What about the crab claw rig?

In my never ending quest for alternative rigs I came across this quote from the Proafile website

The crab claw rig has been tested in a wind tunnel by the famed aerodynamicist C. A. Marchaj:

"Even more startling is the extraordinary performance of the crab claw sail… which demonstrates its superiority to a Bermuda mainsail right from the close-hauled condition. Its superiority increases when the boat bears away, and on reaching, with the heading angle 90 degrees, the driving force coefficient of the crab claw rig is about 1.7, whereas that of the Bermuda rig is about 0.9. That is, the crab claw delivers about 90 per cent more driving power than the Bermudan rig."

Next, the obvious - how come there aren’t more of these rigs around? Are we perhaps so westernized or set in our ways that we overlook these things?

I’d love to hear from anyone with experience of this type of rig. Are there any drawbacks? Anyone have any links for further reading?

Your input always valued!

Take care

 

I think racing rules discourage lots of innovation in boating. Fred Shell uses a crab claw rig on his catamaran, and from the reports I've seen it works well:

http://www.shellboats.com/

The designer of the simple monohull called "trilobyte" uses a modified crab claw as well:

http://www.triloboats.com/intro.htm

 

Thanks for that info, Ken.

Sad to see that potential progress is stifled by racing rules – but, that’s the western way I suppose! My purpose is cruising so I might just incorporate it and blow past some of those hot machines!
After more searching, I also came up with this site which is interesting for anyone else interested in the crab claw concept.

All the best!

 

Trev
The comparative studies can be misleading when you consider that rigs are compared with fixed sails whereas the great versatility of the Bermudan rig is the great combination of sails that can be used.

The CrabClaw is a very efficient rig, but overall the performance is very similar to a Bermudan rig when tested practically.

Remember that the sail driving force is balanced by the hydrodynamic resistance. That resistance increases as the square of the boat speed, so you need to look carefully at the trade offs of the actual factor of increase in sail drive compared with the overall VMG of the vessel. As soon as you are into wave making resistance the sail drive forces need to increase proportionately with the resistance curve and if they are 25% better they don’t result in 25% more boat speed but closer to the square root of this . This is not so in light air but then a Bermudan rig will fly a different (bigger lighter) sail to suit the relative wind. In fact selecting the appropriate Bermudan headsail for the wind speed generally provides drive that is about equal to or actually exceeds the Crab-Claws performance when you compare VMG.

The Crab is a supremely simple low-tech cheap and forgiving rig with many advantages for low-tech human communities but overall a Bermudan rig with interchangeable sails will out perform it and that is why racers don’t use it.

The wonder of it is that for such a cheap simple rig is that its performance is up there with the modern Bermudan, for straight out steady sailing and it would make a very good cheap robust and simple setup at a fraction of the Bermudan rigs cost.

Presumably practical size limitations would revolve around tacking the sail and spars, hence the bipod ideas although you could drop the mast and all into special boom gallows arrange everything and re-hoist.

Has anyone here ever sailed with one?

 

Now that would be the acid test! to find someone who has actually used one! You can play all you want with your theory and maths; statistics will tell you anything you want (e.g. 20% of all road deaths are caused by drunk drivers, ergo 80% are caused by sober ones be safe drink/drive)! the true test is use one in varying conditions with vessels of other rigs in competition, we may get some different answers, depending on the ability of the crew to use the rig!

 

Quote Safewalrus "Last edited by safewalrus : Yesterday at 10:17 PM. Reason: usual problem - my computer can't spell!"
So you found the spell checker at last, did you?

 

spell checker? whats wun o' them then? Do it do magic squares then? want one o' they!

 

Crab claw rig practicalities

I have been involved with several Flying Proas the first was a dreadful 25 footer built to compete in a raft race, the less said about her the better.
The second was a 36footer built from 3 ex USAF grp drop tanks cut & shut together and fitted with a 180 square ft standing lug rig, she went like the clappers and was a superb sailing machine.
The last was a 20 foot flying proa built by Richard a friend of mine, she was fitted with a crab claw rig and i have sailed her quite a few times.
The rig is deceptive and people make the mistake of regarding it as an areofoil sail. Though it can function this way It generates its best drive when operating as a dual vortex delta wing. It is a rig best suited for long reaching points of sail and should not be expected to perform to windward.
Something else that is overlooked by westerners is that it has a lot in common with a kite rig the top yard is often kept at or nearly on the centreline of the canoe and the lower yard can swing out and upwards as the canoe bears of downwind. this leads to the rig generating a huge amount of lift on the "bow" of the canoe.
When we were playing with Richards proa she proved fast and the rig was very powerful and huge fun, however it isnt possible to reef it though the proper way to do reduce drive is to allow the yards to come together thereby destroying the vortex generating potential of the sail.
Downwind the rig generated huge weather helm that lead to some exciting wipeouts and much macho posing on the steering oar!
I feel that we should not look at the flying proa and crab claw rig as separate things but rather that the two represent a supremely elegant solution to the problem of fast ocean sailing. I think there is much potential for development with the crab claw rig and i would love to build another flying proa, anyone got an old rowing eight they want rid of?

 

That's better some practical advice - Keith that sounds some system, you tend to glance at these things and comment - 'some parochial rig' then dismiss it as not worthy - maybe it is, definately more research needed on my behalf! Thanks for your input! Much appreciated

Mike

 

Thanks for those kind words Mike! With regard to development of the rig CA Marchaj suggested that for maximum vortex generation the edges of the spars should be sharp edged, also that the sail should be set as flat as possible, When setting up the rig on Richards proa we found it very difficult to get the sail to set flat and eventually fitted a jacking strut (for want of a better name) between the spars near the mast, this had a powerful tackle and allowed good control over the sail camber.
Taking this idea further, Consider yards made from foam cored Glass cloth or carbon, sharp edged with the sail spread between them or even covering them like a microlights wing.
There are several ways a jacking strut could be incorporated possibly even inside the sail if double sided.
I do not suppose such a rig would be practical for cruising or everyday sailing but if you wanted to sail really fast it might be worth an experiment or two!
Has anyone else got any ideas or experience?

 

Phew! as a simple sailor that stuff is way over my head, just looking at what is appearing to be an interesting but simple rig! I don't really want go in too deep (I confuse easily - bit like bruising really!)

 

I just splashed cold water on myself i was getting carried away, the truth is proas are best as dayboats for having a blast with your friends they can be as low tech as you like which means cheap. For instance our 36 footer the Voodoo child was built in 1984 for a total outlay of about £200 with a high proportion of her parts coming out of skips. Richards proa was cheaply built and is still in commision though fitted with a gibbons rig (reason being he singlehands her and the crab claw was too much of a handful.)
Regarding the far out concept outlined earlier i am building a model to test the practicalities and will get some pics up asap

 

yeah that load carrying problem could be a big problem, unless the boat was really gigantic! But for days out with all the family etc I could see it being real fun!

 

scanned some open books with crab clab rig's i presume

 

this gets more exciting all the time! Your going to sell me on this rig soon WOW!!!