optimist rig on raft race outrigger canoe

Discussion in 'Boat Design' started by Tiny Turnip, Jul 5, 2014.

  1. Tiny Turnip
    Joined: Mar 2008
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    My boys are in the throes of building a clone of Crusher, featured on this thread below;

    http://www.boatdesign.net/forums/boat-design/camping-kid-build-raft-race-28311.html

    for this year's Gigha raft race. It worked pretty well for them 5 years ago, and they are 5 years bigger now, enough to manage the build entirely themselves. Its quick, cheap, dirty, and we're not expecting more than the vacation pottering around in sheltered waters, mild conditions.

    We have an optimist sail, rudder and centreboard which we can add to the sailing canoe, after the race. We have to make the mast and boom. We will use the centreboard as a lee board, being used to this on my Solway Dory sailing canoe, and it avoids building the centreboard case.

    Boys will butt joint two 8ft lengths of 4mm ply to make the sides and bottom, and make the joint with a 1ft square patch over the joint, with PUR adhesive and plenty of tiny screws.

    My questions are about positioning the mast and lee board.

    I plan two middle bulkheads, one immediately fore and aft of the 1ft patch, with the mast step and brace (mast unstayed for simplicity) spanning between these bulkheads.

    The construction will make it easy to position the leeboard exactly amidships, or there abouts, where the sides are already reinforced by the ply patches and the bulkheads. - does this sound reasonable? (the 5" diameter plastic pipe outriggers, 6ft long, as in the pictures, will run from the stern bulkhead to the midships bulkhead) How much might the outriggers drag the Centre of Lateral Resistance aftwards? (the plan is to have them set to skim the water when the boat is loaded and level)


    I guestimate the centre of effort of the Optimist sail to be roughly 20-24 inches behind the mast. Yes/No/other thoughts?

    Assuming that I've got the C of Lateral Resistance exactly amidships (no allowance made for the pipe outriggers) this set up would set the C of E some 18" aft of the C of Lateral Resistance, on the 16ft hull.
    I'd like modest weather helm - does this sound ok, or too much/ too little distance between CofE and C of L R? Effect of the outriggers? (I suppose they will pull the C of L R a little aft, but

    I'll probably have some more questions about the mast and boom in due course - I've got some great lengths of knot free birch rectangular section, planed to 35mmx48mm. Big enough or a little light? I could consider laminating two lengths to get a larger section, but

    1) I suspect the lam wouldn't be fantastic - I'd use PVA and do a rubbed joint, then clamp.

    2) its only going to see sheltered conditions and isn't really expected to last longer than the vacation.

    Bottom line is that it really doesn't matter if it doesn't work at all, but hey, if we've got a shout at a fun build that will sail a bit too, then its worth thinking about a little.

    Feel free to take the piss if I'm totally overthinking this!

    Many thanks all.

    Adrian
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Your estimate of the CE for the Opti sail is about right unless the sail is blown out and then the CE will be a little farther aft. Go for 24 inches for starters.

    It's hard to establish exactly where the CLR is going to be. Board position, rudder, rocker, draft, chine type, and all that make it fun to estimate. Not to forget that the position of the crew weight is going to affect the actual CLR.

    Set the mast is such a way as to have the CE and the CLR in about the same vertical plane. That'll give the boat some weather helm. That is counter intuitive until you consider that the sail is never over the center line of the boat when it is actually drawing. The trimmed sail is to leeward of the boat center line and that will, of course, set up a turning couple. It is common practice to set the CE, a little bit aft of the CLR when fine tuning the helm reaction. On a small boat, the crew can do what is irreverently called "butt tuning" by experimenting with fore and aft weight distribution of the crew.
     
  3. Tiny Turnip
    Joined: Mar 2008
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    Thanks, Messabout - that's very helpful. If I can set the outriggers to just skim, then I'm in with a shout of a fair guess at the CLR. I'll get out a napkin and do a few sums on the back...
     

  4. Tiny Turnip
    Joined: Mar 2008
    Posts: 643
    Likes: 103, Points: 43, Legacy Rep: 743
    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    Well the boys built 'The Melonator' and the crew won the raft race resoundingly, in very damp conditions. We did the Opti rig conversion; I must have got the CL and CE about right - it was reasonably mannered, went to windward fairly well; even tacked and gybed. However, plenty of outrigger buoyancy for four 10 - 13 year olds is not enough buoyancy in the outriggers for two well ballasted 50 year olds and a sail. Seemed to be an issue with insufficient beam in the accommodation too, somehow. Suffice to say that never have we experienced more fear in flat conditions at under 3 knots... With luck, and our support, Thanks for the photo, Paul. You've our full support, and we're all hoping you'll be back on the water after the therapy...
     

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