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Anke and I live aboard WAYWARD, and wrote about it's design and construction at

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Fair winds!

Dave and Anke
triloboats swirly gmail daughter com

Sunday, June 2, 2013

CrabClaw Rig: The T-Modification

See the Crab Claw Sails?
(PS. I couldn't find/read the artist's name... anyone know?

Ahh... Crab Claw Rig! Spread before warm winds, the very wings of a pacific dream! In a thousand forms, they carried the peoples of the Pacific to explore and inhabit seas both wide and wild.

On a fringe of their world, Anke and I got to share a beat of their flight.


A quick overview of CCrig... I don't know the real terms, so I'm making this up:

Generally triangular sails, with limbs along two edges. The leech (by which I mean the unlimbed edge) is opposite the apex (where the limbs converge), and may be hollowed from slightly to extreme.

The theory I've heard is that CC sails generate vortexes near the apex, which then run along the limbs in turbulent balls, generating lift. 

Thus, the mid-fabric can be cut away without much loss (though the amount varies radically). A deep-cut sail will 'self-reef' as the turbulent balls blow off the limbs, reducing drive to the small patch of fabric near the apex.

Shape may be spoiled on any CC by drawing the limbs together, reducing drag.

Camber and draft are more conventionally present, but - I've heard - take a back seat to vortex lift.


This schematic shows some of the evolutions possible along 3 axies.

The problem with (many forms of) traditional CC for lazy sailors is that it's generally set on one side of a mast, and must be shunted across with a fair bit of effort and skill. Not a problem for Poly/Micronesians who are sailors born and bred, and often sail with family-sized crews.

One approach has been the A-Frame mast, but it is relatively heavy and limits the range of motion.

Some genius -- possibly John Rowland, or Art Lane of HSS (Horizontal Sailing Systems)? -- came up with the what I'm calling the 'T-Modification'. 

A yard is set, spanning the Crab Claw limbs. It is fixed to the top of a short mast with a universal joint. Now the sail can rotate around THREE axies (vs the usual one)!! 

Around the vertical axis, it acts like a normal sail -- haul in, let out.

Around the thwartships axis, it may be stood up or laid down!

Around the longships axis, it may be rolled over the top!! 

This last, easy maneuver solves the tacking problem. AND, as the sail approaches horizontal, its force vectors are directed upward, effectively reducing power (converting drive to lift!). AND, in horizontal position, it makes a great bimini / rain-catcher!



TRILOBYTE under Crab Claw
I first saw the T-Mod in a tiny thumbnail in Multihull Magazine whose rigging details were indecipherable. I'd been sketching up a weird little boat called TRILOBYTE (the ancestor of TriloBoats)... it seemed a match made in heaven!

Rigging was tricky, but I was able to combine control of the vertical and longships axies with port and starboard sheets. 

Here's a vid posted by... umm... ra274jags, which shows its workings: 

Anke and I cruised the rig for a couple of months in Sitka Sound. Unfortunately, we were trying out lateral resistance systems that proved insufficient. While we pointed well and developed plenty of drive, we slipped sideways...didn't actually make any progress to windward.

The rig seemed to have the potential for good handling, though when wind picked up and turned gusty, everything happened fast. We never knocked down (which TRILOBYTE is designed to take in stride), but our reactions lagged well behind conditions.

Overall, I consider the modification worthy of exploration as a cruising rig. But I'm not up to it, myself (at least not in cold waters), and we've settled on Junk Rig.

For beginners, I'd say it's either a poor choice (steep learning curve) or a great one (beginners have few preconceptions).

If you're interested, my advice would be to mount a small one on a PDRacer to try it all out, and assume that you'll end up in the water (life jacket and/or wet or drysuit where necessary).

I'd also consider a little yawl type mizzen (aka spanker or dandy) to force a round up to windward. There were several times where we were scratching our heads while racing down or across the wind before figuring it out. Luckily, we had sea-room in those cases!


I get a lot of questions about the universal joint... 

On the prototype, we used a rope grommet, seized in the middle to form a figure 8. The lower loop led through a hole in the top of the mast while the upper loop was fixed around the yard.

In the final plans, I specified a lanyard with its lower end glued into a hole bored vertically into the top of the mast. Its upper, free end is led through a close-fit hole in a plank yard. Stopper knots below and above the plank cushion and fix the yard, respectively. I've not tried this, to date, nor heard any feedback on the system.

I like the idea of a line based joint as it is cheap, quiet, easily inspected and replaced.

Look here for an interesting approach (looks similar to the rigging I worked out, but could be convergent evolution).


So that's what I know and don't know about Crab Claw Rig... 

It's intriguing, powerful, beautiful and likely to dump you if you're not careful!

For more on Crab Claw Rig, start here?


  1. So, what what were the diameters of your spars? Were the diameters appropriate after trying out the rig?

    1. Hi,

      For the T16x4, we used a 3in diameter mast, a 2in yard, and limbs about 2in tapering to about 3/4in -- all from spruce in-the-round.

      Yard and mast were fine, but the limbs were too light... they would start to deflect at the top end of normal sailing breeze, spoiling the sail set. This is a safety feature -- to spill gusts -- but I'd guess 25 to 50% more diameter would help stiffen them without making them rigid.

      I don't have enough experience to recommend rules-of-thumb, but since we first looked into it, there's been a lot of interest and new info coming online. One good source is James Wharram (and Hanneke Boon). Others are Holopuni Canoes and any of the many, indigenous sail projects ongoing among the Pacific Isles of Poly-, Micro- and Melanesia.

      Hope this helps!

      Dave Z

  2. I noticed your CC rig seems higher up than a traditional CC; meaning, the apex on a traditional proa is touching the boat yet on your trilobyte the apex looks like it's 3-4' above the boat. Could this effect the performance? Center of effort? Knock down potential?

    1. Hi,

      The apex is pretty high when the sail's mid-line is lowered to near horizontal. But it can also be raised to near vertical, and then the apex approaches deck level. You can see this effect in the schematic drawing in the right hand sketches.

      In practice, the sail seldom goes fully horizontal while sailing, but the axis is tilted down diagonally. So the apex stays pretty low.

      If the apex were left high, I imagine that it wouldn't draw as well?