Crab
Claw Rig experiments continued..... |
Using info from from this page which details the research of C. A. Marchaj into the aerodynamics of the crabclaw rig, as a jumping off point, I made a simple crabclaw rig for Summer Breeze. |
Page One of Crab Claw
experiments.
May 8, 2002 This is Day Three of "Further Developments", and I'm switching to the "A" frame crab claw rig. I had two 10 foot anodized aluminum spars from an old lateen rig. (I think from a Styrofoam Snark?) As you can see, I'm about to lose a lot of sail area. I'm hoping the reported efficiency of the rig may make up for this. |
The apex was already bolted together. I tied the ends of the spars to determine the width of the sail. I'm using the same cross spar but with truckers hitch out hauls to tension and shape it. |
Here it is with clips every 4 or 5 inches. The clips are barely visible from the top of the sail. |
Here it is on the boat. I think I've figured out how to manage the lines. |
I'll let you know how she worked after the wind comes up.....
May 10, 2002 more further developments....
The wind did finally come up, and it was interesting. I'm having a good bit of trouble getting her to point very well, but the sail isn't very balanced either. I'm experiencing pretty much lee helm, which improved very slightly when I turned the mast head mount to point aft. Also sailed with Julie and Mocha aboard... tacked better, I guess due to more "carry" from the weight. I want to make a larger rig with the CE in the right place. This being 25 ft and my lug being 63 I'm not comparing apples to apples.
Sure cuts a funny profile when running down wind. |
The geometry of the lines and sail control is getting clearer. I think this sail could be made to work well on a canoe or kayak. Here are the changes I'm testing now.
2. Bow Line - I noticed the sail likes to try to trim off center with the peak pivoting outboard of the hull. If the mast height geometry is correct the point line could travel to a fair lead at the base of the mast on the forward side, around to a captured clam cleat (West Marine $2) Combined with the friction disks the mast and sail can now rotate freely controlled by the play in the main sheets. This can now rotate freely like a swing rig or aerorig.
After trying this, I realize it's much easier to get the rig oriented on center for going to windward with the bow line instead of the mast base.
3. Main sheets: Right now I have them running to a fair lead hole in my tiller that is directly above the pivot point so it is not effected by the steering. If this were mounted in a kayak these two lines could run to a fair lead on the mid line aft part of the hull, then run up to the cockpit to cam cleats on either side of the paddler. I'm thinking sea kayak here with foot controlled rudder.
4. Spars: The anodized aluminum spars seem to work well. It's sold in sizes that telescope, which would help create stowable rigs. It is about $1 a foot in the size I'm using. It's sold for building antennas.
http://www.texastowers.com/aluminum.htm
The cross spar and the mast could be wood for weight and aesthetics. 1 1/4" closet pole for the cross spar - 2" round for the mast. 2" aluminum is also available for about $3 ft. in 6 foot lengths. This will take a 2" PVC 90 degree elbow well. Since the sail will not be sheeted to the hull except by the main sheets, and since the sail can generate significant lift, the mast would pass through a retaining collar with a thumb set screw, below the mast partner (or deck on a kayak) to prevent the whole rig from lifting off. This could be made of PVC.
5. Sail material: This is one of the great advantages of this system. The sail material is easily replaced poly tarp attached with PVC "C" clips. Assuming for the moment that the "vortex theory" may be all hooey... and that this sail works via its wing shape like other sails... get this. The spars are tensioned on the cross spar (prior to attaching the tarp) to create a sort of fat "A" shape - sides of the A bowing out. The tarp is trimmed and clipped to the spars. Now, when you relieve the tension on the outhauls of the cross spar, (just slightly) you get instant camber that looks sail loft tailored. Loosen for light air, tighten for heavy.
6. Transport: When the outhauls are released on the cross spar- the long spars scissor closed and the sail rolls around them. The mast and cross spar might be in the 4 to 5 foot range for a canoe or kayak set up. These can go inside the hull and the sail bundle lashed to the outside of the hull in transport. If the long spars (or all of them) telescoped it might make a very small bundle. The sail also might stay mounted most of the time... see shade, next.
7. Shade: A big part of my desire to research this rig is it's ability to offer shade when not being used for propulsion. When the sail is trimmed horizontal it seems amazingly impervious to wind forces. Also, in the case of canoe or kayak use it could be used as a bimini while paddling in shaded comfort, at the ready to convert to sail position in seconds when a breeze blows up.
It seems to me this sail/shade system could be manufactured and marketed. Or plans could be sold. Or kits with the harder to find parts ready made. Or patented and sold to some big kayak company - if there is such a thing?
The experiments continue.... | Well, here is the (not so) simple solution I tried to my
tilt control. Dual friction disks and fair leads. It sort of works, but
the tension of the disks is critical, the lines escape and if the friction
is insufficient to the wind your sail flips vertical in a gust.
A better solution can be seen (staring me in the face) mounted below the disks. Yes, those fancy modern store bought cam cleats. I borrowed these from one of my Force 5 s for the peak line. Works great. They come in a fair lead cam cleat version that keeps the line captured when it pops out of the cleat. Two of these would take care of the tilt lines. I may consider a pair of permanently set "preventer" lines mounted higher on the mast. These would be to limit the tilt on the sail even if the control line got away from you. I have not found it advantageous to have the sail perfectly vertical, and once it is, it's tricky to get it to tilt again - as once that spreader bar is pointing at the sky, you're pulling straight down along the line of the spar wit no leverage. Just a thought. |
Rique Warkworth is doing some great crab claw experimenting on his lovely outrigger canoe he designed and built in New Zealand. Check out his article.
This page created May 6th, 2002. Copyright David J. Beede 2002. |
Feel free to email me, David Beede simplicityboats
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