Rudders & plates along centreline between hulls on Cats

Discussion in 'Multihulls' started by Sailor Sam, Oct 16, 2007.

  1. Sailor Sam
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    Sailor Sam Junior Member

    Can anyone explain why on catamarans, rudders and plates are not fitted along the center line between the hulls, but on each hull.

    thanks
     
  2. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Air

    Hello Sailor Sam

    Some of the earliest cats had centrally mounted board and rudders - a few still do. One of the problems is that if you fly a hull then the foils will be out of the water or reduced in waterplane area.

    Surface piercing foils are not as efficient as underwater foils. This is why many cats and tris have underhung rudders even when a transom hung rudder is much easier to make. The underhung rudders work very well in rough conditions.

    On the other hand there are a few cats that have the usual twin rudder set up with a pivoting centreboard hung from the bridgedeck. Not a bad idea for cruising cats as you shouldn't fly a hull anyway and you can have a really big board with few space limitations.

    cheers

    Phil Thompson
     
  3. Sailor Sam
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    Sailor Sam Junior Member

    Hi Phil
    Thank you very much for yoiur speedy reply. Your info. is very interesting paticularly your last para. Can you provide the names of the cats with C/L arrangements.

    cheers

    Colin
     
  4. Bruce Woods
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    Bruce Woods Senior Member

    center line arrangements

    Sailor Sam,

    To name a few with this arrangement:
    14' arrow- neil fowler design.
    11' arafura- neil fowler design
    16' quick cat- charles cunningham design
    16' tathra- similar to quick cat with jib.
    the cruising cats I can think of off the top of my head with all foils on the centreline are predominantly kelsal designed i think.

    Regards
     
  5. Sailor Sam
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    Sailor Sam Junior Member

    Hello Bruce
    Thankyou for your input.
    I will look at these cats.

    regards.
     
  6. doug kay
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    doug kay Junior Member

    I am rebuilding my ground effect after extensive damage due to a twister and it struck me that a centre board in the centre of the bridge deck would be a good idea. The case can go down to the W/L but it's location with respect to the mast could be a problem. Because i'm 78 years old i reduced the aspect ratio of the rig quite considerably so flying a hull would be doubtful.
     
  7. Sailor Sam
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    Sailor Sam Junior Member

    Hello Doug
    Some low altitude hull flying may not be a problem because as the windward hull lifts the leeward hull sinks deeper so compensating to some extent by lowering the whole boat in the water. A simple sketch shows this clearly. Taking the centre casing as low as possible is a god idea as this better distributes the side forces from the plate. I see centre line plates as a labour saving device when tacking so you might find that aspect usefull.
     
  8. Petros
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    Petros Senior Member

    It can also be done on large high performance cats, check this one out:

    http://www.wsl2009.com/canais.asp?id_canal=66

    It has not been in the water yet, but I would assume the designers know something about cat design.

    I think on smaller cats it is assumed you would fly a hull at times, and you would not want to go sailing with little or no center board or rudder in the water.

    I had also considered this and if you are not designing for sailing with a hull flying, you would not need more than one center board or rudder. The bottom of the hull also provides end-plate effect and make them more efficient, but I suspect that one larger center board and rudder would have less drag than two smaller ones, unless the end plate effect is more important (add a surface plane on a center mounted keel and rudder?). The fewer the parts the lighter and less complex it would be.
     
  9. grob
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    grob www.windknife.com

    You should be able to arrange it so that when the catamaran is sailing level with both hulls in the water the boat is balanced. i.e. center of effort CofE and Center of Lateral resistance CLR are in line.

    When the hull lifts and the center rudder is lifted out of the water the CLR moves forward, and so the boat luffs up, the boat de-powers and the hull drops back into the water.

    Gareth
     
  10. brian eiland
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    brian eiland Senior Member


  11. doug kay
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    doug kay Junior Member

    you have some excellent ideas Brian, I shall have a good look at the double centreboard idea over the winter. I tried to use a foil instead of the normal sail rig but the mast needs a deep bury and on a cat. that is difficult but I did look at the idea of counterbalancing the rig with a plate and the mast able to spill wind as a mono does you may be able to dispense with the shrouds also if not a frame of aluminium would do the trick.
     
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