Adding dagger boards to polyethylene amas?

Discussion in 'Multihulls' started by llamalookout, Jun 17, 2007.

  1. llamalookout
    Joined: Feb 2006
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    Location: NZ

    llamalookout Junior Member

    I really want to add dagger boards to my polyethylene windrider17 tri, by using a couple of dagger boards from a 16 to 18 foot beach cat.

    I've thought of all sorts of weird and wonderful ways of adding a board, but thanks to the info on this forum I have decided to stick to the normal approach of having trunks through the amas. This will keep the board out of the jib & cockpit and its easy to change the amount of board deployed to suit conditions.

    Since nothing sticks to polyethylene I'm guessing the best way would be to make the trunks out of polyethylene and then have them professionally plastic welded in place. But I'm not sure if its practical to create extra parts out of this material or if the plastic welding is good enough.

    Alternatively the trunks could be made from composite, but then there would be issues of how to bond them to the hulls.

    Any ideas appreciated!
  2. Chris Ostlind

    Chris Ostlind Previous Member


    In three words: don't do it

    What do you suppose will be gained with this mod to this boat?

    The ama to aka joining system is not engineered to accept that kind of lateral loading when the boat is powered-up and cooking along in swells and chop.

    Poly plastic is famous for what is known as "creep". This means that the plastic will deform as it is loaded and any fixturing, sealants and otherwise, will eventually fail due to enlargement of the mounting/attaching holes.

    First signs will be serious leakage of water into the amas and then total failure with the ama/aka mount fixtures ripping right out of the plastic as the amas fill, get heavy and place unbelievable loads on the whole system.

    Here's a diary of an F Series trimaran that was modified to have ama mounted angled foils. Look over the photos, enjoy the written material and let your common sense be your guide.

    This is a collection of the gorey photos from the aftermath of sticking foils into a pair of amas without the benefit of re-engineering the ama/aka connections.

    I will say that later, after the boat was repaired and proper connective tissue was installed at the critical joints, she sailed well and fast with her new foils.

    Chris Ostlind

    Attached Files:

  3. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    Don't do it two

    I haven't owned a Windrider but did have one for a couple of weeks to test and went camping in another one. The hull material is very subject to creep as I found when I left the main hull up against a pole - it warped!

    I think the Windrider has been very well designed by Jim Brown as a low loading and safe boat. The lack of powerful vang and short rudder act as safety valves and adding big foils may make the boat harder to handle.

    One of the aspects of the Windrider that I find amazing is the use of a low aspect fin and lowly loaded rudder to make the boat foolproof to tack. Tacking has long been a bane of multihulls - just watch an overloaded Hibie in light winds. The Windrider has a low aspect fin that stalls until it has way on. The rudder is lowly loaded and the boat always falls off the breeze until the fin develops lift - when you have gotten going again. This feature is really clever and makes the boat so easy to use. I would urge caution about fiddling with this. Bigger and sexier is not always better. If you want more performance a Hobie 14 of similar may be more suitable. Keep your Windrider for camping and the like.


    Phil Thompson

  4. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    I think that's the worst thing you can do. Nothing sticks to poly very well, so they will be impossible to seal properly. Don't breach the hull integrity.

    I would go with leeboards mounted to longitudinal beams which are in turn mounted to the cross-beams. This will avoid breaching the amas and allow you to vary the longitudinal position so as to correctly balance the boat. You can also vary the size of the boards. The leeboard should lie alongside the hull when down so the hull can help brace it against the side load.

    Tacking the boards is no problem - just shape the boards with a horn projecting above the pivot and angled a bit back, lead a shock cord forward, and a pendant aft through a turning block to a cleat convenient to the cockpit. Pull the windward board down just before the tack, and release the new windward pendant when the tack is completed. You get twice the board area during the tack, and the shock cord neatly flips the windward board up and holds it there (thanks to the angle of the horn). I used this method to add leeboards to a canoe and it worked a treat.

    Lastly, you can shuck the whole kit if it doesn't work out for some reason, and you haven't ruined your boat in the process.
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