Trolling motor powered surfboard: prop problems

I said 4" becuase someone mentioned that it might work for me.

Looks like my trolling motor runs about 1200 rpms at 12v. I'll assume that it doubles at 24volt. I've tried running at many different voltages, but I'd rather run 12volts to save battery weight

I considered a weedwhacker engine, because I have a used weedwhacker I can part out. But after reading on this forum, it seems that they don't have they power to idle in the water. Maybe a smaller diameter prop could help with this issue?

 

I don't know if RPM doubles with voltage, or the wiring is different. Increasing voltage without the motor having the wired-in capacity to run at the same wattage would probably fry it. i've seen how they sell the trolling motors and it appears on the surface that voltage equals power, but it really has nothing to do with power. Higher voltage in bigger motors is keyed to the fact that they consume more energy, and are likely to be paired up with multiple batteries, which can be run in series to get the higher voltage, which alows for less circuit resistance in cables and internal motor wiring.
Generally, higher voltage (24) suffers less voltage drop in the same cable that 12 volts would, so long as the motor amperage is halved. The net result is the same power, but more efficiently delivered.
Voltage times amperage equals wattage, or real horsepower. So any trolling motor available in both 12 and 24 volts would also have two different amperage ratings. The 12 volt unit would have twice the amperage rating of the 24 volt.

 

The only reason I ran the motor at 24 volts was to see if it would help increase my speed, which it did. If the motor were to fry, I figured I could just rewind the rotor with fatter wire and try it again. Used trolling motors go for $25-50 around here, so frying one for the sake of science was a risk I took.

I think the weedwhacker idea sounds awesome, but I'm concerned with trying to get the prop down deep enough, without extending it too far behind the board. Seems like a bend would be required in the shaft, and I do not know how I would accomplish that

 

Take it apart, fill the tube tight with fine sand, and bend to suit. Make sure to seal the ends when you bend it.

 

The prop photo is of a high efficiency prop for a human powered boat. It is optimised for 150W input at 10.4kph. The blades are 215mm long and 40mm chord. Prop efficiency is a function of the apsect ratio of the blades. The longer and narrower the better. With boats there is usually draft limitations and stress levels that tend to force lower aspect props.

Basically aeroplane props are more efficient than boat props so if you don't have draft and stress constraints you can make high efficiency boat props that look very similar to aeroplane props.

So if you have the room to spin a long thin prop from a model aircraft then this will be more efficient than a typical boat prop. However the most important thing is that the prop suits the application. It does not have to be large diameter but you lose efficiency potential if you reduce the diameter.

If you cannot get the motor and board data then you could buy a 16" by 12" prop for a model aircraft and see how it goes. This will give you some insight.

Rick W.

 

Rick-
Could you please post a link to your video of your human powered catamaran
in action, the long stainless shaft one? I am contemplating a similar problem with running a honda four stroke GC160 5hp air cooled engine with a cheap slip clutch thru an 8ft stainless longtail shaft, unsupported mostly, using the self centering characteristics demonstrated by your video. Thats how I found this thread, im trying to use a minn kota disposable prop from wallyworld on the end of the shaft, a very small pitch, so I can achieve some decent rpm if needed.

 

Not exactly sure which one you mean. This is my OC1 with the unsupported shaft (is a 2.6Mb file so will take a minute or two to download):
http://www.boatdesign.net/forums/at...8374-prop-shaft-systems-v7_strutless_prop.wmv
The prop is mounted on a 1/4" spring steel shaft that is 1.2m long and has a rigid tubular section about 500mm long from the gearbox. You can see the silver flash of the prop and watch it move about as the boat turns. This shaft is good for a thrust up to about 10kgf.

The shaft takes a bit of design effort.

The main reason for using the curved shaft is to avoid vibration from the prop when it runs on an angled shaft. If you have a low efficiency prop then there is little advantage having a curved shaft.

Setting up an unsupported shaft is not trivial. It has to be thick enough to take the thrust load without buckling and yet flexible enough to avoid fatigue failure.

If you tell me a bit more about the hull you want to push and what speed you are after I might be able to make some suggestions.

Even on my curved shafts I do not normally run them unsupported because they dive when I reverse and hang deep in the water until I start spinning the shaft.

