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Thread: Scale Catboat question

  1. Default Scale Catboat question

    I grew up sailing in my dads eighteen foot traditional, though fiberglass, catboat. For anyone who doesn't know what a catboat is, it has a single gaff sail, with a very long boom thats even longer than the hull itself, and a very wide hull in proportion to length, with a retractable centerboard and inside ballast. catboats are very hard to steer going downwind in a strong breeze because of the long boom which puts the center of effort way out to one side. This is helped a little by pulling up the center board which moves the center of lateral plane aft to compensate.

    Sometimes in my quiet moments I think of making a working scale model of a cat-boat. I love their character and personality more than their sailing qualities, and a model would take me back to when I was a kid.The problem of increasing the stability in proportion to size, means I need a significantly deeper outside keel, though not as deep as other types because of the extra beam. The question is, how much will this keel cause the model to steer badly downwind, or "Weathercock"

    I know that the best way to figure this out is to make the the thing and give it a try, but I also enjoy thinking of of various design solutions on paper purely for the sake of discussion. I thought of a pivoting deep fin and bulb that could be allowed to pivot, about a virticle axis, in order to follow the direction of the flow of water when going downwind, and then fixed in the middle for going upwind. The question is how to make it strong enough, and how to make a servo keep it pointing exacly fore and aft when going up wind.
    I have Ideas on that too

    Nother Idea is to just make the rudder deeper or bigger. Problem with that is you need one hell of a huge rudder to move the center of effort any significant distance aft.

    Nother Idea is a deep skeg with a long skinny ballast extending foward from the bottom of the skeg, and a conventional retractable centerboard just like the real boat.

    Ideas anyone?

  2. #2


    you should talk to dick lemke. he know alot about cats and multi hulls. he is also a nice guy
    but that is just my opinion. dick does an article with the amya
    long live the cup

  3. #3
    lorsail Guest


    John, you might consider a centerboard with a thickened section toward the bottom so that it is streamlined when the board is down and streamlined when it is up; a bit more wetted surface than a bulb but should work- and the slot for the trunk would have to be wider aft but you could use a gasket..
    Garry Hoyt is building a catboat and I believe(not positive) he uses endplates on the top and bottom of the rudder to improve its power while still retaining the shallow draft advantage.

    Doug Lord
    --High Technology Sailing/Racing

  4. #4
    Join Date
    Nov 2003
    St. Louis, MO, United States


    Just for simplicity sake, I'd go with a skeg or full(er) length keel instead of a centerboard. Seems like you could eliminate a lot of tracking problems, even with the moment generated by the cat rig. I'm not sure, but lengthening the chord of the rudder might make it more resistant to turning while keeping that cat-boat feel to the profile.

    Good luck with it!


  5. Default

    I thought of the weighted centerboard Idea but it seems to me with the boom as low is it is on a traditional Catboat, you don't have much room for a daggerboard type weighted keel unless you make it much wider to get enough lateral plane, which means it could jam easily when its pulled up and down, and also end up possibly not deep enough. Thought of the pivoting weighted board idea but I couldn't think of a way to get enough weight low enough on it.

  6. #6
    lorsail Guest


    John, try playing around with thickening the lower 4" or so of a pivoting centerboard even to extending the front end(when its vertical) and maybe even the back end a little.(sort of a hatchet shape) I bet you could come up with ashape that could hold quite a few pounds and work vertically or horizontally.

    Doug Lord
    --High Technology Sailing/Racing

  7. Default

    good thought, but maybe the pivot point itself would also have to move fore and aft so that the the boat would float the same with the "hatchet" up or down. Hmmm....but then the foward end of the trunk would have to be much further foward and you'd run out of room. It seems th only way is to physically take the boat out of the water, remove the keel, and replace it with inside ballast every time I want to go downwind.

    Nother Idea:
    two "Hatchets" one foward one aft so it stays level at all times.
    Nother Idea:
    a very deep rudder with a lead weight on the end of it, and a small retractable centerboard up near the bow with a weight on the end of it.
    Nother Idea:
    fix the deep ballasted aft rudder permanently centered, and use a bow-rudder insted of the stern rudder. This would need a mechanism to allow the bow rudder to swing freely except when control inputs are given to it.
    Nother Idea:
    A deep fin keel atatched at about whare the skeg is that angles foward, in profile, to whare the lead wants to be down deep..........

  8. #8
    lorsail Guest


    John ,if you have enough room from the centerboard pivot to the transom to get the keel deep enough when pivoted down the ballast being way aft when retracted may not be a bad thing esp. in heavy air since every boat loses pitch stability when scaled down.
    Alternatively, your idea of rotating a fixed keel would work-I just like the weighted centerboard better .
    A guy in Europe invented the VARA rudder system the puts a foil in a cylinder that rotates within another cylinder. This could be adapted to make a strong yet easily rotatable keel. Actually the cylinder part only has to be at the top and bottom on the INSIDE and the outside cylinder could go up in the boat above the waterline and be supported athwartship. Probably a brass tube inside a brass tube would be friction free enough if you could get one large enough. Or a delrin inner cylinder fitted to a glass or carbon tube; I could help you with that if you wanted to go that route. I have the McMaster Carr catalog that has every tube known to man in it and I'm sure I could find a series of matches. You could order the catalog from them by calling :1-404-346-7000
    My suggestion is to do some heavy brainstorming on the weighted centerboard; you could even consider just enough ballast in the centerboard +20% to right the boat from a 90 degree knockdown and use a little sliding tray hidden inside to move weight side to side-that could be very efficient if you put the battery on the tray.

    Doug Lord
    --High Technology Sailing/Racing

  9. Default

    I definitely like your Idea of the 20% weighted board and the sliding tray on the inside. I think thats getting warmer. You're right that having weight aft is a good thing going downwind particularly in this case. I will continue to brainstorm

    Thanks again!

  10. Default

    here's an Idea:
    Link the movement of the sliding tray to the movement of the weighted centerboard, thereby eleiminating one control function for simplicity!
    Hey this is cool, maybe I should actually build the thing!

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