11' sail dinghy.

Discussion in 'Sailboats' started by Deadeye, Jul 12, 2013.

  1. Deadeye
    Joined: Jan 2009
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    Deadeye Bender of Nails

    One of the projects I've been working on for a while (off and on) is to add a windsurf rig to my 11' rowboat. I have the hull, I have the rig, what could go wrong ? :D:D
    I've gotten to the point where I need to get some design input that's beyond my own experience.
    First, some pics of what I'm working with:








    From what I've read, it seems like the rule of thumb is to locate the CLR of the daggerboard about 10% of the waterline length aft of the CE of the sail. Is that what I should be using here or am I overlooking something ? (which is entirely possible, or I could be completely off the mark on that). Since this will essentially be a cat rig, which I've never sailed, I know I'm in for some trial and error. I'm looking forward to tweaking it, I just need to get in the ballpark first :cool:

    It would be most convenient to build in the daggerboard trunk with its aft edge being the forward face of the center (rowing) seat. The center seat needs to stay where it is because the new bronze oarlocks are going into the same place as the old plastic ones.

    The rudder in the pic is sized arbitrarily and deliberately left long - I'll cut it to length when the time comes. I haven't glassed it yet in case I have to make any changes to the shape. What kind of behaviour can I expect if I use that transom angle as the rudder hinge line, or do I need to get the rudder vertical ?

    Being a windsurf rig, the sail goes right down to the mast step. That puts the sail's tack low inside the hull and, with the luff being a sleeve, cuts down my options to secure the mast inside the boat. To deal with that, I'm thinking about a partner in the center of the bow seat with a removable tapered 'pole' over which the ~2" hollow, tapered, carbon mast would fit. I can tie off the cringle to keep the the whole rig from riding up as a unit.

    The first thing I need to deal with is the rotten transom:

    I figure I'll cut the transom skin off and clean up all the rotten ply. It will get 3/8" DF marine ply for the new core, then I'm thinking about mixing up some bear$h!t and bonding the old skin back on vs laying up a new one. I'd lay up new glass if it was a bigger boat or if I was going to hang a motor off it but, with the 3/8" ply replacing the original 1/4", I see the glass being more of a cosmetic/sealant than structural.
    At the same time, I'll make up removable seat boards for each side running between the transom and center (rowing) seats for sailing.

    I'm also getting rid of the cheesy plastic gunwhale mouldings and I'll be laminating new ones from red cedar and epoxy (will probably be finished bright). I've got the full length strips cut and ready to go, I just need more clamps.
    WAY more clamps :D
    After I get the transom replaced, I'll be making up a bunch more of these el cheapo laminating clamps from PVC pipe scraps:

    Thoughts, opinions, suggestions, corrections ?
  2. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    Before you do any major surgery to the hull why not try a leeboard first, and maybe steer with a sculling oar?

    At least that way you'll be able to experiment with the CLR position and see if the windsurfer rig idea works

    Richard Woods of Woods Designs

  3. Deadeye
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    Deadeye Bender of Nails

    Hey Richard,
    The leeboard is a great idea, though I've never actually used one.
    I suspect, however, that I would need to get over that middle seat to move it across when I tack. OTOH I'm sure that, like so much of sailing, that would become habit with practice.
    I guess the idea there is to make a board with a symmetrical outline but a foil in profile so that I can flip it around when I change sides to keep the cambered face to windward?

    I've read a little about sculling a sailboat (admittedly it's mostly anecdotal from reading the Pardeys and Slocum). I have the oars anyway so it's definitely worth trying, even if I have to make up a longer one to do the job.

    Thanks for your thoughts. I can build darn near anything but that doesn't compare to the experience of 'been there, done that'.
  4. gggGuest
    Joined: Feb 2005
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    Location: UK

    gggGuest ...

