A Photo & Discussion Forum for Wharram Design Enthusiasts
How do you design a Crab Claw rig for your boat?
I have seen tons of photos and videos of various types of rigs on different types of boats, but unlike the junk rig Bible - Practical Junk Rig - I have yet to find anything that exists for the oldest known sailing rig, the Crab Claw. In fact, I have yet to find any "How to" articles for designing your own crab claw rig.
Anyone have any resources or ideas?
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Replies to This Discussion
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Permalink Reply by Alex on
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Being low tech, I guess no ones thought to......
Flat cut triangle. Size to boat. Position to c of e.
Some bend required in the spars. Strong enough not to break.
Tacking crabclaw like Hans Klaars or traditional - like on proa's and Wharrams?
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Permalink Reply by Budget Boater on
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That's not really helpful.
How do you determine the CoE of a crab claw sail?
How do you determine the CoE of a crab claw ketch rig without a jib (i.e.: Aluna)?
How do you determine the proper lead of the rig over CLP?
Where is the CoE and CLP on a Narai (or any other Wharram design that does not show this information in the plans)?
How does raking the sails/masts affect balance?
"Some bend." Is that a technical term? Is it measurable?
"Strong enough not to break." Is that a technical term? Is it measurable? Under what conditions?
Since I have about as much access to Bamboo, as the San desert tribe have to North Face polar clothing, spars might be a serious issue, especially when I call up a supplier of some unknown replacement material to let them know that what I need should have some bend, but also should be strong enough not to break...
Alex said:Being low tech, I guess no ones thought to......
Flat cut triangle. Size to boat. Position to c of e.
Some bend required in the spars. Strong enough not to break.
Tacking crabclaw like Hans Klaars or traditional - like on proa's and Wharrams?
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Wharram doesn't include that in plans?
Not that hard to work out though......
Even easier on a triangle for the crabclaw.
Work it out on the narai - there are many internet resources for this.
Wingsail jwd or the original sail plan?
Start with duck works...... It's not that hard. I believe there's a very good article on this.
CLR is not that hard to find either - especially in a double ender Wharram.
Some simple internet homework will find these answers.
Yes bend is measurable - it's worked out often for things like beams etc....
Personally I wouldn't use bamboo though, while it'scheap and works, it needs replacing from time to time.
As for mast rake - no idea?!
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Thanks for the information.
I still have no idea how much "some bend" is. I have not figured out how to measure "some."
Obviously the amount of bend for a given force varies with the material, and then again with variances in differences in each material. But before a viable material can be selected, one must determine how much "some" bend actually is.
For example, I have figured for 36' long spars for the forward crab claw sail. A 3" diameter .125 wall aluminum tube will deflect 10.78 inches with 4.69lbs of force applied to one end. 4" diameter .125 wall aluminum tube will deflect 4.4". A 4" round solid wood beam will deflect 5.9".
So how relevant is any of this information? I don't know because I do not know how much "some" bend should be, nor do I know how much force will actually be applied to the end of the spar, nor how any of those forces that may act on an entire rig will contribute to, or subtract from, the ultimate deflection at the end of the spar.
We can easily go data crazy here, yet get nowhere without having the right data to input.
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Permalink Reply by Robert Hughes on
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If only it were so simple.
The "robot/bending" link is for deflection of a simple beam "built in" at one end with a perpendicular point force at the other. The sort of spar we are talking about has a non-uniform distributed force applied by the sail and wind, plus point loads wherever it is fixed or supported. Even if the formulae in the model worked for this scenario you don't know what the forces are. This is why clever folks at research institutes carry out wind tunnel tests and run complex finite element analyses on computers. The other (often better) way to find out what works is to learn from the experience of people who have tried it in the past. This is why "how to" books and articles are so useful, as long as they're written by people who have enough experience and understand why something works or doesn't...
The "centre of area" article is something that is frequently seen, but again it's an over-simplification of the actual situation. It shows how to calculate the centre of area for a selection of shapes, but that is usually not the same as the centre of effort of a sail of that shape. The actual centre of effort is determined by the distribution of the pressure differential between the front and back of the sail spread across the whole area. That distribution is affected by the aerofoil shape, angle of attack and speed of airflow, as well as other factors. This is why a cunningham is useful on a bermudan rig. Think of an aircraft wing of uniform chord - If the simple model in that article was valid the centre of lift would be halfway between the leading and trailing edges. It's not, it's much nearer the leading edge. Again this is where a "how to" book of hard-won experience would be so useful. In a bermudan rigged dinghy there is a "usual" figure for the distance between the centre of area and centre of lateral resistance, but it's likely that would have to be different for a crab claw.
