Building my own trolling motor.

J

jdricha5

1 µW
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Apr 21, 2012
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I have been looking at rc motors to build my own trolling motor. I am a engineering student and have a 10ft kayak that I am looking to propel. What is a descent cheap motor to start with from a hobby store.
http://www.hobbyking.com/hobbycity/store/__15199__HobbyKing_Donkey_ST3508_730kv_Brushless_Motor.html
Would something like this work with a gearbox?

I am looking for a days worth of use. Any ideas i'm very good at building things and understanding how they work. Im going to be using the motor to propel myself to a fishing spot.

Thanks
JD
 
Nice!

What are your plans on the transmission?
I've looked at it also. Came across a few problems immediatly. A pod construction, like a typical trolling motor, is hard to waterseal. And electrifying a gas outboard only using its transmission will make it loudy.

Came across this video last week. Pretty awesome build. Could use some tweaking though, different prop and such. The poster of the video already mentioned it.
http://www.youtube.com/watch?v=rb2FkCF-e58
 
For a simple low cost trolling motor build i was just now thinking about a system which works like the way long tail boats are propelled (but mini sized, lol! :mrgreen:)

207-034-long-tail-boat-thailand-island.jpg


just bolt a shaft with a decent prop to the rc motor. Mount it on a wooden board, give it the right angle, fix it to your kayak and go!

minn kota prop
http://www.ebay.com/itm/MINN-KOTA-P...ear&vxp=mtr&hash=item2568023b80#ht_1383wt_954
 
I would hope that using a gear box the motor would have enough torque to spin a large two blade prop at a low rpm with out producing cavitation. One of the issues is i am currently looking for some kind of algorithm that will change what thrust in air is to what thrust in water would be.

I would like the design to be two submerged motors in a water proofed tube( similar to a trolling motor) on both the right and left front sides of my kayak while steering is controlled with a rudder. So it would be direct drive besides the gears.

Im looking for a low cost solution. So 100 dollar motors are out of the question. (Otherwise I would buy a trolling motor)
 
I would like to use a brush less motor for more efficiency so I do not have to use as large of a battery.
 
From what I have been able to determine, a trolling motor normally turns 1200 to 1400 rpm. That motor is a 730kv, so on 12v will be turning 8760 with no load and shouldn't be loaded down to less that 75% of that or 6600 rpm. This means your going to need to do a gear reduction of around 4.5 to one or so. The gearbox is where the cost is going to come from, but if you do it, ( especially using a rc plane prop of around 9x6 size) you will likely have a fairly efficient setup.


Dean in Milwaukee
 
That's a pretty small motor, driven at the Limit, it might have quite a lot of power, but i don't think that it'll handle being driven at the limit for a long time (which will be the case in your scenario).

Then there's another issue, since it's an outrunner, it'll be difficult to cool it, if you go for an inrunner, you could add water cooling, which would make it more durable.
 
nioko said:
That's a pretty small motor, driven at the Limit, it might have quite a lot of power, but i don't think that it'll handle being driven at the limit for a long time (which will be the case in your scenario).

Then there's another issue, since it's an outrunner, it'll be difficult to cool it, if you go for an inrunner, you could at water cooling, which would make it more durable.
Click to expand...

Cooling isnt likely to be too big a deal, it will be underwater!
 
It would probably be a lot easier to take a used trolling motor and modify it. Most of them are made for 12v, but I suspect you could run most of them at 24v or more with the right speed controller. Check Craigslist and eBay.
 
fechter said:
It would probably be a lot easier to take a used trolling motor and modify it. Most of them are made for 12v, but I suspect you could run most of them at 24v or more with the right speed controller. Check Craigslist and eBay.
Click to expand...

There's a really big problem with doing this, unfortunately. The power absorbed by a propeller is proportional to the cube of rpm, whereas motor, and hence propeller, rpm is directly proportional to voltage. If you double the voltage then you double the rpm and the power increases by a factor of eight, leading to four times more current through the motor followed quickly by commutator/brush failure.

