Mastless Crab Claw ?

Discussion in 'Boat Design' started by Canada Bob, Mar 10, 2011.

  1. Canada Bob
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    Canada Bob Junior Member

    As a Novice sailor and small boat builder I'm looking at a sail plan for a 16ft catamaran {a home built project boat}.

    I'm intrigued by the crab claw / proa sails but I have in mind a modification that could eliminate the need for the short mast, but not being familiar with the stress and strains involved I thought I'd float the idea in here before I start sawing and cutting :)

    The basic idea is to eliminate the need for a mast by having the top spar locked {but removable} into a sleeve on one {or the other} of the bows and coming up at a {say} 60 degree angle. The free / top end of this spar/mast would be secured to the stern by an adjustable tension line, sort of like an inverted V. The tension lines would adjust the angle of the spar/mast.

    This would leave the deck area clear of rigging and the boom very easy to control. This wouldn't be a rig for a racing boat, just a day/fishing/fun boat, so there shouldn't be any heavy load forces involved.

    As for the spar/mast itself, I have access to a fair number of pole vaulters poles, {used or seconds} with varying degrees of flexibility and a good degree strength.

    The thing is though, I have seen all kinds of rigs around, but nothing as simple as this could be, am I missing something here ?

    Canada Bob.
     
  2. yipster
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    yipster designer

  3. Canada Bob
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    Canada Bob Junior Member

    Hello Yipster.

    Thanks for the link, I'll check it out...

    As to the details of the sail itself, from what I've heard a crab claw / proa type sails are quite forgiving in shape and deployment, can't say that I know that for sure though, it's all new to me.

    Canada Bob.
     
  4. messabout
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    messabout Senior Member

    Bob; crab claws have some usefulness but you will be better advised to plan for something a little more conventional. To keep things simple you could use a sprit rig or lug rig. If you use the crab claw there will be a learning curve while you sort it out. It will not likely be a weatherly rig either. If you are a very experienced sailor and are willing to endure some developement pains then the crab claw will be OK as a novel rig. Otherwise just stick with a rig that is more likely to provide good results.
     
  5. Canada Bob
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    Canada Bob Junior Member

    Hello Messabout, thanks for your comments, appreciated.

    The platform that I work with is a home built 16ft catamaran, made from sheets of closed cell foam sandwiched between 1/4" marine plywood. It aint the prettiest boat afloat but it's one that I can experiment with under power or sail.

    I'll have a look at the rigs that you mention, and take in what you say above, but the cost of a crab claw, and the ease of construction may somewhat compensate for the downsides in performance. A crab claw is one of those things that I've had in mind for a few years, and if I don't run one up it will follow me around for years :(

    Add to that, it won't see anything other than sheltered waters, lots of lakes to go at in Nova Scotia if it works on them I might just dip it in the sheltered coastal bays.

    Other than the prospects of limited performance do you see any flaws in my pole vaulter mast/spar and method of rigging ?

    Thanks for your feedback, valued and appreciated.

    Canada Bob.
     
  6. rwatson
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    rwatson Senior Member

    Well - if something works, its more popular, but it doesnt sound like it would cost you much if the experiment failed, so just go for it, and why bother with the commonly accepted designs right now.

    The islanders use the crab claws because they were, or are limited in available material - and the concensus is, the performance is a lot less than bermudan or traditional rigs that 'most' boats use.

    Nothing to stop you playing around though - thas the fun of boats.
     
  7. yipster
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    yipster designer

    [​IMG][​IMG]
    how about an old cheap triangle deltawing hang glider, than again, 16ft hulls may be to much to pull eh
    anyway, i would check that scaling story at sail performance, talk to others etc
     
  8. PAR
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    PAR Yacht Designer/Builder

    I'm not sure who did the tests about 20 years ago, but the crab claw faired surprisingly and exceptionally well, besting the Bermudian in windward ability.

    This said,considering your novice sailing abilities, any reasonable evaluation of this rig should wait, until you have a better grasp of the fundamentals and a skill set to recognize good and bad habits. Other wise you'll not know if the rig is good or bad at anything, let alone how to adjust it, modify it or correct issues it might display.
     
  9. Canada Bob
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    Canada Bob Junior Member

    Thanks for the feedback, all the comments are appreciated.

