Trimarans with Daggerboard aside the central line...

Discussion in 'Multihulls' started by Skip JayR, Nov 11, 2015.

  1. Skip JayR
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    Skip JayR Tri Enthusiast

    Regularly we know on bigger Trimarans that the daggerboards are front the mast step positioned exactly on the central line (horicontal line between bow and stern).

    With curiousity I noticed first time from a Formula 40 Trimaran (Philippe Cabon Design, see pictures in this thread) that some few (smaller) Tris have been designed and built with daggerboards which lay 1-2 feet aside the central line (see pics attached).

    First thoughts I assume that this kind of construction has three targets:

    • trimming: The daggerboard must be positined behind the mast foot (between mast and cockpit) for keeping the centre of lateral gravity / lateral pressure point in balance.
    • wind courses: fully uplift of the daggerboard possible otherwise it would be blocked by the main sail beam on down wind courses.
    • comfort: creating more space in the saloon as the daggerboard case is standing little bit aside.

    I wonder how it works to trim such a Trimaran ? What about the seaworthyness and safetyness ? Any risks ?

    E.g. having the Daggerboard 1-2 feet more on starboard side I suppose that the weather helm in tendeny is bigger for courses the wind direction comes from port side, while this boat has reduced capability for close to wind courses the wind direction is coming from starboard side. - Right ?

    How to compensate this "assymetric position" of the daggerboard ?
    Are there any benefits (beside the upper) for trimming such a Trimaran ?
     

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  2. Doug Lord
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    Doug Lord Flight Ready

    Well, you have tri's with the daggerboards in the amas and I would guess they work ok-so 1-2' either side of the centerline should be no problem.
     
  3. Corley
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    Corley epoxy coated

    Often it's done just for sensible packaging of limited interior space and making the most of the limited sole space in a skinny main hull. As long as it is aligned fore and aft correctly there will be no discernible performance difference.
     
  4. rcnesneg
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    rcnesneg Senior Member

    As far as I can tell, there is really no downside to having a daggerboard slightly off center.

    I have an 8 foot keelboat which i would think is extremely sensitive to that sort of thing. The mast is completely on the port side of centerline and the daggerboard/keel is completely on the starboard side of the centerline. They are separated by a sheet of 3/8ths plywood. It was a design decision that resulted from wanting an unstayed mast in the same position as the retractable keel/daggerboard.
    You can see a bit of what I'm talking about here:
    [​IMG]

    Long story short, I can't tell a difference at all.

    I suspect a big wide trimaran would be less sensitive to that sort of thing than my little 8 footer, so in other words, there's no or very little difference for that tri compared to having it on the centerline.
     
  5. neville2006
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    neville2006 Junior Member

    Is there a reason Tri's aren't all designed with the boards in the outer hulls?
    This would free up much needed space inside and I would think be easier to engineer as well as potentially safer if you hit something. Board doesn't get in the way of the main boom when raised etc.
    Is there a proven hydrodynamic problem if you have a board not aligned with your rudder?
    I once asked Kurt Hughes about this and he said that you would have to have your rudders out on the amas too but didn't elaborate and I am not sure if that was based on any testing/evidence or just a gut feeling.
    Of course if you were doing a new build that is probably what you would do like Ladyhawke
     

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  6. Corley
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    Corley epoxy coated

    I'd go with a central daggerboard for reasons of keeping things simple. One daggerboard and one case weighs less than two boards and two cases. If the daggerboard exits forward of the mast then there is no clash with the boom. I'd have my doubts about it being easier to engineer you are applying additional force to the float which it otherwise wouldn't see it's manageable of course but might require some additional reinforcement of the float and crossbeam.
     
  7. Skip JayR
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    Skip JayR Tri Enthusiast

    so cool your tiny boat ! :) made my day :D Tks.
     
  8. Skip JayR
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    Skip JayR Tri Enthusiast

    hm... thats a good item I forgot to think about how the hydrodynamics of the daggerboard positioned between mast and cockpit might affect the single rudder mounted on the transom of the central hull.

    Are there some basic formulas about this item ? - Would be great to have a table with a matrix of "distance between central daggerboard and rudder" and "speed".... and the dimensions (length of rudder + length of central daggerboard)

    Anywhere a link ?
     
  9. pogo
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    pogo ingenious dilletante



    And what do you wanna do with your " matrix" , your " table " , your " formula" ?
    Searching in the www. for pix of them, eeh ?

    pogo
     
  10. Skip JayR
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    Skip JayR Tri Enthusiast

    What about the risks of collissions ??

    If the daggerboard case is front the mast, then there might be still a buikhead between saloon/central cabin and foreship. So if this daggerboard should hit something, the case is damaged/cracked and water flows in, then its less risky for "sinking".
    [​IMG]

    While having the daggerboard case in the saloon (as seen in the upper pictures) it might be risky if the boat has a collission at high speed between 15-20 knots the central hull might be ripped and torn up.

    Right ? Or do I spin around with an apocalyptic thinking ? :)
    [​IMG]
     
  11. waynemarlow
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    waynemarlow Senior Member

    Most dagger boards are in the main hull for slow speed manouvering ie the boat will rotate easily around the daggerboard when for example mooring and harbour work.

    There is an additional benefit of the daggerboard in the main hull, it will act as almost a governor on the boats performance, if the main hull begins to lift out of the water then you have less daggerboard surface to resist the sail pressures and all excess energy will cause the boat to slide sideways, rather than trying to bury the Ama.

    Some of the farrier boats have had for many years offset dagger boards and indeed the F85SR plans I have, if you move it to one side, you can get a nice central toilet and shower area between the front cabin and rear living areas.

    There is one other option to consider, Ama rudders and no central rudder, it frees up totally the outboard for harbour work and always will mean maximum rudder depth.
     
  12. pogo
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    pogo ingenious dilletante

    U got it !

    pogo
     
  13. Corley
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    Corley epoxy coated

    To be honest I'd rather have the problem in the main hull. You still probably have two intact floats to keep you afloat. If you have a float holed below the waterline it can be a serious problem as they are hard to get into and assess the damage by comparison.
     
  14. cavalier mk2
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    cavalier mk2 Senior Member

    Kurt dropped the ball on that one. Quite a few tris from Tristars on up have done this. Then there are the cats with central rudders and boards in one or both hulls.

    I agree with Corley though, better to have them where you can get at them and offset is fine. Long list there too, including Kurt. One big case and board is lighter than 2.
     

  15. Skip JayR
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    Skip JayR Tri Enthusiast

    Good point...

    hm.. and yes, on a middle sized Tri probably it wont be an extreme big drama having a leckage in the main hull as by a given draft of 40-50 centimeters in average such kind of hull damage still let you walk around under deck safely in some fisher trousers... :)
    [​IMG]
     
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