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Thread: Crab Claw Sails

  1. #1
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    Question

    I'm building a Dave Carnell $200 Sailboat, aka Bolger Featherwind. I'd like to figure out how to put a crab claw sail on it. Any leads on books or info on crab claw sails?

  2. #2
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    Try Craig O'Donnell's web site:

    http://www2.friend.ly.net/~dadadata/

    He has a lot of info about alternative rigs, and I think I remember seeing something about crab claw sails there.

  3. #3
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    There is a veriation of a lateen rig with curved yards and a claw sail in this book:

  4. #4
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    And Todd as a forumite would thank ye kindly for purchasing it too!!

    Crab claw cat... emultihulls here in Aussie have one for sale just for something interesting to look at
    http://www.emultihulls.com look in the <$100,000 section "40' Polynesian style cat" This is one seriously weird cat sorta like two proas joined together

    Take it easy
    Shane

  5. #5
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    There is an old book called "The Canoes of Oceanus" (check the library) that I used for research when writing my book that has a bunch of Polynesian rigs, including various crab-claw types.

    Most true claw-rigs use a sort of bipod system to support the sail, one half of which is a leaning "mast" and the other is essentially the yard attached to the sail's leading edge - except the lower, forward end of it is lashed to the deck. Confused yet? The yard/sail/boom section pivots on this lashing and the leaning mast keeps the whole thing from falling over. For this to work, the bipod needs a fairly wide base, which isn't a problem on an outrigger but might be on a dinghy. This is why I modified a basic lateen for the faux-crab-claw in my book. With curved spars and a very curvy, hollowed leech, it looks like a crab-claw, but it attaches to the boat and works like any other canoe lateen.

    There is another simpler, similar rig which uses a V-shaped sail bent to a slightly raked (and sometimes curved) mast with the boom sticking up in the air aft and the hollowed "leech" on top. It would be much easier to rig, though they have to have pretty long spars to get much sail area. I'd post a couple pictures, but I haven't done battle with Imagestation yet. If you can't find any photos, drawings or diagrams, let me know and I'll e-mail you a couple.

  6. #6
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    I found a couple pix of the simple type that are pretty good. Check out: http://www.wharram.com/Melanesia_images.shtml

    T.E.B.

  7. #7
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    Thanks for the wealth of info.

    AngWood: you're right. Craig's got a ton of good stuff on proas and crab claw sails.

    I found another good site, (http://www.schachtdesign.com/proafil...crab_claw.html),
    with a great digest of C.A.Marchaj's research that says the crab claw is superior to any other rig in a number of ways, including great power, no need for auxilliary headsails downwind, easy to trim, low center of effort, low cost, low stress rigging, no need for reefing (unusual brails depower the sail enough), and it works best with the sail cut flat!

    Most of the proas require shunting (reversing direction & flipping the mast aft to get on the other tack). But Craig shows some silouhetts (?) of traditional proas with crab claw sails, including a tacking "half-claw" from Tahiti. That one has an unraked vertical mast, with a boom that curves back, then up, creating a U-shaped sail with spars on each edge but the top. If this gives all the advantages of the crab-claw, plus tacking, it would be great. But it looks pretty darn tall to put in a monohull.

    Greg: I finally bought Canoe Rig. What a fabulous book! If you like little sailboats, it doesn't get much better than this.

    Shane: That is one far out boat! Independent thinking has no limits.

    Todd: Thanks for the leads. I went travelling after my first post, took the Schachtdesign printout with me, and spent a lot of time sketching. If you want to be able to put a crab-claw on a monohull, and to tack instead of shunt, the options seem limited. To get the crab claw's aerodynamic advantages ("vortex lift!?!?"), it sounds like you just need a sail that makes the wind cross a spar, the sail, and a second spar.

    First idea: If that's right, I wonder if a plain old Sunfish style boomed lateen is a crab claw sail, either rigged normally, or at least when you tilt it forward so the boom is angled. Your "Bird of Paradise" almost falls in this category. If you squeeze the yard and boom together for a longer & skinnier triangular sail, then it starts looking a lot like those proa silouhettes. The spars are now long and the rig sure looks tippy, but maybe it isn't really, due to the mysterious "low center of effort" of true crab claw sails?

    Then, if you rake the mast, you should get some upward lift (like a windsurfer) as soon as you ease the sheets, and great reaching and running performance.

