Hull design - CAD drawings

Discussion in 'Boat Design' started by congellous, Sep 3, 2013.

  1. congellous
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    congellous Junior Member

    Hi, I have a hull design that needs some advice, it's a Cat with a Hydrofoil to be made in GRP from a mold. The idea is it floats pretty much on the waterline but has the stability of the two submerged hulls and then at lowish speed starts to lift out onto the foils. Any comments on the design ? I'm thinking of a mid keel based propellor.... Thanks
     

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  2. BMcF
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    BMcF Senior Member

    Catamarans and trimarans have been built (Westamaran, Fjellstrand, etc) that operate as fully foil supported high speed vessels, with varying degrees of success. They require a fairly robust flight control system to properly manage the lift from the foils, almost always using trailing edge flaps.

    I did not understand what you meant about the "mid keel based propeller"....
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    If the intention is to have it entirely foil-supported at speed, what control will there be over trim ?
     
  4. congellous
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    congellous Junior Member

    Hi thanks for your comments, I was proposing a keel midway along the main hull with a propellor deep enough to keep it going "airborne"
     
  5. congellous
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    congellous Junior Member


    Wing tip flippers and a mid propellor but this is very much a draft proposal I'm looking for general advice on the shape and feasibility, cheers
     
  6. rwatson
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    rwatson Senior Member

    Umm - have you done any measurements, displacement, weights etc at all ?

    the totally horizontal foils indicate a severe lack of research on he concept.

    The hull shape doesn't show any immediate benefits.

    There is no point discussing feasibility and shape unless you have a bit more than pictures.
     
  7. congellous
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    congellous Junior Member

    Research, that's why I'm here. It's a river boat hull so the proportional shape is fixed, following reading on here I thought I'd make it a Cat and then wondered if it could even be a hydrofoil from the video's and design data. It's not going to be used on the seas, however, it may be transited like a barge and maybe moved around in coastal waters. I wondered wether it could be used as a hydrofoil at slow pace just to save drag as it's an uneconomical shape ? Thanks
     
  8. BMcF
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    BMcF Senior Member

    Actually, the presence of the foils and struts increases the drag of the vessel at "lower" speeds. Only once you've lifted the hull nearly clear of the water is the resistance potentially decreased relative to the bare hull.

    At what speed did you desire/expect the foils to be supporting most or all of the vessel's weight?

    We built and tested a trimaran that was assisted by fully submerged foils such as you showed on your sketch. It was 13.5m LOA and displace about 9 tons. There was a very pronounced "hump" in the resistance of the vessel, around 12 knots, that made taking off difficult at times. Once the vessel exceeded 14 knots, the speed would then quickly come on as the drag reduction benefits of being on foils became apparent.

    But we've also been involved in foil-assisted projects where the vessel never made it past the drag hump and foils were only a source of excessive drag.


    As an aside...controlling/maintaining the trim and roll of a concept such as yours is quite difficult to do with simple manual controls.
     
  9. BMcF
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    BMcF Senior Member

    Given the hull shape and intended use, perhaps you should look at adding skirts fore and aft and pressurizing the cavity between the hulls with a small centrifugal blower.
     
  10. congellous
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    congellous Junior Member

    Great that's more like it -
    At lower taxi'ing speeds it doesn't matter that much about the drag and it would provide a stabilising effect I assume.

    Take off as low speed as possible, I have read a few threads and know that there are issue's.

    It's 10'6 beam and 41'4 length, enough speed to get it onto the foil is all that is needed and otherwise just walking pace.

    Intended uses canals, rivers and Island hopping in fair weather at most
     
  11. frenette
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    frenette Junior Member

    Could you give us more information about the goals of the boat?

    Sheltered or open ocean?

    Target weight aka what is it going to carry?

    By mini keels I'm assuming the prop comes off of the aft posts?

    There are others with more experience designing foils. I normally get the off the shelf designs.
     
  12. congellous
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    congellous Junior Member

    Hi, sheltered, fair weather cove hoping maybe, in Greece or similar or canals, river, leisure holiday cruise style.

    Weight - as light as possible, lighter than equivalent size yacht or motor cruiser at the moment less than 1000kg and I wouldn't expect it to go past 1500kg

    I'm open to suggestions on the props and yes some tried and tested foil design, nothing fancy needed, it's not a performance boat, timid little cruiser.

    Thanks for your input
     
  13. congellous
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    congellous Junior Member

    I might even drop the hydrofoil idea, however, I was wondering wether the channel between the hulls could be shaped to entrain the water to a central propellor ? Is this what you are suggesting
     
  14. rwatson
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    rwatson Senior Member

    That can work. If you design the tunnel correctly, it will actually get some uplift from the air compressed into the tunnel at speed.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Channelling the water, which will be aerated, into your propellor I would doubt would be prospective.
     
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