Dimensions of trolling motor prop

Discussion in 'Props' started by terhohalme, Mar 18, 2017.

  1. terhohalme
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    terhohalme BEng Boat Technology

    I have been wondering lately why is it so difficult to find a trolling motor propeller for another purpose than trolling. All propellers seem to be designed mainly to protecting manufacturer from warranty issues more then making customer satisfied. No matter what trolling motor or what size the boat, always the same low pitch propeller. And if you like to use the full power of the trolling motor and you have calculated dimensions (diameter & pitch, or even area ratio), there is no way to find suitable propeller.

    The propellers have parameters like weedless, high thrust, high speed, silent thrust, big fish, fastest in lake, biggest balls of fisherman and all kind of nonsense. But ask just diameter and pitch and you get nothing. Why is it so difficult to print these two numbers on the propeller package? Like 11" x 4".

    If you happens to have a slim light boat, you'll get the same low pitch propeller and depends on the revolves of the shaft, you'll get max 6 mph of propeller wake speed and max 5 mph of boat speed. And use some hundred watt's of power. Now, you buy more powerful trolling motor, you get the same low pitch propeller, speeds up to 5.2 mph and wonder what went wrong, when the new motor uses almost the same electric power. It's as efficient, right? But you wanted more speed...

    Manufacturers of trolling motor propellers, please print the propeller dimensions (Dia x Pitch) on the sales package, show it in your net shop specifications and carve it in the propeller hub.
     
    Last edited: Mar 18, 2017
  2. ondarvr
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    ondarvr Senior Member

  3. PAR
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    PAR Yacht Designer/Builder

    The reasons are pretty simple, first is the desire to keep proprietary things that isolate their products from others. The reason you don't see other prop dimensions is the market, which is interested in trolling, mostly. The other segments of the market are so small, that it's just not worth working up a product run, for the cost/benefit analysis justification.
     
  4. gonzo
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    gonzo Senior Member

    You could calculate what propeller size you need and adapt one from an outboard.
     
  5. ondarvr
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    ondarvr Senior Member

    Easier to just take a stock prop and heat it up and reshape it to what you want.
     
  6. terhohalme
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    terhohalme BEng Boat Technology

    PAR, you are probably right, but with the "normal" propellers you can buy by dimensions.
     
  7. terhohalme
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    terhohalme BEng Boat Technology

    Gonzo, small outboard propellers are designed for double rpm. The diameter is also too small to be efficient.
     
  8. terhohalme
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    terhohalme BEng Boat Technology

    ondarvr, how can you increase pitch in the blade-hub joint?

    The problem is just here, the trolling motor propellers are designed for 4 mph speed. The world is full of small light boats which could go much faster with electric trolling motors, if they just could have the right or better propeller. Torqeedo has done something better, but their motors are far too expensive.
     
  9. ondarvr
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    ondarvr Senior Member

    The props are plastic, so adjusting the pitch with heat is not much different than doing it with metal. You're not going to change it from a 4 MPH prop to a 50 MPH prop, but you can probably increase the pitch enough to make a difference.

    Now if you're actually very serious about this, you can always put up the several hundred thousand $$$$ needed for the project and make them yourself. But if you aren't willing to do it, then why should they be.
     
  10. PAR
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    PAR Yacht Designer/Builder

    Again, the market for trolling motor props is 4 MPH, with a very small percentage looking to do more than they're designed for. I used to make trolling motor props, for just what you're envisioning, but the market was so small, it's just not worth the bother. Bring me an order for 50 props, for each of the most common trolling motors and I'll tool up for a production run again.

    On the other hand, making a prop isn't all that tough, as it can be cast or hand laid up. Of course you'll need to design one suitable and choose a building method.
     
  11. terhohalme
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    terhohalme BEng Boat Technology

    PAR, thank you for sharing your experience in the issue.

    I think, this should be in the interest of manufacturers of trolling motors. Every day, more and more people would like to use electric motors instead of petrol ones in their small boats. Trolling motors are often passed over because of this 3 knot barrier, better propeller could offer 5-6 knot which could be OK. There are enough power (500-2000W) in the trolling motors for small displacement boats. 2 kW electric (or 1 kW at shaft) per 1000 kg is sufficient to achieve the "hull speed" in calm using suitable propeller. Perhaps not with the wide flat bottom bots though.

    Yes, the propeller design is not that difficult. Just engineering. Need to get this 12"x 8" propeller 3D-printed from some tough plastics like poly-carbonate.
     

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  12. gonzo
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    gonzo Senior Member

    What do you mean that outboard propellers are designed for double RPM or that they are too small?. Calculate the pitch and diameter you need for the target speed and choose a propeller that fits your calculations. An outboard propeller has a larger hub than an inboard, so will probably be easier to fit. Otherwise, you can use an inboard propeller too.
     
  13. terhohalme
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    terhohalme BEng Boat Technology

    Sure, it is possible to find diameter (12") and pitch (8") quite near, but the blade area is about 5-8 times more than needed and efficiency gone that way... OK, I'll take this how it is and check if 3D-printing is available. Have to prove to myself that I can do better than trolling motor manufacturers...

    Actually we made 3D printed propelles four years back to our solar racing boat (https://www.facebook.com/MidnightSunFinland/), but they are all at the bottom of Gulf of Finland.
     
  14. gonzo
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    gonzo Senior Member


  15. Joakim
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    Joakim Senior Member

    Torqeedo has a rather large selection of propellers: http://www.torqeedo.com/en/products/accessories/propellers-and-fins

    They don't give pitch. I think diameter is 12", at least for most of them. They give too parameters V is speed in km/h and P is power in W. You can reverse engineer the pitch, since they do give shaft rpm of their motors.

    High speed and so low power that a typical OB propeller has 5-8 times too much blade area is a very special case. No market for that.
     
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