10 ft "cabin" cruiser

Has anyone any experience with a "crab claw" sail for a small monohull ? I made a sketch and to me the combination of this sail and my "soapbox" looks appealing......taste differs....

It's not a "true" crab claw as used on the Pacific multihulls but a variation of the concept.
Opinions about the effieciency of such a rig are not unanumous positive....

At first glance, a single sail (balanced lug for example) on such a small boat can offer some advantages.
Why not use a balanced lug than ?, good question, but for now I have no other answer than that the very short, stayed or unstayed, mast seems attractive to me.

i welcome your opinions

 

Search the forum and you will find several threads regarding crab claw sails.

I did an advanced search, searching titles only, with "crab claw" and came up with several specific threads.

Here is one bit of information that you should find useful.

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-39.html#post258947

Go to post #388 if it doesn't come up right away.

 

Thanks !

yup,Marchaj sure shaked some "naval tree branches" with his "observations & conclusions" about the crab claw sail.

Probably very few people with on hands experience to share I guess.

 

Maybe you can apply this to designing a crab claw.

http://cpl.iphy.ac.cn/EN/article/downloadArticleFile.do?attachType=PDF&id=43316

 

Thanks LP but those alpha's and bèta's and Re numbers ain't my kind of cake I guess. Way above my hat.
Still very interested in a crab claw sail for my little boat though. From what I understand (very little) about the crab claw is that it can be cut flat so that should make it a bit simpler to put together for the backyard builder.
The sail I have in mind is only 3.29 sqm but I have no clue if such a small sail would be enough for my ,relative, heavy boat.
Guessing will be the main task I assume to determine the dimensions of the spars for example.

 

Here's a sketch of what I have in mind for the spars of the crab claw (by lack of knowing the proper name for it (boom-luff ?).

The intention is to glue up 3 wood laminates ( no clue about which wood to use) in a large curve. Carve/sand the spar to an eliptical shape of 60 mm x 40 mm. well or not covered with carbon fibre.

Comments welcome.

 

Off the top of my head. 3.3 m^2 seem a bit small and double that might be more appropriate. Again, that is off the top of my head with no knowledge of the stability characteristics of your boat.

The greek in the paper is not really that complex. It's just engineers trying to make their work sound more complex than it really is.

Alpha is the angle of attack of the foil. The plane of the foil vs. the direction the wind is coming from.

The inverted V is the sweep of the foil leading edge. The angle is taken normal to the airflow in plane with the foil. Normal is engineering again meaning perpendicular to the flow. So...sweep is taken xx degrees from 90 degrees to the flow. No wonder it seems complicated.

And then for Reynolds Number (Re). I can't stand Re. It's a dimensionless number(already complicated....again). In this case though, it just a set of variable and fixed numbers. From Wikipedia, Re(for airfoils) =

R = V x c / v, where

V = is freestream velocity in meters per second (m/s),
c = chord where chord could be several things, but in the case, I believe root chord is used so that would be length of the foil in the direction of air flow at it's base or the longest length that is aligned with the wind.

And,

v = in the kinematic viscosity of the fluid that the foil operates in or the kinematic viscosity of air at sea level or 1.46x10^(-5)m(^2)/s. More engineering code. In layman's terms

v=0.0000146 M(^2)/s

(big V's and little v's have completely different meanings. More freaking confusion!)

So if the wind is blowing 10 MPH, roughly 4.5 M/S and the boom on your crab claw is 2 meters long....Re=

4.5 x 2 / 0.0000146 =

6.17x10(^5)

I believe I have made all of the calculations and conversions correctly. This places the crab claw sail well within the parameters of the delta wing, dual vertex generation theory explained in this paper. It's encouraging and also very interesting to see a completely different form of lift production than I have ever thought about before.

 

One of the bits of information I drew from the paper is the fact that air flow is not attached to the foil surface. With this in mind, rounded spare sections might not be the best choice. It's hard to get away from conventional thinking and rounded spars come to mind naturally. Your flow should still separate with the elliptical section, but I think it will try to attach itself at first before separating. I am in no position to this is good, bad or otherwise. I could have a negative affect on foil performance or no effect at all. I would think, as counter intuitive as it is, that triangular, or even square section would promote the desired flow separation needed for this type foil. I think that round might even be better. I'm just thinking out loud so don't take this as criticism.

I've never thought much about the crab claw and learning about it has really sparked my interest. I've always wanted to put a sailing rig on one of my kayaks and this rig might be fun to play with.

It looks like this is an "over the top" rig where the lifting side of the foil is the same side of the foil as opposed to a tacking rig where the lifting side changes every tack. You could take advantage of this and tune/design the foil surface for maximum efficiency. By the way, I've read one paper on dual vortex theory so now I'm an expert. Triangular spar sections would give you a sharp leading edge and if you wrapped the sail cloth around from the lifting side, you could have a completely flat lifting surface that would be most conducive to lift (vortex?) generation. Just my two cents. Happy designing.

 

It's indeed an "over the top rig". At the top of the mast is a hinge that contains a bearing block through which the short spar (cross spar by lack of a better name) , that connects the 2 sail spars, can turn. The hinge can swivel from star-to backboard only.
lines control the position of the cross spar, going from parralel to the mast on starboard to horizontal, to parralel with the mast on backboard side..
When the the cross spar is for example in the horizontal position, the sail can be rotated from the "bimini postion" (parralel with the deck) to the full upright position like an inverted spinakker.....if that makes any sense....
When the cross spar is tilted to one or another side, the sail can be rotated around this spar so the lenghtwise angle of the sail can be changed.
I realize this all sounds very complex....which it probably is, but to me it seems simple.
Will this work ? I don't know....

 

Why don't you just pick up an old windsurfer for $100.00 (or less), and use the mast, boom and sail. Would be just about the right size for your craft.

 

Would be the most sensible thing to do I guess. I still have a lot of time to think about it before that little boat ever hits the water.
I think a Cadet class sailrig could be usable too and perhaps there are dozens of small boat sailrigs that can be had for cheap.

 

Here some mast and rig ideas from Bernd Kohler's K-designs, the KD 122 Voyager, or one of these DUO 800 parallel rig.

 

In past experience, asymmetric hulls with no boards and parallel rigs, have proved to be unpopular for various reasons.
Over the last half century, symmetrical hull sections, vertical water foils and the
Bermuda rig have proved themselves, to be the most effective, and thus most popular design features.

 

Thanks for the info !!

 

I think you're right, but all of this was developed by people who stuck out their neck because they thought things available at the time could be improved. If all would settle for this now there wouldn't be any progress anymore. Progress depends on those who persist in seeking for it . . . :idea:

Bernard Shaw (1856–1950) --> Man and Superman (1903) --> Maxims for Revolutionists

BTW, the links were posted for the mast and rig ideas, not for the catamaran idea or the chosen hull form there, and of course by ‘‘one’’ was meant to look at ½ the DUO 800 parallel rig.