    I guess I should clarify the novice sailor bit, I have my "ticket" for small craft in Canada, and I've sailed on sheltered waters {lakes and coastal bays} for some of the last 20 years, but...

    The total hours per year don't usually exceed 20-30 hours, and there's years when I don't get on the water at all. Much of the time that I'm on a craft of one sort or another I'm fishing rather than sailing, and the places that I sail don't really test anyone's skills, the furthest I've been out in open waters would probably be 3 miles off shore, {and that's on clear/calm days}, so I can't compared myself to some of the black & blue :) water sailors in here.

    I suppose I could say I've "20 years experience" {some folks would claim that}, but it's more honest to say I have 1 years experience 15-20 times over, enough to keep myself out of trouble but that alone don't make me "an experienced sailor".

    No sure if I'm doing a good job of explaining my abilities, but knowing that even when I'm in or on a boat that 80% of the time I'm holding a rod not the helm, might better indicate my "skills" or lack of them. Having said that, I've only turned 1 boat over so far, {a West Wight Potter}, and that was 15 years ago.

    So {respecting the ocean, & the squalls I've seen on Grand Lake} I still consider myself a novice, I stay well away from challenging situations. I appreciate what PAR says, more often than not a proper sailor would have a lot more sail up than I deploy, but I manage to find my way back to the shore that I set out from, rather than Nineveh :)

    All in all a boat to me is a platform to fish from, but I do take notice of the boats I see around me, I'm curious about their design and I've spent more time making mods to small craft than sailing them, hence the appetite for a clear deck crab claw.

    So, at the end of the day, I don't race {or even enjoy going fast} in any boat I've had, it's the peace and tranquility that I like when I'm on the water, the only challenge that gets my attention is to try something that is a little bit different.

    If a pole vaulters stick might lend itself to useful purpose for a modified crab claw then that's my challenge for this year. I just wondered if the basic idea of using an unstayed {sort of} pole as a mast/spar might have some inherent fault ?

    As far as being able to evaluate the rig, well as you say Par, that's beyond me, all I would know would be if the design was functional as a sail, or simply an umbrella :)

    Does anyone see a flaw in the idea ? if not then that's my little project for this year.

    Thanks again for the comments and words of caution, fully appreciated, if you get things wrong on a boat it may well be the last thing you get wrong :(

    Canada Bob.
     
  10. messabout
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    messabout Senior Member

    Bob; you have revealed that you are not merely a rank beginner and you do like to experiment. In that case you will have fun doing the crab claw project.

    Several years ago, Wooden Boat magazine did a lengthy piece about various sail plans on subsistence fishing or transport boats in third world countries. The study that was written up was sponsored (financed) by an international agricultural commission of some sort. Every concievable sail plan that was appropriate for the sort of boat under consideration was tested and evaluated. The crab claw plan did exceptionally well in certain instances. In fact it bested the marconi in several situations. I suspect that Par was referring,above, to this article.

    The vault poles are intentionally limber and that might cause some problems with sail shape. There are advantages to limber poles as well as disadvantages. The poles will bend to leeward in gusts and thereby dump some of the sudden wind force.

    A crab claw rig is sort of like a lateen rig except that it puts its larger area higher than the lateen. If you build the claw you can also fiddle with the lateen configuration without changing much of the gear. In either case it can be done on the cheap and that is recommendation enough. Go for it.
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    This is something I have been looking at, more from the perspective of a Lateen than a Crab Claw. I want to try it on a sailing kayak; I can easily paddle upwind if necessary so windward performance is not a life-threatening consideration for me. Apart from the factors noted above, here are some more to consider -

    The boom attachment point has to be well forward, like a catboat, for adequate sail area and balance. For the kayak case I found it necessary to attach it to a bowsprit because of the forward cockpit location - which introduced structural problems. If it is impractical to have standing rigging the boom must be stand-alone, and it must be longer than a vertical mast, therefore it will need to be stronger and heavier.

    If the boom is pivotted so it can move off-center to trim the sail its weight may be a problem, although less so for a catamaran. The forces where it steps to the deck will be considerable, and more complex than for a mast.

    For the loose-footed lateen sail the boom can be stationary but that may not work for the Crab Claw as the weight of the lower boom - which will want to return to center - may cause handling problems in light airs. Interestingly, the lateen sail becomes a lifting sail when used in this manner which is helpful to me because of the limited stability of a kayak, but it doesn’t do much for a catamaran.

