Troll motor setup as Emergency "get home" device

[Captain Ben]

I am in the market for a Trolling motor to serve as a "get home" device to my 4.3L Merc.


I have a 2 battery setup (switch for 1, 2, both, off), which of course in the "both" position, places the batteries in parallel and maintains 12V

I see Bass Pro has a guide for choosing the correct trolling motor based on boat size. (I have a 20ft Sea Ray I/O)

http://www.basspro.com/webapp/wcs/s...ode=article&objectID=29748&objectType=article

From this article, they reccomend a 74lb thrust. I beleive I could go with slightly less, considering the nature of my intended usage.

I would like to run it off my existing batteries, but would need to run them in series (12+12V) for any 24V trolling motor...

Side note: For a 24V motor, I would picture running a separate on/off switch off the batteries, with one terminal, setup for fast connection to the trolling motor. In this setup, the main 12V 1/2/off/both switch would be turned to the off position for emergency propulsion setup. I haven't thought deep in to safety concerns with this, but it seems logical. (not my first choice)

I see there are 50 and 55 lb models for 12V motors (not sure if it goes higher). The main question i have is whether 50-55 lbs is enough thrust to move my boat (assuming good charge on 2 newer deep cycle marine batteries, sea state 1, and a nominal .5-1 knot current, negligible winds) I also intend to keep a additional battery on the boat)

Thanks!

 

[Silvertip]

Re: Troll motor setup as Emergency "get home" device

You are inviting disaster regardless what your thought process is. There is far too much 12 volt electronics on a boat to risk accidentally placing 24 volts on them and turning them to toast. But beyond that, the size troller you need to get you back to shore depends entirely on how far from shore you are, wind, load, current and wave conditions. If you are several miles off-shore it will be a long lonely journey back and plan to paddle a bit of that distance. You would need at least a 24 volt motor and more than likely a 36 volt unit to serve as a backup and even then, more than a few miles leaves you short of your destination. For backup install a 9.9 - 15 HP gas.

Getting back to your switching scheme: Setting the dual battery switch to OFF merely cuts off current from either or both batteries to the COM terminal. So you add a switch (and it better be a big one) to series connect the two batteries. You not only need to jumper NEG on BAT 1 to POS on BAT 2, but you also need to DISCONNECT the NEG cable on BAT 2 from ground. Failure to do so will create a grand fireball as the minute you flip the switch you have shorted BAT 1 directly to ground. Trolling motors in the 55# range (12 volt) draw up to 55 amps. 70# 24 volt motors will pull about 46 amps and 100# motors at 36 volts will draw in 42 amp range. Those reqjuire big capacity switches and big wire.