Rick W

 

I ran a troll motor at 2x rated volts for many years without any damage as confirmed upon internal inspection. The key is not to exceed design CURRENT to prevent burnout. You do this by cutting back on the pitch of the prop so it is less aggressive. You get 2x power and rpm. Only thing to watch is if the prop tangles to where it stalls- you must cut power immediately or have a breaker in place to do it for you.

As for the prop, may try the the torquedo see efficient electric boats #129, to see how the current measures up. Model air props are good and cheap for some cases, but trolls don't have enough torque at low 1200rpm (unlike geared down motors) to drive the really large efficient ones, in my experience.

Hope this helps.

Porta

 

Regarding weed whackers, I saw a guy down at the boat ramp who tried to put a trolling motor prop on a weed whacker. He got out in his little 12' aluminum boat and started it up, but it died the instant he touched water with it, even a little bit. The RPM is WAY too high and the torque way too low. If you came up with a gear reduction or belt reduction, it would probably be fine.

Regarding pitch on a prop: There is a standard way to measure it, but the more accurate you want to be, the more complicated it gets.

First, the pitch is defined as the distance the prop would move forward in one revolution, with no slipping. I use the metaphor of a swimming pool of jello.

As well, the measurement is to be taken at 0.75r, so for your 4" prop you would move out 1.5" from the center and take your measurement there. That's because not all props have an accurate pitch all along the blade, which is called a true-pitch prop. That means if the 4" prop moves forward 4" in one revolution when measured from the tip, it should also move forward 4" at 0.5r and at 0.75r as well. Since not all props do that, prop pitches are measured from 3/4r station.

So in your case, you have a 4" prop and are measuring at 3/4 diameter or radius, so you have an imaginary 3" prop. The "distance traveled" is 2*pi*r, so 3*3.14=9.42" to travel for that revolution, that's around the circle.

The project is to measure the angle of the prop. Figure the prop is going straight up, shaft is upward. The 9.42" is the distance around the circle, we measure the angle from our "horizontal" (remember the shaft is pointing up) to the blade. Now extend that angle for 9.42", and measure the distance from the bottom line. That should be 4" if your prop has a 4" pitch.

Now, the next problem is that the shape of the blade matters. The angle of the blade is not intuitive, you can't just slap a ruler on the bottom and have that be it. The pitch is measured from what's called a "zero lift line." There is a calculation you can do to find it, but it really has nothing to do with the angle of zero lift. It's just a line used during the design process, and if your blade is cupped at all then it makes no sense at all.

If your eyes are not glazed over, I can tell you how to figure zero lift line.

However, now I want to go into the hobby thing.

You might find that to use a hobby prop, it might be easier to use a hobby motor too. What's the goal here, are you trying to make a surfboard go really fast, or are you trying to make it into a fishing platform?

If your goal is speed, then going with all-hobby parts might be a good idea if you can swing it. You can get a servo controller to control speed if you use an RC speed controller. The RC brushless motors put out incredible power for their size and weight. If you don't get enough thrust from one prop/motor, you might try two.

I'm really interested in this project too, it sounds like a blast.

 

Joeforte,

Is this what you had in mind?

www.powerski.com/content/psi_index.php OR www.ballerride.com/2008/03/31/monojet-the-40mph-surf-board/

A buddy of mine has one similiar to it but the engine is further aft.

I'll see if I can get more info from him about it if you want.

Tom

 

Higher voltage higher speed

Interesting post, regarding voltage, yes higher voltage means higher RPM

The current is max at stall and minimum at top RPM.

Double the voltage at stall will mean double the current so this is where things can burn out)
But at top RPM, the current will be the same as with 12V, but top RPM will be double. (roughly)

The motor keeps accelerating until its' generated voltage approaches the applied voltage.


I am going to run a 24V 80lb thrust at 48V
I will need to prevent the current getting too high at stall or under high load but it should be able to obtain much higher RPM.
(note this will be done with PWM control and current sensing)


I'm using a 80lbs trolling motor from ebay, it's good.
Probably made in the same factory as minkota.
Uses the same wire and metal as a torqeedo I'm sure.
(just 2500 dollars cheaper)