possible motor source...

[mpittman]

from what ive read here and on a few other diy boards , i pretty much assume for dpv application a lower rpm higher torque dc motor is in order.
i started looking at my best friend's dad's old electric wheelchair .... 24 volt , high torque , low rpm , no gear reduction.... hey this might be halfway close to what i need (at least closer than a high rpm low torque trolling motor i think?).
he told me those particular motors run about 500 bucks each (i think he's half full of it) , i started thinking about other commonly available machines that use similar motors.... anyone notice how weve been overrun recently by the electric scooter craze???
ive sourced a few online scooter dealers who also sell just the motors , moderate rpms (around 2500-3000 at rated voltage) , not sure on the torque , 24 volt , and around the 350-500 watt range , now without getting into all the huge details over bench loads and actual water and prop loads , does this sound "halfway" close to what were looking at for dpv use? if so i can pick em up for under 100 bucks each (well more if you really feel like you have to spend alot on something that may flood the first time out) seems like other than sourcing the right props , might be a cheaper route than buying and modifying new trolling motors....
as always any comments , or ideas for or against are appreciated.

 

[herman]

Why lower RPM/ higher torque? That combo requires a larger prop for the same speed/thrust which is going to require you to increase the size of the prop and associated guards. Special props are going to be hard to come by and expensive. Seems to me that a trolling motor with some form of electronic speed control would be an ideal canidate. It's already somewhat sealed and comes with it's own prop. By enclosing the tm in a housing and extending it's shaft somewhat, you would have a motor that would withstand a flooding with no problem. Yard sales are a good source of used Tm's. I have 2 here, a 10lb and a 35 lb thrust that I have gotten for $10 and free, both of which should work nicely. Speed control is going to be another problem. A regular DC motor is going to require some form of speed control and a way of interfacing the control to the operator, more expence. An inexpensive TM uses multiple winding and simple switches to accomplish different speeds. A simple relay system using reed switches for 2 or 3 speeds could very easily be constructed and by using magnetic coupling for the speed control circuits it would be simple to keep them watertight. I have no idea what the recommended thrust for a DPV is but an old rule for boats was 1lb trust for every 100lbs of boat and based on that, a 10lb TM should make a diver fly. Another rule of thumb you might find useful is 1lb of thrust= 1 amp@12v.

 

[mpittman]

Why lower RPM/ higher torque? That combo requires a larger prop for the same speed/thrust which is going to require you to increase the size of the prop and associated guards.

the reason i have stated high torque /low rpm is on standing designs for similar uses. speed boats typically use high revs and a lower blade pitch to generate great amounts of speed with a streamlined hull. conversly tugboats when tugging (not very hydrodynamic) are a very close parellell to what you dpv is.. an underwater tug. they dont go fast , but they do manage to drag other waterborne objects many times their size because they use engines with alot of torque and not so many revs . so far as prop size goes.. you can take advantage of low revs high torque with far more than just prop size.. a deep pitch prop will move just as much water as a larger prop with a shallower profile. also the amount of blades on the prop can be used as a factor to take advantage of all that torque . i remember seeing an old popular mechanics denoting 2 props designs for speed , 3 prop for multi use , and 4 and 5 prop designs for sheer pulling power.
putting all this together in my head it would seem to me that the reason it makes sense would be the directed flow of propwash . a shallow cut blade at high speeds would direct it back into what was being pulled (you for instance) where a deep cut blade moving at a lower rpm the water would still be displaced but the angle of a deep blade would push it more at an angle outward and to the sides of the prop then directly behind it. not to mention slower moving water wouldnt create a "wash" quite so far back making its effects on the tow behind more negligable. some of this is conjecture and assumption off of current designs , if anyone can add to , or take away from this , as always please do.
not sure if the 10 lbs per 100 lbs of boat applies to dpvs... divers arent nearly so hydrodynamic as the hull of a boat not to mention a very small portion of the boat is being pulled through the water whereas the entirety of the divers weight is being slogged through the drink. i saw a spreadsheet not too long ago comparing lbs of thrust across various dpvs and im wanting to say (though i may be off some here as this is from memory) most dpvs hitting 2mph were putting out roughly 35-40 lbs of thrust. these motors im looking at im not sure what theyd generate on a prop but they do pull a 200 lb man and a 100 lb bike to 25 mph or so with a single engine @ 24 volts.
for a single torpedo dpv yeah id be down with using the trolling motor idea.. im actually half thinking of building something thatd fall somewhere between a torpedo and a 1 man wet sub.. thought about how cool that plastic tow sled on the other topic down a bit would be if it was self powered. yeah its gonna be heavy (prolly 150 lbs on land) but thats what a bow mounted cherry picker on a pontoon boat is for

 

[tachyon]

My marine architecture course is far in the past, but high RPMs in water causes cavitation. When you have the energy to spare from an engine, cavitation can be an acceptable energy loss. In a battery powered SDV, any loss due to cavitation will be unacceptable, as well as the fact that cavitation will cause bubbles that will obscure vision if excessive. The same is true of the losses suffered in a reduction gear transmission without the issue of visibility. Thus the best choice would be a low RPM high torque motor with an appropriate prop. In this case, the term is appropriately a propellor rather than a screw or impellor because it will be larger and slower to provide thrust without cavitation.

This is assuming a battery powered electric propulsion unit. A unit running on another form of power that is more efficient or has a larger amount of energy available would present other engineering requirements. Steam or Hydrogen Peroxide systems are unfeasible as SDV powerplants, but offer examples of alternatives.

tachyon

 

[divesmith]

You might want to look at motors for electric golf caddies. (Kangaroo, Club runner and about half a dozen others) There are a number of them around and used older ones can be found rather cheap. Since you only want the motor the condition of the caddie doesn't matter and buying the whole thing would probably be less expensive than ordering a new motor from one of the manufacturers. Most of them are 12 v dc. You can easily build a solid state pulse control to add variable speed which also conserves power.