    Second idea: Take a regular leg-o'-mutton, rake the mast way back & tilt the boom up, and you've got a long triangle crab claw silouhette, with just two (long) spars. This sounds like the simple version v shaped rig you mention, but the Warram/Melanesia site pics are a little hard to make out.

    Third idea: Modify the u-shaped Tahitian half-claw enough so you can unship it easily. Since it doesn't get wider at the top, maybe it's more stable than the others.

    For a monohull, it seems like the big question is whether there really is something special about crab claw sail aerodynamics that places the true center of effort below where it would be on a normal sail the same shape.

    If yes, then the crab claw should be a great monohull sail: put most of your sail area up high, get the power of the crab claw vortex lift effect, enjoy stability due to low CE, etc. Any ideas on the center of effort issue? If I find out, I'll post it.

    -Brett

    [ 03-07-2002, 01:03 AM: Message edited by: Brett Herr ]

  8. #8
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    As far as I know, the C.E. is the C.E. and there is no way around it. In the case of the claw-rig, the studies seem to show that it gets a lot of bang out of a rather low-aspect buck, but I don't know of any reason why it would behave differently in terms of helm balance or heeling lever than any other rig of similar proportions, and those two things are what the C.E. is all about. Of the three or four claw-rig variations that I have seen (1.-single mast leaning inward and forward, 2.-double, shear-legs-style bi-pod mast, 3.-straight but raked mast and 4.-curved, raking mast) the most interesting are those where the tack corner can be moved (#1 and #2) changing the aspect ratio and C.E. location (relative to the hull) as it does. These both require a fairly broad base in order to get the lower part of the mast outboard far enough from the sail that it doesn't interfere with the yard. This isn't a problem on a multihull, but could be on a monohull. In such a case, the boomed lateen, like a Sunfish has, may be about as close as we can get. Cranking up the tail end of the boom might help eliminate the major weakness of that type of sail which is the inability to ease the sail when heeling and being overpowered - conditions where the clew corner of the sail is actually skipping along on the water's surface, preventing you from easing any more. All you can do is hike hard out the windward side and bring the boat back up flat and then ease. With a claw-style, angled boom, you could probably go over as much as 45 degrees or more without the sail hitting the water.

    The leg-o-muttin variant with the aft end of the boom (straight or curved) cranked up, and a straight, raking mast is currently in use and no doubt, has been for quite a while. A friend sent me photos last year that he took near Panama of two natives in a dugout powered by such a sail. Some of the Polynesian boats also used it. I've even got a smaller, solo version of the Bird of Paradise drawn-up with two of them in a ketch rig. If I ever get time to build another hull, I'll try it. Im not sure that the entire world will want to jump on the claw-rig bandwagon, but it's certainly worth playing with.

    [ 03-06-2002, 07:20 PM: Message edited by: Todd Bradshaw ]

  9. #9
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    On the CE/heeling lever, I read somewhere that the crab claw has upward lift (they used another term) like a jib or a windsurfer sail, because the business side of the sail faces slightly downward. Their idea was that pulls up on the boat, counteracting the heeling lever that's trying to tip you over. Makes sense to me.

    Those bipod masts seem very straighforward, but look industrial to me. MAIB had one on the cover, on a Crab Claw Cat just released by Shell Boats, a few months ago. Maybe if they wishboned the bipod it would look better.

    Thanks for the tip on the lateen sail corner in the water. I never noticed that when I sailed in a Sunfish. Probably because we were teenagers and all we ever did was count how many times we could capsize the boat before we had to go home. We didn't get any practice recovering from gusts.

  10. #10
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    If you buy that "upward lift" stuff I have a bridge to sell you.

  11. #11
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    Dadgum, it's me -chuckle-.

    ===
    There is an old book called "The Canoes of Oceanus" (check the library) that I used for research when writing my book that has a bunch of Polynesian rigs, including various crab-claw types.
    ===

    Canoes of Oceania, by haddon and hornell, very difficult to find but you may be lucky via interlibrary loan.

    published by the Bishop Museum, Honolulu, they may still be able to sell you a copy as it has been reprinted a few times.
    : Sinepuxent Ancestors & Boats
    :: The Proa FAQ
    :::: Canoe Sailing Resources
    :::::: Ebooks Online
    :::::::: &c., &c. www.thecheappages.com

  12. #12
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    Thanks, Craig. I must have sailcloth on the brain - that or I'm getting a bit foggy due to old age or toxic fume buildup.
    T.E.B.

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