Trolling motors wiring up contactors?

[ryanonthebeach]

Hi All

I have a pair of trolling motors that I have turned into bow thrusters
see:
http://www.psychosnail.com/SailingArticles_Thrusters2.aspx

I have a suspicion that the controllers for these motors aren't all
that efficient, they are drawing less amps than they should, and would like to just run them directly through
contractors.
Would this work? are there any dangers (to the motors)?

Thanks!!
ryanonthebeach

[alan white]

First, efficiency has to do with loss in the form of heat. Second, a contactor will only give maximum thrust, withy no in-between.
If that would satisfy you, then by all means just use a contactor. You may save a tiny bit of energy, although losing the variable speed might not be worth it.
There would be no danger at all to the motors, since you can't exceed the specified voltage without adding more batteries in series.
However, bow thrusters don't run much except in maneuvering, so why mess with what is already a useful and efficient set-up?

[Rick Willoughby]

You say you have two 6V 125 batteries. You claim these will be 235AH total. That is only that case if connected in parallel. I get the impression you have a 12V system. So in reality I expect you have 125AH batteries rated at the 20 hour rate. Drawing near C1 rating will reduce capacity to about 90AH and the voltage delivered will be under your expected 12V.

You need to measure the voltage as close to the motor terminals as you can. Likely to be well under the expected 12V.

To pull 100A from lead batteries you do not want to pull more than about C0.2. This means something like 500AH battery and that has to be 2 by 6V 500AH, 1 by 12V 500AH or maybe 2 by 12V 250AH or other similar combination that might come in a manageable size. You need roughly 4 times what you now have.

What does the boat weigh? Unless the prop performance was done for your boat it is meaningless. You are expecting a lot from a tiny prop. The thrust available from any given power is a function of the prop diameter. So for low power you want to swing as large a diameter prop as you can possibly fit. It needs to be pitched to achieve the full power demand at rated rpm.

Rick W

[ryanonthebeach]

thanks Rick and Alan
I bought some more efficient props a little while back for the motors and it did make a difference.
I'll try hooking up the contactors and see what difference it makes.