    The sailboard rig on a dinghy bit is often tried and never works well. However if you don't need it to work well (ie all you really want to do is to lazily waft down the river and are happy to row back if the wind is in the wrong direction) then that's not really a problem is it:)
    The challenge for a board rig with a full size sail is that the mast isn't really going to be strong enough to be an unstayed mast, because its designed for all the pull to come where the boom goes some feet further up. You probably want to arrange for stays there which can be readily attached when you've got the sail on.
  5. Deadeye
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    Deadeye Bender of Nails

    It's funny you mention that - when I was mocking it up I pictured basically a gallows that that would come up from the sheer on each side and clamp the bare mast at the opening in the sail's sleeve where the boom would normally ride. That would be in lieu of of mast partner in the top of the bow seat and would increase the distance between the two points ('gallows' and step).
    Unlike a stayed mast, the loading would be transmitted to step as in an unstayed mast...rather than shrouds and stays carrying those loads. That's what led to the idea of an upright 'pole' or spud that the mast slides down onto. It ought to be simple enough to have it come up to the height where the boom normally sits.

    If you hadn't mentioned that, I might have had a "D'Oh!" moment, so thanks !

    And yeah, I'm not planning on winning any races with it - I look at this as more of a test bed that I can play around with until it's dialed in. It's more about what I'll learn from doing it than any real need for it to be a success...which is also why I want to build what I need rather than just buying a solution.

    Do you recall offhand what complications show themselves ?
    It seems to me that it's all about getting the geometry right, which is probably true of any rig, and is exactly what I'm trying to address by starting this thread.
    Thanks again for your thoughts on this.
  6. Skyak
    Joined: Jul 2012
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    Skyak Senior Member

    "Do you recall offhand what complications show themselves ?
    It seems to me that it's all about getting the geometry right, which is probably true of any rig, and is exactly what I'm trying to address by starting this thread.
    Thanks again for your thoughts on this."

    The big two are that windsurfers apply righting forces to the boom -the mast lacks the strength for the cantilevered mount. The second is that windsurf sails are designed to be heavily tensioned by the wishbone boom bending the flexible mast. Without this tension the shape stinks.

    These problems can be solved but most don't because they are just trying for a quick solution
  7. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Thats a sweet looking little dink Deadeye. Everyone seems to want to make a sailboat of a rowing boat and visa versa. That includes me and most of the rest of us. We just can't seem to leave our boats alone. That's 'cause we love them.

    The lead between CLR and CE is an elusive thing. Consider which direction the lead ought to be calculated. Ahead of CLR or aft of it. Somewhere between 5 and 10 percent of DWL works out OK most of the time. Notice that a lot of designers lay one on top of the other and then tune with mast rake.

    Make some rough scale sketches and locate the two positions. Give the two positions some vector arrows. Which way does the boat want to turn? Now ease the sail to broad reach or dead run position. The CE moved forward big time. Observe the vectors. What happens now? Do not let the boat have much, or any, lee helm at any point of sail. Lee helm is dangerous in a blow and counter productive in any air. Lee helm makes it hard to "feel" what the boat and sails want. Notice that when boomed out there is a substantial turning lever effect.

    Most of the windsurfer sails are cut so that the luff and its sleeve is very curvy. If you put that sail on a straight mast the sail will be so full as to resemble a blimp. Weather perfomance, not. The sail and masts are, or ought to be, matched. The sailboard mast is deliberately bendy to match the luff curve of the sail. On a sailboard, the mast is supported part way up the luff, as noted above. That, of course,.influences the bend curve both fore and aft and sideways too.

    The location of the booms end, though not critical on a sailboard, is pretty important on a conventionally sheeted layout. If the sheet runs through an ordinary traveler or bridle, it will be well below the boom end. Now there is a substantial down force on the boom end. That will overtighten the leech and bend the mast more which will take the draft out of the sail. You can build a tripod to elevate the sheet lead. That's a clumsy fix but it works. Ask me how I know this. The same problem exists on a sprit boomed rig. Actually a windsurfer rig is sprit boomed.

    The cute little dink can forgive your misplacement of CLR/CE to some extent. Moving the skipper weight forward or aft will change the trim of the boat and consequently change the location of effective CLR. Not good enough for we perfectionists. We need to let the dink run on its happier lines. So we gotta' get the layout right.

    Build a bridge that will clamp to the sheer. Leave it adjustable. Also install a mast partner with several alternative heel locations. You can fiddle with positions now. You can ask me how I know this one too. Build a stiffer mast, recut the luff of the sail to have less curve. Build a tripod for sheet leveling, and do not under estimate the strain on the transom that is imposed by the too big rudder that you probably should build.