The airflow over a crab claw sail looks like a fascinating subject for a PhD project, with so many variables, and that's before you move on to stress analysis of the structure. It would be interesting to know whether it's better to have a simple triangle or what effect the "horns" have. What effect does increasing curvature of the spars have and is there an optimum? Is elliptical curvature (as on the wing of a Spitfire) best? It would be a serious bit of work though. If I was planning to build one I think I'd start by basing it on known designs evolved over the years by people who used them. Maybe there are a few good "how to" articles out there somewhere?
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As I stated earlier, there seems to be no "how to" books/articles for crab claws, even though it is the oldest known sailing rig (4,000 ± years old.) You would think that with a rig as old as the crab claw, that there would be a book. I guess the ancient Polynesians didn't have writing, and passed down the knowledge via word and deed.
That being said, I don't think that anyone who has actually applied a crab claw rig had access to, nor data from research institutes, wind tunnels, nor used complex finite element analysis on computers. Therefor, the data is out there, it is just not found on the internet. Maybe I just need to spend $5,000 to take a trip to Tuvalu for a couple of weeks and have a talk with some people there so that I can install $500 rig on my boat...
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Try to get hold of Othmar Karschulin - his site is multihull.de
He has done extensive testing and wind tunnel analyasis and published it - in German unfortunately.
Also has extensive experience first hand.
Try proa sites, as that's where the experience seems to be.
Re bending - you could start with bamboo as structural strength?
Hans Klaar uses branches joined together on a very big crabclaw.
Obviously the forces can't be huge.
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I still have not found much useful information on the Crab Claw rig, although I was able to develop CE for the rig and hull. Proper lead is still elusive, and I believe I will solve this problem by using masts that I can rake forward and aft as needed to achieve the required balance. Once I find the balance, I can then make the stays more permanent.
Proper spars will be more of an issue. At this point, I am considering attempting to replicate bamboo with Douglas Fir. Instead of laminating Doug Fir strips together, I plan to create a rectangular spar and install "H" bracing between two 36' long pieces of 1"x5". The "H" bracing glued between the two pieces will simulate the vascular bundles of Bamboo, thereby also simulating the internodal regions of Bamboo. It is the spacing that I have to figure out. I am not sure what the distances between vascular bundles are on 4" diameter Bamboo, but I am guessing that the spacing of the "H" bracing will need to be 30% less in order to get the required stiffness out of the spar.
So what will start off as a rectangular spar with internally spaced stiffeners and approximate dimensions of 3" x 4.5" x 36', will have its edges well rounded over to create a long oval spar, that will hopefully be stiff enough to use for a 436 square foot main sail.
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Permalink Reply by wakataitea on
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on what typ of boat will the crab claw be used? if it is on a tiki 38 and bigger, i can recoment oiur system. works since 25000 sm with out any probems and modifications...
PS: do not use bamboo if you want to sail seriously wit it...
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A Narai MKIV.
However, I do not want to run a jib. I would prefer to run a simple ketch with only two crab claw sails. The sail's spars are the biggest issue at this point. I would prefer to use a readily available material, like aluminum tubing, but I have no idea of the stresses.
I am not sure why you do not recommend Bamboo since it has a higher compressive strength than wood, brick, or concrete and a tensile strength that compares to steel, and at a fraction of the weight. Not that it matters much since I cannot get adequate culms in central Texas.
wakataitea said:on what typ of boat will the crab claw be used? if it is on a tiki 38 and bigger, i can recoment oiur system. works since 25000 sm with out any probems and modifications...
PS: do not use bamboo if you want to sail seriously wit it...
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....Bamboo since it has a higher compressive strength than wood, brick, or concrete and a tensile strength that compares to steel, and at a fraction of the weight.....
this is very very theoretically...... forget about this. think practical....
bamboo will bend to much too and the sail will use its shape. it is not easy to get real good bamboo too. there are many different types of bamboo... hans klaar used bamboo for a long time till he was fed up with fixing the spars.... bamboo , if not 100% treated will have bugs inside and will not last longer then a year. check how the wharram guys prepair there bamboo for the lapita boats.... big job and this are small sails....
alu will be fine till a thirteen stress and then will bend or break. a solid wood spare will handle all this much better. our spars are 10.5m long and have a diameter of 100mm. two layers of yellow cedar glued on each other with a 2mm thick unidirectional layer of fiber glass in the middle.
don't forget that you have to access the end of your spars on your besan mast when you lower the sail.... you drawing looks nice but i think it will in real not work out. you sail area will be to small. the jip is important to point or sail higher upwind and to sail the boat through the wind when tacking....
our sail area is 60 sqm instead of 95sqm original. the jip hast 15 main 45.
if you want to get some where with your boat, you have to make your drawing more in a scale... cheers hans -
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