If you fit a limit stop to the controller, so that the motor rpm cannot exceed the speed it runs at on the original supply voltage (usually 12V) then you can run on higher voltages. In fact, if you want more range this trick works well, as it reduces the current drawn on the supply side and so improves battery performance, particularly with lead acid batteries. A friend is doing this with a 24 V brushed motor controller and the result so far look pretty good. All you have to watch is that you don't accidentally over-power the motor, as they don't have a lot of reserve power capability as a rule.
 
I was thinking if your trolling motor was orginally designed for a large bass boat and now you put it on a small kayak, the load will not be that much greater. The propeller load will depend quite a bit on craft speed. If the controller has a reasonable current limit and the limiter works properly, then the motor will just go as fast as it can go for the given amount of current (and never burn up). Overheating the windings will be unlikely with water cooling, but I could imagine commutator failures.

I think a kayak might be pretty fun with an overvolted motor. :twisted:
 
fechter said:
I was thinking if your trolling motor was orginally designed for a large bass boat and now you put it on a small kayak, the load will not be that much greater. The propeller load will depend quite a bit on craft speed. If the controller has a reasonable current limit and the limiter works properly, then the motor will just go as fast as it can go for the given amount of current (and never burn up). Overheating the windings will be unlikely with water cooling, but I could imagine commutator failures.

I think a kayak might be pretty fun with an overvolted motor. :twisted:
Click to expand...

The effect of boat speed on propeller load is pretty small, the overwhelming effect is that of propeller rpm and that dreaded cube law. Many people have tried over-volting trolling motors, most have ended in tears, usually with the brushes and com burnt out.

If you limit the motor current to that which you'd get at 12 V, then over-volting works well at increasing battery life, as I mentioned before. The main problem is that these motors are built right down to a price and are very crude internally, with poorly designed props and often just a switched series resistor speed control.

If you want performance, then look at the link I posted to some posts by MCDenny on my long running thread on Boat Design that I posted before about trolling motor mods: http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-17.html#post305142 , posted in this very related thread: http://endless-sphere.com/forums/viewtopic.php?f=39&t=38897
 
vax is it possible for you to post some pix of the Audi starter planetaries with a ruler in view? Might be useful for other applications on the board!
 
Jeremy Harris said:
The effect of boat speed on propeller load is pretty small, the overwhelming effect is that of propeller rpm and that dreaded cube law. Many people have tried over-volting trolling motors, most have ended in tears, usually with the brushes and com burnt out.
Click to expand...

I stand educated. Interesting about the airplane prop and how much improvement a simple fairing made.
 
Thanks for the pix. The ratio looks like a little over 4/1 holding the ring gear. That is a useful ratio. The planets have needle bearings also, another positive. Looks like powder metal technology. Challenge driving the sun gear...

The packaging looks useful. Is it a standard Audi/VW starter that stayed the same for a number of years, or a very select model to find it?

Just noticed, is this puppy a two stage? Each stage the same? So about 16/1 overall ratio?
 
BM,
I have a similar planetary drive starter motor, but in a much smaller package since it came from a Yamaha motor bike.
The only markings is "Mitsuba -2 " , its 65mm dia and 150mm long,..of which only 15mm is the gear train.
Only 3:1 reduction, but a lot of torque even on 12 volts.
I use it a a direct drive ( crankshaft) portable starter for 2 stroke race kart motors.
A little smaller and lighter than the Audi unit Vax has, but this little sucker spent its first life cranking a 1100cc sport bike with typical Japanese reliability !

Just found it on Alibaba !
http://www.alibaba.com/product-gs/339374039/Starter_Motor_for_YAMAHA.html

If its any help, i know that Toyota also used a planetary reduction starter on some of the early Camry's .
 
My gears came from Audi A4 V6 engine, mine is 2.4, but I think 2.6 and 2.8 engines use same starter motor.
Also I have one gearset from Lada Niva starter (made by Bosch), that's at the second photo. However, the ring gear is plastic.
The Audi's sun gear is more usable, there's shaft for bearing at the one end and thick round part at the other end (former starter shaft, about 12mm diam.), that can be drilled for direct motor shaft mount. In my case there will be also standard lip seal on motor shaft to keep oil in.
 
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