    Better than all that, find a small rig with a boom located at the foot and have a mast that matches the sail luff curve....... or visa versa. In any case the 11 footer is not going to be a fast boat so maybe a bunch of less than ideal factors will not be too objectionable.

    There. That is the dissertation relating to the mistakes I have made in the past. I'll almost surely make some more mistakes in the near future ....if I live long enough.:D
  8. Deadeye
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    Deadeye Bender of Nails

    Thx for reading and for helping out, guys: you've given me plenty of food for thought.

    At the moment, I think I'm more in the latter group than the former until I get it into the water and can start tinkering with it.
    I think this is the first summer in 20-odd years that I'm completely boat-less: not a happy place to be !!
    I'm not too concerned about the difference from 3 1/2 to 5 kn at the moment: right now I'm looking to get from zero to 3 1/2 kn.
    What you've said about the forces on the mast is my primary concern about using a 2-foot(ish) vertical spud that the mast slides onto. Right at the top of that spud is where the mast's ending forces would be concentrated (I'm thinking about the failure mode of hollow masts with internal bulkheads)

    With what you've said, messabout, fabbing up a stiffer unstayed mast might be the way to go. Besides, I've always wanted to try a tapered bird's-mouth mast :)
    But that's a project for another day. I was going to try to avoid using the existing boom, but what you've said now has me thinking more about trying to include it in 'the plan'.
    One of my criteria for this project is that it can't cost much: I've got a few irons in the fire already.

    I got the rotten transom ground away today:
    They used 1/4", 3-ply ext for the core and it was soaked. The skeg has about an inch of water in the bottom and the ply was soaked. It looks like the culprit is the four bolts where the transom pads bolt through - no surprise there. Next I'll use the skin I cut out as a pattern to cut the new core. I'm also going to gel the inside of that compartment and put in a drain.
    Then I'll go ahead and lay up new glass. Despite my need to keep costs down, re-using the existing transom skin (per my earlier thoughts) seems like a false economy so I'll lay up a new one. I'm thinking of two units of 10oz roving and 3/4oz mat.
  9. CT 249
    Joined: Dec 2004
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    CT 249 Senior Member

    The main issue with windsurfer rigs on dinghies is that most people use modern windsurfer rigs, which are very low drag but also have quite low lift. For most windsurfers, where the idea is to plane around on a very low-drag shortboard in strong winds, that's a good combination. Dinghies are quite different.

    That sail is a Windsurfer One Design sail which is designed for the '80s renovation of the original 12' 21kg Windsurfer. It's deeper in draft and has a tighter leach than a modern windsurfer sail. That means that it is actually more powerful for its area than a modern sail (and has more drag) and that makes it better suited to a larger longboard or to a dinghy.

    That sail appears to be in very good condition for something 20+ years old...if it was around here in Oz I'd be running at you with a couple of hundred bucks as we still race those boards down here and in Italy and my club-racing One Design sail is pretty dead! They tend to delaminate around the tack first, so make sure you tension the downhaul after tightening the downhaul when rigging, and release the downhaul before releasing the outhaul when de-rigging.

  10. Deadeye
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    Deadeye Bender of Nails

    Thx CT, it's good to know it's a decent bit of kit. 12' and 21kg sounds like the board that came with it. It's in pretty rough shape, but I'm going to take it down to beach and play around with rigging it to see exactly how to make it work in its new home.

    The picture doesn't lie: the sail does indeed look brand new - it even had what looked like scraps of tissue stuck to the vinyl in a couple places. I assumed that it was to protect the vinyl from sticking to itself during manufacture and shipping. The first time it gets wet it would disintegrate - which, combined with the lack of wear anywhere, makes me think that it's never been used and been stored out of the sun. I do have another (well-used original, I expect) sail for it as well: same size, lighter weight, and no battens. The construction is interesting: it looks like the same kind of clear vinyl used for the windows was also laminated to the Dacron on one entire side of the sail (the back side on the pic I posted).

    Thx as well for the rigging tip - I've been reading about how to rig one of these but that sounds like a tip that comes from experience...which I don't have.

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