2 kayaks to make 1 cat ?

Discussion in 'Boat Design' started by fabrice, Mar 23, 2009.

  1. fabrice
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    fabrice Junior Member

    Hi everyone, I'm owner of a pair of "twin" kayaks. My wife tried my sea kayak and wanted just the same !
    So, I was wondering how to get a cat out of these.
    2 PVC pipes and ratcheted webbing does a very efficient job to create the double hull.
    I then fitted other pvc pipes (all 5 cm diameter) to make a mast support.
    Mast is made of wood, so are crab claw spars.
    The overall design may look crappy, but it worked without snapping in 3-4 b on the nearest lake. (sorry no pics for the moment)

    First of all, I'd like to hear from your background on this peculiar design.
    Second, I'm now working on an major improvement :
    It's obvious that my yaks aren't built to do a beating or even a reach, and slipped a lot.

    what's the best design ? A leeboard on each side ? Just one leeboard ? Maybe a centerboard attached in the middle of one of the pvc pipes ?

    Would a rudder be enough ? After all, cats don't even got any board, if I'm not mistaken ?
    Same questions then with the rudder : centered ? each kayak one like any cat ? ...
    Gee, so many questions, so less time left to live ...

    (Hope my english is readable, don't be afraid to correct the weirdest faults, I'm inclined to improve myself every time I can)
     
  2. fabrice
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    fabrice Junior Member

    never tried this concept ? anyone ?
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    I have tried a sail rig on a kayak and it worked well, but the sail area was too small for working upwind well. Your "catyak" can carry more sail area.

    Cats usually have daggerboards or sometimes angled foils to prevent sideways drift or leeway. You will need a board to prevent leeway on a reach or when beating into the wind, because kayaks do not have much resistance to leeway. The location of the board in the side to side direction is not very important while both hulls are in the water; the inboard of either hull should be OK. You can start with the leeboard directly under the mast and move it until you find the best fore or aft location.

    A foot-operated rudder would leave your hands free for sail handling; do the kayaks already have rudders? If they do you could use either one to steer, and mount the leeboard on the same hull. A kayak rudder may be too small for sailing but you can attach a piece of plywood to it to make it larger.
     
  4. alan white
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    alan white Senior Member

    A twin cockpit kayak with two floats (aka a trimaran) would work better.
    More waterline, more speed, and not subject to losing feeboard when heeled.
    I've seen single tris but not a double. The board would go behind the back of the forward seat.
    How much board area relates to how much sail and how much vee the hull (s) have, and Dave Gerr's .book, The Nature of Boats, should help you to determine the size. The book is great for all kinds of calculations and rule of thumb.





    , .
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    The tri would be faster and simpler than the cat. You can also have 2 single-seater tris of course. If you go that route, the floats or amas can cause more drag at speed than is desirable. There are two solutions to that:

    1) sail with the amas out of the water, which requires you to balance the wind force on the sail and is not easy to do in a kayak

    2) design the amas to plane at speed, which requires a more sophisticated shape and construction

    I took a 3rd route: I used a Bruce foil which is a leeboard mounted on an outrigger at an angle so the sideways force of the wind produces lift from the foil, resulting in zero heeling force. It is not as efficient as a vertical board when beating but it worked great on a reach.
     
  6. alan white
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    alan white Senior Member

    AK--- did the foil also make drag as well as lift? Did it skew the boat (cause weather helm)?
     
  7. CrazyRU
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    CrazyRU Junior Member

    Kruger canoe


    Kruger canoe
    http://krugercanoes.com/images/krugerSeaWindsCarib.jpg

    has built in hardware to convert two boats into catamaran for rougth water paddling and sailing.
    They normally use bi-plan twin sail rig and leeboards however it is faster to paddle the boats upwind.
    [​IMG]
    There are some European companies sell kits to transfer two kayaks into sailing catamaran
    http://www.variocat.de/index.php?Variante_FaltbootCat
     
  8. Chris Ostlind

    Chris Ostlind Previous Member

    Attached Files:

  9. fabrice
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    fabrice Junior Member

    Thanks a lot for all your answers !
    food for thoughts, I guess.
    The variocat seems to be the nearest concept, and comes with a large board and rudder. I might stick to this.
    I'd like to stress that I'm not trying to sail my yaks, but actually use the available hulls. I really enjoy paddling a kayak.
    If only this comes to a happy ending, I'll post pics and give backup info.
    humm, eventually, it could end in buying a used hobie, but I should have learned a lot in the meantime I hope. :D
     
  10. fabrice
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    fabrice Junior Member

  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Yes: the Bruce foil was a bit draggy, but it was low aspect ratio (1:1) and I just used a flat piece of ply - not a proper hydrofoil profile, so efficiency was not to be expected. I knew even less about sailboat design then than I do now.

    I did not notice significant skewing. It would make some progess upwind but not as fast as paddling, but I only had 15 sq ft of sail area, and it was asymmetrical with both the foil and the mast offset to one side and a canted sail as well as the foil. Biggest problem was tacking because the kayak took too long to turn and often got into irons. Just an experiment but lots of fun.
     
  12. rwatson
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    rwatson Senior Member

    Try looking up James Wharrams designs. He does a cat that is designed as two individual canoe hulls.

    Its a plywood design you can get plans for
     
  13. fabrice
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    fabrice Junior Member

    rwatson, this link is great.

    Hitia makes sense to me, although hitia hulls are V shaped : coming more from polynesian canoe than kayak.

    Thanks !
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    I would imagine a Vee shaped hull makes sense in a dedicated catamaran to minimise the increase of wetted surface as one hull takes the entire weight when heeled with the other hull out of the water. Such a Vee shaped hull would be useless as a monohull, and a Hitia, although small, is far from the 2-kayak concept of the first post.
     

  15. fabrice
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    fabrice Junior Member

    humm, I read from other sources that the V shape is more wetted than U or elliptic.
    I'm new to these concepts, so I believe what I read (maybe misread, indeed :D )
    So far, It comes to my mind that the V shape is more likely to be accurate without any "whatever"board.
    Most trad/ethnic canoes I see sailing on the web roughly take the same V shape.

    So ... I guess you're right from a hull designer point of view.
    Hulls apart : same LOA (17), symetric, simple design.

    Maybe inflatable cats could be nearest in concept and usage. I just ran into "smartkat".
    --> centerboard, centered rudder.
    Inspiring.
     
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