
good in that wind.
I've seen conversion factors to convert thrust to horsepower ratings that
were extremely optimistic. I think the best way to get an idea of the
equivalent horsepower of a trolling motor is to look at the spec for
maximum current draw. That is around 50 amperes for my RT70S. Multiply that
times the voltage to get watts. At 24 V, then, my motor uses a maximum
input of 50 x 24 = 1200 W. One horsepower is 746 W, so the motor has an
INPUT of less than 2 hp. The output power, of course, has to be something
less than that. It feels like it would be equivalent to about a 1.5 hp gas
motor.
I used AWG4 tinned cable with all terminals soldered and crimped in order
to minimize the voltage drop from batteries to motor. I used two group-24
size deep cycle 12 V batteries with one battery on each side of the
centerboard trunk. The batteries cost about $45 at Costco and are rated at
95 ampere-hours. I bought a smart charger at Sears for about $90. It has 12
V and 24 V outputs at three charging rates and accommodates regular, deep
cycle, and gelcell batteries. I'm very pleased with the charger.
I also have a British Seagull Plus Forty gas outboard rated at 3 to 3.5 hp,
and it drives the boat very well, through wind and chop, and achieves hull
speed in smooth water. It is noisy and messy and has no neutral or reverse.
Occasionally it quits or won't start, but it is generally reliable and has
never been disassembled or been in a repair shop, and it is 32 years old.
If I have a long way to go and can't depend on the wind I still use the
Seagull. I used it on a recent all-day sail from Redwood City to the south
end of the Bay to see the wooden sailing ship that is stranded there. We
motored all the way down and part way back. If I had used the MinnKota, I
would have had to use maximum speed to keep up with the other two,
gas-powered, boats, and would have surely run the batteries down. I did
carry the MinnKota stowed as a backup but didn't use it.
But the electric is wonderful, almost totally silent except for a slight
vibration. It's on or off instantly, with a turn of the tiller handle, in
either reverse or forward. It's super clean. After use, I just hose it off,
wipe it dry, and spray it with Armor-All, then stow it in a closet, unlike
the Seagull which lies on the garage floor leaking grease and fuel. And of
course there is no fuel to mix and empty and store, and no gearbox to top
off. Offsetting those advantages somewhat is the mess of the wet-cell
batteries. I leave them in the boat, recharge them promptly and check the
electrolyte once in a while. The batteries are strapped down.
Here in the Bay area, there is almost always wind for sailing, so the
MinnKota is my preferred auxiliary for routine daysails or for relatively
short excursions anywhere on a windless day. It's also nice to be able to
motor on lakes that don't allow gasoline motors.
Overall I can say the MinnKota has lived up to my expectations or exceeded
them. Last year I wrote a detailed account of one of my first excursions
into the Bay with the MinnKota. I ran the motor for about 4 hours that day,
and it never slowed down noticeably. It also proved to be rugged. I still
have the post and will resend it to anyone who wants it, or look in the
archives for "Manatee Rams a Destroyer."
Harry Gordon
P14 #234, Manatee
Gunter-Rigged, Lateen-Rigged, and/or Electric or Gas Powered
Mountain View, CA