Foil assisted multihull design

Discussion in 'Boat Design' started by groper, Sep 29, 2013.

  1. groper
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    groper Senior Member

    Hi, not sure if there is already an appropriate thread discussing this, but im interested in the design of foil assisted powered multihulls like the hysucat type system.

    The specific design im interested in relates to a foil assisted 11m catamaran powered by twin outboard engines.

    The full design displacement is 4 tonnes. Lightship is 2.5 tonnes.

    From the typical hysucat type designs ive read about, the aim is to have about 60-70% of the vessel supported by foils at its running speed, ideally around 25kts.

    The main foils extend into the tunnel level with the keel just forward of the CoG.

    The rear foils id like to explore the option of running a custom made foil bolted to the cavitation plates of the outboard engines to support a lesser fraction of the lift and provide adjustable trim via the hydraulic trim tilt on the engines.

    Does this sound like a viable set of parameters to develop a workable design?

    If so, what are the main design considerations and problems to consider?

    Any links to further reading on specific foil design consderations for this type of application would be appreciated.

    Thanks
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    :eek: I assume this is being contemplated for your current build ? There is a big gap between your two hulls, will the structure cope with a substantial load a centreline strut will impose ?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Incidentally, I suspect the reason trailerable power cats are not foil-assisted, as a rule, is the difficulty of having the boat sit safely on the trailer, with that hardware in the way. Easier in a 'split' tunnel hull, of course.
     
  4. groper
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    groper Senior Member

    There are many different ways of approaching the problem... but yes a full tunnel span foil with center strut is but one option, although i think the span is too much and there would be too much lift, ultra high aspect ratio and very small size would be difficult to build with sufficient strength... inside hull edge to edge span is 3.6m. i was also considering 2 partial tunnel span, eliptical planform foils, perhaps with endplates, mounted in cantilever fashion from the inside edges of the hulls... have yet to do calculations to determine the ideal foil geometry and thus determine the structural requirements for an unknown quantity... working out the optimum foil geometry is something i would like further help with... any takers?

    Appropriate position with respect to CoG? - Would it be appropriate to assume 80% total lift from the main foil and 20% from the rear foils and position them according to their respective moments?

    How to achieve the minimum drag configuration at designed speed and load?

    What about angle of attack, aspect ratio, foil area, and section shape of shallow running foils?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    You could go the Ben Lexcen-style winged keel, with raked foils fitted to a central raked strut, nothing touching your two immersed hulls, would require a beefy structure in the deck though to take the loads. I like the idea of everything being raked, must have seen too many docos about the advantage of raked wings on jet planes, water being 800 times as dense as air it may well translate to the watery medium.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Oh......and if everything is raked, and there are no connections to the demihulls, no problems with kelp and weed and plastic bags hanging up on it !
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I take it you have factored in that you don't want any foil projecting below the bottom of the existing structure, or you may not sit flat when dried out, or scrape the bottom with your foil at anchor. It may even get snagged under a log ! There are definitely disadvantages to them, I wonder whether your boat is going fast enough to make it worthwhile, certainly until you are going fast enough to produce lift, they will only slow you down with additional drag. I suspect dynamic lift generating foils are an unusual mix with a displacement cat.
     
  8. sottorf
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    sottorf member

    Hi Groper
    I specialize in design of hydrofoil systems for power cats -both new builds and retrofits to existing vessels. Roughly the parameters you mention for the main foil are correct but they vary depending on speed of the boat and hull shape. There is no easy rule of thumb about where to fit the foils. You will need to calculate it out or alternatively work it out by trial and error.

    You can use foils bolted to the cavitation plates of the engines. There are off-the-shelf products you can buy for this. These work but will not give you as good pitch stability as true Hysucat stern foils.

    Mr. Efficiency:
    Hysucats can be trailerable as the foils are located at or slightly above the keel. Even if they have dihedral it is possible to trailer them. There are a large number of 5.5m - 10m Hysucats on trailers in South Africa.

    Not attaching the foils to the hull reduces the efficiency of the system significantly as now the foil flow will be 3D in nature with tip vortices being generated. I don't recommend that unless it is really not possible to attach the foils to the hull. Kelp and plastic bags do not hinder the foil more than they would the props so their is no added disadvantage to using foils.

    IF you need assistance with your design, you are welcome to contact me.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Hmmm.....can't see any great problem there, there would be two tips to produce vortices, the same as your set-up, with the two short foils you have right aft, connected to the hull at only one end.
     
  10. sottorf
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    sottorf member

    When the foil is attached to the hull, the flow is practically two dimensional because the hulls act as massive end plates for the foil. If you don't attach the foil to the hull you end up with a short wing of small aspect ratio with has poor lift to drag ratio and poor structural strength.

    The rear foils efficiency is not that important as they are small and do not generate much lift.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    You still have two ends producing vortices, which you say are bad.
     
  12. groper
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    groper Senior Member

    Sottorf,

    The catamaran im building is very similar to your eCat design shown on your website, its 90% structurally finished in infused foam sandwich panels, hence the favorable length/displacement ratio. i can add the modifications for foils immediately.

    My design goal is for outright best efficiency at any speed above 15-20kts, in terms of fuel burned per distance travelled.

    Hull shape is a simple symmetrical slightly flared U section, with block coefficient of 0.89 continued right to transom, forefoot is shallow ramp with elliptical waterlines to fine entry bow.

    Pic shows shape and build status;
    [​IMG]

    [​IMG]
     
  13. sottorf
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    sottorf member

    what is predicted weight of your boat? That is going to determine if foils will be useful for your application. Foils only provide benefit above a certain critical Froude number.
     
  14. groper
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    groper Senior Member

    Weight = 2500kg lightship and 4000kg full displacement.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Methinks the America's Cup racing has been exercising groper's grey matter !
     
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