Claw Rig experiments
|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
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's a close up of the truckers hitch. There is a dowel inside the 1" PVC. I tied a constrictor hitch around it, then went around the spar and to the loop of the truckers hitch. I moved the two little nylon fair leads that were on the spars to capture this tension line so the cross bar can't slide up or down the frame.|
|After trimming about 6 inches
outside the frame I begin folding the tarp over double then rolling it
around the spar and clipping it with PVC "C" clips. This time I
made them from 3/8" thick slices of 1/2" schedule 40 PVC. I
remove about 1/4 of the circle to create the "mouth" of the
clip. I'm putting them about every 9" to start with. (My tip of thumb
to little finger hand span.)
The leach.... bottom of the "A"... I left as a factory edge.
|I alternate sides to maintain tension, and use a couple of clip clamps at the apex to help hold the tarp tight while I popped on the clips.|
|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'm running the two corner sheet ends through the fair lead hole in my tiller. This allows me to control them both with one hand. Pressing down on the tiller pinches them effectively like a jam cleat. Lifting the tiller and I can let either or both sheets out. One goes out and one comes in to tilt the sail both go out with the bow/point line is tightened.|
|Though I knew the tilt sheet was continuous I had it so long that they were acting like two lines. I shortened the tilt line to be more of a tight loop with little slack in it. In function when you let out one side you are taking in the other so they need no independence. In the photo I'm using a clip clamp for now. It could run through a lever type deck line clamp (Or a cheap home made variation there of...<grin>)|
|The bow/point line is pretty straight forward, so runs through the towing eye in the top of my cutwater stem. Could go through any small block or fair lead. You can see in this photo how the tilt lines are running through inwale slots. It might be better to have them running to the base of the mast so you could trim the sail hard to the center line of the hull.|
|My trusty test pilot patiently awaits... Wouldn't you know as soon as I get this rigged every breath of air died away. I did take a few minutes to row around under the "bimini" version of the sail. It worked wonderfully. The shade from the mid day Florida sun was particularly welcomed by Mocha - she can become a hot dog pretty quick!|
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.
I ended up not using the small inner disk. I'm letting the line slide over the bolt and the plastic disks apply the friction.
|1. Friction Disks: This is a stack of plywood and plastic disks mounted to the aft side of the base of the mast such that the tilt line travels from one end of the cross spar, to the friction disk assembly, once around it and back to the other end of the cross spar. There is a wing nut to adjust the tension on the line. At it's most simple this would be set at a working friction and the line could be grabbed anywhere along its run to tilt the sail. Slightly more complicated could be that this line works as a sort of preventer to control the speed of the tilt while other lines did the actual tugging. I hope that isn't necessary.||
I tried straight from the spreader ends to the mast, but slack is created as the sail tilts. This is less pronounced when going through the inwales, but I still want to work it out mast only - so it could mount on a canoe or kayak.
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.
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.
|Here you can see the lines are getting more centrally located and the mast does pivot pretty well. This is good as some of the seemingly most efficient points of sail have the entire rig slanting across the hull with the peak of the sail pointing to windward.|
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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