Easy to build, cheap, foils?

Discussion in 'Multihulls' started by Rolf Nilsen, Jul 19, 2007.

  1. Frosty

    Frosty Previous Member

    Oh yes Im sorry I meant 1'x2'4''. Bits coming off and hitting the prop are a concern. I have surface props and although bronze and sturdy im not sure what would happen . Thats why I wanted to keep the foils small so that there might be a chance the props will not be damaged.

    I might be able to get an old helecpoter rotor ,I think it is a NACA four digit foil, I cant remember now.

    I would have thought that 1 inch ply in 1 foot square would have been strong enough, or perhaps a little shorter on its chord.

    I was'nt aware that the length of the foil was a concern. However laminating two section of 1 inch ply would not be difficult, I could even shape it to fit the hulls better. 2'' ply foils would surely be strong enough?

    Can I just ask you what is L/D and ci

    Attached Files:

  2. Guest625101138

    Guest625101138 Previous Member

    L/D is the lift to drag ratio. This is important as there is a cost in getting the lift which is the drag. If the foil does not work better than your existing planing surface then you will go slower.

    You are planing 14T at 20knts. This works out at 180kW for propulsion with 8:1 L/D for the plasning surface. If you allow 60% overall efficiency then required power is 300kW to drive at 20kts. If you have less power than this then the L/D is better. If it is better than 12:1 then the proposed foil will make you slower not faster.

    I would be wary of fixing a foil under your hulls. They look quite flat so they will interfere with a foil unluss it is a couple of chords lengths below the hull.

    Rick W.
  3. Frosty

    Frosty Previous Member

    Sorry Rick-what is 300Kw? I have 2x250HP

  4. Guest625101138

    Guest625101138 Previous Member

    300kW is 400HP. How much throttle do you need to do 20kts? If you are using all 500HP then the crude foils might make some difference.

    I would be very cautious about mounting a crude foil to your hull below the waterline.

    You do not see many hydrofoils in operation. There are some practical issues with the foils like running into things. They are quite prone. They are inclined to catch any long trash in the water and this really slows them up. I guess it depends on your area of use.

    I would be trying to devise a method of testing a foil that did not involve through-hull connections.

    I think you have posted before about a foil between the hulls. This might make more sense for a trial. You could set up for and aft clevises on the cross beams between the hulls and mount struts from these to support a single foil that extends under each hull. It could then have a chord of say 6" and it might be 3m wide. It would only need to be about a foot under the surface to be effective or say 6" under below hull. A well designed foil could easily get better than 30:1 lift to drag with this sort of aspect ratio. (Same reason gliders have wide wings with short chords - they are more efficienct.)

    The thing about the clevises is that you could easily demount the foil if you want. It would allow you to play with different designs. You could make quite good foil shapes using different steel sections like pipe for the nose, an "I" beam along the line of maximum chord, say 1/4" flat plate on the bottom and 1/8" plate for the curved top surface. This would have the strength to do some serious lifting. Mount arounf the CoG of the boat so trim is still controlled by the hulls. The lift will reduce as the foil gets closer to the surface.

    Make sure you have clear water for testing because it could make a mess if you hit something solid with it at speed. Check the insurance is paid up before testing.

    I forgot to answer the other Question last post - Cl (or CL) is coefficient of lift.

    Rick W.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.