Electric hydrofoil prop

Discussion in 'Props' started by cmaas, Sep 8, 2020.

  1. cmaas
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    cmaas Junior Member

    I have built an electric foiling catamaran using Hobie 18 hulls and a Tohatsu 15 outboard converted to 10kw electric. I am now reliably foiling but I need to limit speed to 11 knots because, I believe, the prop is creating vertical lift when I try to increase rpm which pushes the stern up and the bow down. I can compensate for this somewhat through foil control surface adjustment but ideally I would use a prop that has no, or very little, tendency to change it's depth below the surface with changes in rpm.

    I read that blade rake plays a roll in this and am wondering if there is a way to make a vertical lift neutral prop either through changing rake or some other way?

    I did add some cup to the tips which seems to help but if it did the effect was small, difficult to quantify and possibly imaginary.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    How much of those Hobie hulls are in the water, when you are foiling ?
     
  3. cmaas
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    cmaas Junior Member

    They are about 300mm clear of the water. This is a fully foiling boat.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Beats me how you keep the prop in the water. Have you got any pictures of the set-up ?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Oh, and I am interested to hear more about the conversion of the outboard !
     
    Will Gilmore likes this.
  6. bajansailor
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    bajansailor Marine Surveyor

    +1 re Mr E's comments above - and if you have a video of your Hobie up on her foils, could you post this as well please?
     
  7. Will Gilmore
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    Will Gilmore Senior Member

    Have you considered enclosing the blades? shrouded propeller vs nonshrouded opeller https://www.boatdesign.net/threads/shrouded-propeller-vs-nonshrouded-opeller.24756/

    However, I would try to drop the props below the foils. Let the forward thrust add to the upward tilt by pushing against the resistance of the folds below the horizontal with a bigger lever arm. If I am making my meaning clear.

    The foils have a center of longitudinal resistance. The prop is a forward force trying to overcome that resistance. If the prop is above the horizontal of that resistance, it drives the boat in an orbit over to top. If the prop is below the horizontal of that resistance, it drives the boat in the reverse orbit around that resistance. You need to find the balance.

    -Will (Dragonfly)
     
    Last edited: Sep 9, 2020
  8. cmaas
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    cmaas Junior Member

    Thanks for the reply. I see what you are saying. I'll try to post a picture to show you what the set up is. The prop is more or less in line with the horizontal foils. I would rather the prop only provided forward thrust, leaving the vertical lift and ride height control to the foils and their surface sensing wands.
     
  9. cmaas
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    cmaas Junior Member

    Hope this works.. these photos seem a little large.

    This is what we're working with. She's no beauty. This is not a commercial venture just a quick, cheap and dirty way to have some fun.

    So, the prop. Do I understand correctly that more blade rake decreases stern lift and vice versa?


    IMG_0207.jpg DSCN8356 (1).jpg
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Fun project! When a propeller operates close to the surface, the inflow of fluid into the propeller disc becomes asymmetric. As a result, there is a difference in fluid aceleration between the upper and lower halves of the disc. This is manifested by a vertical force, ie lift, ultimately resulting in "propriding", where the propeller seeks a balance between lift and submerged disc area.

    It can be counteracted to some extent by "raking" the blades backwards; creating an upward acceleration of the flow from the lower half of the disc. Now, any deviation from a horizontal acceleration includes an added loss; it might be better to use the "original" lift force by reducing the rear foil lift, unless that generates take-off problems. And stay away from shrouded props, they generally have higher losses than an open screw.

    Edit:
    Btw, you now use a propeller designed for central xhaust outlet. If you ventilate this opening or not, may influence the behaviour.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Fascinating ! This motor is capturing my attention, no end. As baeckmo says, the prop is creating a little drag behind the hub, but it may already be ventilated. This drive appears quite professionally executed, I am impressed.
     
  12. cmaas
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    cmaas Junior Member

    Thank you. That makes perfect sense and now I see why I hit a speed limit when the boat is at its designed ride height. Interestingly - to me anyway - is that the stern foil is providing no, or close to no lift once flying even though about 40 kg of lift is required. So that lift is being provided by the prop, which is very annoying as it is not consistent. I have tilted the outboard back so the the prop shaft is tilted 3 degrees negative from horizontal. The stern foil aoa is 2 degrees negative with 10 degrees of negative flap and still the stern lifts! Of course I can keep adding negative lift to the foil but the inefficiency of having the prop and foil lifts opposing makes me crazy. Yesterday I added 2 more degrees of negative tilt but saw no improvement. In fact the boat became slightly less stable in roll for reasons that are not apparent to me.

    It seems like the props that are available for these small outboards only offer choices for pitch and diameter, forget rake. I would be happy to hear otherwise.

    I did bend another 5 degrees or so of rake into my prop blades, as well as cupping the tip and trailing edge. It has become quite ugly. Though it does seem to work it can't be optimal.

    I see that prop design is a complicated process. In my perfect world I would be able to obtain a prop that minimized lift but was still wonderfully efficient in my application. Building a composite prop is not out of my skill set but designing one certainly is.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    No way, that prop is not giving you 40 kg of lift, and is giving virtually no lift when running deep.
     
  14. cmaas
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    cmaas Junior Member

    The conversion is fairly straight forward. My biggest mistake was to choose a controller that is so wonderfully sophisticated that it requires an expert to program it for a particular motor and controls.
    If by ventilated you mean air is introduced in place of exhaust, no I didn't do that. I had thought to remove that flared bit and fit a tail cone but that is far down the list at this point.
    You wouldn't know it by the crude construction but I have been a professional composite boatbuilder for close to fifty years. I can do nicer work that this, really!
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Just leaving the exhaust passage open to the outside would ventilate it. How much did the conversion cost you, and what is the output of the electric motor ?
     
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