Trolling motor powered surfboard: prop problems

Hey Everyone. Awesome website BTW> This seemed like the best forum I could go to with my problem, so here it is:

I've managed to mount a trolling motor on my 8 foot surfboard. It actually works pretty good, but I need more speed, and less torque.

The reason I say this, is the motor was designed to push a heavy boat with poor hydrodynamic qualities. The surfboard is lightweight, and has great hydrodynamics.

I managed to change the pitch of my prop by heating the blades and bending them, and I'm also running 24volts now, which seems to have doubled the motor speed, but it is still too torquey, and has little top end.

The board pulls HARD from the start, and tops out almost instantly.

I think I need more prop pitch, or more RPM or both. The motor is not even coming close to using it's full power potential, due to this lightweight design of my craft. Where can I look for a "faster" prop, or even a faster trolling motor to hack up.

I tried a trolling motor with more thrust, but it made no difference, because like I said, they are designed for thrust, and not speed.

I'm also open to suggestion on other motor options, including:
2-stroke powered
Thrusters? Like they use for small submarines
Jet-ski type motors, ect....

Also, do they sell large pitch props for trolling motors?

 

It is unlikely that you will find a standard boat prop to do the job if it really is low drag. It has to be getting on the plane to be low drag for such a short board.

You could look around for model aircraft prop. If the drag is as low as you suggest then these will handle the thrust without too much distortion.

You really need to know the power output and rpm of the motor to select the best prop. The prop selection will depend on the drag and speed.

It is easy to make a suitable prop if you have a welder and hand grinder. Takes about 4 hours.

Rick W.

 

I do have a grinder and a welder, but I thought a plastic prop would be better for a trolling motor.

Is there anywhere I can find a prop with a larger pitch?

Is there anyway to figure out what pitch I should use? Any formulas or estimations I can use?

 

May sound nutty, but there are probably model RC boat props close to what your looking for. You would have to adapt the hub, but that's not a big deal.
Some model boats are pretty damned big and fast.

alan

 

I'll look into that. What effect does prop diameter have? Seems they would be smaller than trolling motor props.

Keep the suggestions coming!

 

would it be possible to post some shots... im interested in making something similar...

Thanks

Brandon

 

To do a prop design you need to know the motor power outut and rpm. Then estimate od the drag versus speed of the boat.

If the board is getting on the plane then the drag can be estimated at about 1/8th the total weight.

The attached photo shows a a stainless steel prop for a high efficiency human powered application. It has the stregth to delivery about 1kW in a low drag high speed application.

Rick W.

 

Pitch makes for mph, diameter for applying power to move water, RPM to determine pitch, and horsepower to determine diameter.
Dragging your board behind a boat with a man on it with a spring scale, you would see that at a certain point, the rated thrust of your motor was met. That speed x 1.25 should determine your pitch number if the prop will be 80% efficient. RPM at load speed (80%) is used to get the pitch along with your target speed. Now only diameter has to be figured, but I'd carve gradually until it's right. If the thrust and RPM, and hence pitch is known, the only refinement would be diameter. If the motor won't make load speed, the diameter is too large. If the motor RPM is too high (like 90% of no-load speed on an 80% efficient prop), the prop needs to be larger.
I think.

Alan

 

Wow, Thanks a lot. With my physics background I should be able to get a rough estimate. I wish I knew a way to figure out the rpm of the motor, although that is variable due to voltage, I still have no clue.... not even an estimate.

That picture of the prop looks like an aircraft prop. Do these work for boats? Seems like most boat props have more surface area.

I was looking at RC boat props and the largest diameter I found was 4" which seems too small to me, but maybe not?

One more thing: How is pitch measured? I've seen it as a decimal, and also as a size in mm.

I'll try to get pics of my setup on here soon.

 

Pitch is distance travelled per revolution. A simple pitch measurement could be to rig up a stationary tube aft of and to one side of a prop, oriented parallel to the prop axis, and insert a rod into that tube, so that rotating the prop will push the rod through the fixed tube at a rate relative to degrees the prop is . Measuring the distance travelled by the rod in, say, 5 degrees of rotation allows you to extrapolate how far a full rotation would push the rod.
That is why props twist---- no matter where you put the tube and rod, the distance travelled will be the same--- hence the outer blades are flatter--- they move faster, and so they have a smaller angle.
Pitch can be designated in inches or centimeters.
Four inches might work for you--- you are at the very bottom of the power requirement scale for high speeds. The pitch, no matter what, will be calculated from your target speed backwards. RPM will be a part of that calulation, so do some homework and find out what the RPM is on your motor.
Call the manufacturer if it isn't on the label or the literature.
I think your pitch will be about 20", just a guess at how fast your board will go (20 mph plus). The question is how big the diameter.
I am no prop expert at all. I only know some basics. I do find this very interesting though.
Airplane props would have too high a tip speed---or be too small to grip the water--- forget it.
Keep trying...

 

A weed wacker engine might work. High RPM, thats for sure. And it & fuel would weigth less than an electric motor & battery.

Alot of stuff on the forum about the above.

Tim

 

[QUOTE=alan white;153543 A simple pitch measurement could be to rig up a stationary tube aft of and to one side of a prop, oriented parallel to the prop axis, and insert a rod into that tube, so that rotating the prop will push the rod through the fixed tube at a rate relative to degrees the prop is . Measuring the distance travelled by the rod in, say, 5 degrees of rotation allows you to extrapolate how far a full rotation would push the rod.

What?? could you explain that again please.

 

Wow, I've already learned a lot. I think maybe I can put a strobe light tachometer on the blade to get the RPM.

Now I understand why a prop has different angles on the inside compared to the tips.

I used a torch to melt my blades into a steeper pitch a few weeks ago, but now that I think about it, I propably did not design them very efficiently. I bet they also deflect pretty badly because they are plastic.

So I think I might start by buying the largest RC boat prop I can find, with about 4-8" of pitch. I'll whittle it down and see what happens.

Keep the input coming. I'll try to get some specs.

 

With that pitch, you'll need a lot of RPM. Do you mean diameter?