Trolling Motors

[Subscuba@aol.com]

I use on my Sub a Minnkota 70 pound thrust 24 volt Trolling Motor ($169 each).
I examined the shaft seal and found it is a simple "Cup" seal in which the
shaft rotates in a rubber seal. Thus the shaft does abrade the rubber. This
degradation is a function of differential pressure across the seal and for low
pressures is long lasting.

When I was talking to Minnkota, they stated the seal wasn't good for much more
than about 30 feet of fresh water before it could leak. I believe his response
was conservative but their seal certainly would not work at 700 feet and if it
did it wouldn't last long. I ended up pressure compensating my motor with a
somewhat simple SCUBA regulator such that the differential pressure across the
shaft seal was less than 2 feet of seawater and was positive. This means that
any leaks would leak air out and not corrosive seawater in. At least I hope
so, I still have not made sea trials yet although the my Wet Sub is ready.

The point of this is that since the Evinrude Trolling Motor is in the same
competitive market I personally doubt that their shaft seal will reliably
operate with 700 feet of sea water differential pressure. Pressure
compensation, I believe, will be required.

If someone is planning to use the Evinrude Trolling Motor, it would be
critical to call the vendor and get real data on their shaft seals. I know I
did on the Minnkota Trolling Motor. I spent about $20 on the phone calls. I
also got design information on the Motor Controller I designed. High surge
currents can demagnetize the Trolling motors permanent magnet stator.

Good Luck,

Ken Martindale