hydrofoil for low speed motorboat oddity

Discussion in 'Hydrodynamics and Aerodynamics' started by Tommifin, Mar 8, 2018.

  1. Tommifin
    Joined: Oct 2014
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    Location: Finland

    Tommifin Junior Member


    First I'd like to give a little background.
    This boat of mine is forever work in progress platform for doing odd and hopefully working things.
    It's an old (~70's) sailboat hull without any interior and keel removed. Weight approximately 500kg.

    I have made an outboard motormount 80cm wide( foam/epoxy grp) which extends to flush with the hull about where the back of the keel used to be. it has a narrow V.

    For stability I have added little side pontoons which barely skim the surface.
    Now last summer I have done several daytrips, and I get to max 9,5kn with me and 2 kids on board.
    Outboard is now 18hp mercury 2-stroke. The motorwell part is planing but the center of the hull is not optimal shape for planing and pushes water to side s quite heavily.

    I have allways planned to get this foiling. Now the idea is to put a v-foil on roughly the keel position.
    Foil is almost done. \_/ shape foil with 200mm horizontal bit and 40 degree v.
    Profile is H105 ( thanks Tom!). Chord is 180mm.
    For the aft, I plan to do 2 foils from the pontoons to the motorwell. horizontal and surface skimmers to adjust angle of attack.

    Is there any hope to expect this to get foiling with the 18hp, or 40hp?
    K Matveev's exel for lift wold give me approximation for lift as 277kg for 2 x 900mm foils, 220mm chord, 9,3kn. But I have no idea how this would correlate to real life with e.g. H105 profile.
    Would I assume correctly that under 10kn this is a no go situation? Bigger engine to get to around 13-14kn? My target would be to get into 13-15kn speed range.
    For sure the front foil would need to get the hull out of water first.?.
    Larger foils foils (250chord) to the rear and perhaps also bigger to front? IMG_20170708_143102.jpg IMG_20170708_174555.jpg

    Any suggestions? I'll ask for more if anyone could comment on my plans, otherwise this will be a novel:)
  2. jorgepease
    Joined: Feb 2012
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    Location: Florida

    jorgepease Senior Member

    even if you don't lift off completely, you might get high enough up to overcome the drag of foils and go faster than you are now, you should try it and see what happens, I'd like to see myself ))
  3. Tommifin
    Joined: Oct 2014
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    Location: Finland

    Tommifin Junior Member

    I think this would be a good idea, to install first the front V-foil and get it to lift the center of the hull slightly of the water at a target speed. It will go a little bow up but will be interesting to see what will happen to the speed. I havent seen much data of hydrofoil assisted like hussycat if I remember right. But it might be a thing to try. Secondly stage would be to check if the aft will need foils as well for either take of some weight or to redesign the planing area to carry the aft wit as little wetted area as possible.

  4. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    Have you considered the possibility that when lifting the stern the propeller is in an area (depth) not suitable for its work?
    Last edited: Mar 15, 2018
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