I don't think it will work exactly like that. Half the voltage is not necessarily half the current because you are not dealing with a purely resistive load and the load will change with speed in a non linear fashion. I think, anyway. Approximate, probably. But thats sort of who cares. But what I would do is run the existing motor with the existing battery pack, using a PWM controller. Just get a cheap Chinese one for $40 or so. If it goes up in smoke you can still hook directly to the battery like you been doing, to get home. Be sure to fuse the controller, both input and output, appropriately. The nice thing about using a PWM controller is you can hook up say a 10k pot as a throttle, and have constantly variable speed, At high speed just bypass the controller for slightly more efficiency but most PWM controllers are around 90% efficient I think. Install a voltmeter and ammeter in the controller output so you don't accidentally exceed the motors rating. I just made my cheap 12v trolling motor have a really cool underwater explosion pushing it a bit too hard, and I WAS monitoring voltage and current. Well, I was sort of in a "here, hold my beer and watch THIS" frame of mind.
You should also test your current system and see what voltage and current are on your feed cable without a controller, since you know it is safe to run it at that power level from experience. It will be a good benchmark to go by.
A bigger trolling motor would be a nice upgrade. A big 36v model would be great, and just wire up a series bank for it. Better trolling motors use a PWM controller built right in, for higher efficiency. Cheap ones are efficient when ran wide open but not so efficient at reduced speeds because of the inefficient control system.
---In electricboats@yahoogroups.com, <fishcanoe@...> wrote :
Hi all.
Have a small sailboat. Bolger Oldshoe (rare enough but mine has an aluminum hull...probably the only one in existence). Also have a trolling motor...about 50 lb thrust. Well, the control electronics died....so I just bypassed those and ran 12 volts directly to the battery. Worked pretty well. Pushed the boat nicely along at 2.8 miles per hour...and given this a kinda boxy shaped boat with a wide full length fixed keel that is only 11.5 feet long AND about 5 feet wide and is probably like 800 pounds with 2 people in it....well, thats pretty good. And that was with a moderate headwind...but sails furled and very small chop with no waves to speak of.
Anyway did find out that setup would NOT work against a stronger headwind...or significant waves....well it sorta did...but 1 mph probably would't cut it. I do suspect a 100 lb thrust trolling motor and 3 or 4 deep cycle batteries would be workable in all but the worst conditions....but that would get pricey and still not be enough in the worst of the worst....so ended up getting a 2.5 HP gas outboard. The boat hauls ass with that thing even at half throttle (well, for that boat in any case).
However, it still makes more noise than I'd like (glad we got the water cooled one! ). The handful of times I used the trolling motor it was much nicer and was more than fine for fighting light winds or having no winds.....and worked really well when the wind was working with you but just wasn't quite enough by itself.
Okay, having said ALL that....
I'd like to use the trolling motor still BUTTT I'd like to use it at a lower power. Doing that I could get by with probably only getting one deep cycle battery (got the gas motor for when I NEED range speed).
So, I could run the motor on 6 volts. Or perhaps 8 volts direct.
Here is my basic engineering reasoning. Half the voltage will approximately be half the current. Voltage times current equals power. So 6 volts would be about 1/4 the thrust than used at 12 volts. And 8 volts would be roughly half.
Does that sound about right?
Thanks for any input !
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