    What I have said above doesn’t prove it cannot be done successfully of course, but these things need to be taken into account in your design. So keep us informed, this is interesting.
     
  12. yipster
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    yipster designer

    was going to mention shunting and toptacking but this is a good idea imho
    yet on the foot(s) there is still some figgering out as well me thinks

    http://en.wikipedia.org/wiki/Vortex_lift
    http://www.proafile.com/view/magazine/article/rig_options_crab_claw/
    http://lukawskiyachts.com/mona/crab_claw_sail.htm
    http://adg.stanford.edu/aa241/highlift/sstclmax.html
     
  13. Canada Bob
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    Canada Bob Junior Member

    Thanks to everyone for the insights that you outline above, valued and appreciated, I'll be taking them into account.

    The objectives of this little project of mine were based on clearing the deck of as much rigging as possible, and being able to deploy a simple / stable sail plan.

    I guess I've had that in mind for a few years, so when the pole vaulters poles came my way a little light went on as to how I might be able to use them.

    The concept is, to have the first 1ft or so of a pole slotted into a sleeve at the bow of a boat {or a Cat, in my case}. The sleeve would be inclined {not vertical like a Nonsuch}, probably leaning the pole/mast back at around 60 degrees.

    From there I would have a back stay line going up like an inverted V to secure the mast, these back stay lines could control the angle/shape of the mast.

    Messabout mentioned that the poles would be flexible enough to dump out a sudden gust, that was my initial thought too, but once you have the mast under tension from the back stays there would be little flexing, if any...

    Not sure if I'd need a second pole as a bottom spar, or if all I'd need would be a sheet line ? I would use this to trim the sail and to dump any gust that came along.

    I guess {but what do I know} that this simple inclined mast configuration could be applied to a mono hull, but in my case I'm looking at how best to apply it to a Cat, the ideas include...

    You could have the inclined sleeves in 6 different locations, both port and starboard bows, so on a Cat that's 4 points already, and then central positions between the bow and stern.

    The idea of the sleeve {on a Cat} is that as Yipster mentions, you would be able to shunt, that's if this concept works at all :)

    Add to that, having 3 forward points at any given time might give provide more stability or more points of sail ?

    To shunt the craft all I'd need to do is slacken off the back stays, then pull the mast from the front sleeve, walk it to the back, then do the same thing with the stays, put the tension back in, pull on the sheet and away we go, hopefully not just out with the tide :)

    Seems simple enough, and it may just work, as a fishing platform that is, I wouldn't be expecting to go anywhere fast with this rig, but to me sailing isn't about fast, it's about being out there...

    Am I missing out on something in my thinking above ?

    By the way, thought I'd copy you guys in on when I turned Lionheart upside down, it'll give you a laugh...

    http://potter-yachters.org/stories/lionheart.html

    Canada Bob.

    PS... Just having a pint with a buddy of mine here at 53-50' 53" N by 03-3' 12" W and now further enlightened and encouraged by his comment that...

    If the worst came to the worst sailing my rig, I'd at least be able to grab the mast, cut the back stays and be flung 3 furlongs to the shore !!!
    I'll have what he's drinking :)
     
  14. messabout
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    messabout Senior Member

    Bob; The tale of lionheart is well and humorously done. There are times when Poseidon and Davy Jones conspire to test your mettle. We are glad that you did not succumb to their evil prank. I once encountered the wake from a big ole tanker or bulker in Tampa bay. I did not capsize but did need a change of underwear in the aftermath.

    Your drinking buddy has a great sense of humor too. Vaulting ashore is it?
     

  15. Canada Bob
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    Canada Bob Junior Member

    Hello Messabout...

    Lionheart was a good little boat, had many an adventure on Lionheart, mind you that was the scariest encounter I've had on a boat, and I hope to keep it that way.

    As for vaulting ashore, that could be an unforeseen and added benefit, hurled across the water to dry land, yea, I'll take that over having to swim for it ;)

    Have to say I've had a bit of a charmed life, I once had a contract taken out on me, but I managed to dodge that bullet. Never thought I'd meet up with Casper Weinberger, {Secretary of State for Defense, at the time}, or Neil Armstrong, Ted Kennedy, Carl Levin et-al. I'll send you a PM link to that saga, it was a HELL of a time...

    Canada Bob.
     
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