Crab claw sails: the mast to tack connection

A key question in the design of a crab claw sail and rig for your sailboat is whether to fix the tack of the sail aft of the mast, or to let it go forward of the mast.  The advantage of fixing the tack aft of the mast (e.g. with a yoke) is that one doesn’t need to move the sail and spars to the leeward side of the mast each time you tack.  There is also no longer a need for a snotter, so you have one less control line.  The disadvantage is that you have to engineer a yoke at the forward end of the lower spar and then decide how the upper spar is going to be attached.

Wakataiea

Over spring break (2016) we had the pleasure of cruising aboard Wakataitea, a Tiki 46 now owned by Mike and Catherine.  It was a treat in part because I’d never been up close and personal with such a large crab claw sail.  With spars 10.5 meters long, a 10m leech, and a total area of 45 square meters, Wakataitea’s main sail is 2x bigger than the biggest crab claw I’ve constructed and sailed.

After struggling to get the right combination of size, spar stiffness, and fore/aft positioning of the center of effort with the 20 m^2 crab claw on the Hitia 17 in the 2015 Race to Alaska, I was eager to see how Hans had built Wakataitea’s main.  He laminated his spars with yellow cedar with a layer of biaxial fiberglass sandwiched in the middle.  They 100 mm in diameter with a round cross-section.  The sail is sleeved over the spars with gaps for the halyard and mainsheet connections.  The tack is held just aft of the mainmast with a stainless steel yoke-hinge welded up by Hans.

I had admired these “jaws” a few years ago when I was rigging a 13 m^2 crab claw on our Tiki 21 Milagra.  Hans seemed to have the right idea: a yoke to fit the mast and a 2-D hinge that would both allow the spars to close together parallel when the sail was struck, or open fully with the upper spar either parallel to the mast, or offset to leeward if the halyard was less than 2-blocked.

Here are some screengrabs of the “jaws” from this 2008 Vimeo video of the construction of Wakataitea:

Upon examining this SS wonder in person (in 2016), however, it became clear that the forces and torques exerted on the tack-mast connection over ~7 years had fatigued a key weld in the jaws.  The connection had failed between the horizontal hinge-pin on the forward end of the lower spar and the vertical hinge-pin on the lower end of the upper spar.  The cause was probably twisting torques between the two spars.  (A repair had been in place for at least a year: an upside-down rusted steel eye-bolt replaced the vertical rotation pin with amsteel lashing the eye to the horizontal pin, and PVC+rubber/tire pads added where the spars worked against each other or the mast.)

Florida Tiki 38

I don’t know the full story yet, but I gather a similar fate befell a welded aluminum yoke-hinge on a Tiki 38 based in Florida that carries a crab claw main sail.  Here are a few images of the tack (presumably prior to the failure):

Twisting torques

What causes these twisting torques is a great question!  I suspect it has to do with the fact that the halyard and mainsheet are attached at fixed points: on the upper side of the upper spar and lower side of the lower spar, respectively.

crab-claw-torques

The above sketch suggests that the outward bowing of the sail when it’s loaded up by the wind is what causes these twisting torques.  It might be interesting to make the halyard and sheet attachments via rings or loops of line so that the spar can be pulled without twisting it…

Tiki 21

If we can’t fully get rid of the twisting torques, what we really need is a 3-D or universal joint where the two spars meet the yoke.  On the Tiki 21, I accomplished this (mostly) by lashing the bamboo spars side by side.  The twisting torques still turned the yoke, but the play between the yoke and the mast usually could accommodate them.

This combination of a lashing to connect the two spars and a yoke/snotter to connect the tack and the mast seems to work pretty well.  The first yoke we tried, made from a forked madrona branch lasted a couple seasons, but eventually broke with a BANG during a downwind run in ~20-30 knots — I think by a combination of twisting torque and forward force.   A new yoke fabricated out of 5/8″ teak planks has lasted 5 seasons so far.  The hardwood was able to take the stress without breaking (though in high winds the teak got slightly crushed where it made contact with the aluminum mast).

An improvement to our system might be to allow the yoke to rotate within the lower spar, thus preventing the twisting torques on the lower spar from causing the yoke to come out of orthogonal alignment with the mast.

Other good ideas

From Glenn Tiemann:

Glenn has a slightly different solution on Manulele.  His foresail tack appears to be held together with a lashing line (threaded through a hole in the upper spar?) and held aft by (double?) snotter lines that run aft from the lower spar at the tack.  This has the advantage of not needing a yoke, but the disadvantage of adding tripping hazards.

In other iterations (2 photos below from Wharram site) have bamboo spars lashed together and a suite of snotters (4 of them?) that appear to be able to haul the tack aft and/or outboard towards the bows.

Manulele bamboo tack Manulele bamboo tack

Here’s a couple of closer views from Kim Whitmyer, though these appear to have a single snotter with 2:1 purchase —

Manurere foreclaw tack furled
Manurere foreclaw tack furled by Kim Whitmyer.
Manurere crab claw tack sailing
Manurere crab claw tack sailing by Kim Whitmyer.

From Hans Klaar:

It’s hard to find good still photos, but Hans uses trees, wood, and line to construct and support substantial yokes on the lower spar of the big crab claw main on Ontong Java.  I think he’s gone through multiple iterations of this sort of solution, but this 2013 Vimeo video about Ontong Java has the clearest views I’ve seen.  Here is the best screengrab:

Ontong Java tack yoke

It appears that he’s used a symmetrical fork in a large tree as the yoke and the lower spar!  This yoke rests on a hoop afixed to the mast and is also supported by a line that runs up and around the mast (sort of like a snotter, but easily adjustable).  The upper spar comes down on one side of the mast and is also attached to the line that runs around the mast.

Ontong Java mast loops to tack
Notice the B&W line lashed around the mast which has clearly experienced a lot of downward force on the aft-port side.

Previous iterations may not have had the mast hoop, and may have involved a pin hinging the upper spar to one side of the yoke…

And lastly, here’s a shot from Jan 2013 that shows slight revisions to Klaar’s tack-yoke system, now on the “Mareva” (re-named from Ontong Java).

Mareva tack-yoke set up
It seems the yoke may still be a tree fork, but it may be scarfed or otherwise lashed into the lower spar. It no longer rests on a mast “hoop,” has loops of line instead of yellow plastic chafe gear, and may only be held up by a few turns of line around the mast (tenuous?)… The upper spar is now on the starboard side of the mast, is not obviously hinged with line to the mast, but instead is in a complex cradle of lines hitched around the mast (possibly to allow it to hinge well above the lower spar, thereby accommodating the furled sail?)…

From Beat Rettenmund:

On the Tiki 38 Aluna, Beat accurately describes the situation and then his solution:

Problem is the boom (lower spar) needs to be able to rotate left and right depending on which tack you are on (sail pulls out on top, sheet pulls in on bottom). On Aluna the yard is attached with a rope to the mast, while the boom is tied aft, the two spars also connected by rope. Like that the tack is suspended and doesn’t touch the mast, neither does the yard, also solving the problem of chafe.

Here’s a photo taken when Budget Boater visited Aluna shortly after her launch:

Aluna foremast-tack connection Aluna foremast-tack connection

But this much more recent shot indeed shows the tacks nicely suspended aft of the masts:

Aluna crab claws

Does anyone have a close-up of this more recent configuration?

From CLC Outrigger Junior

Though they allow that tack to go forward of the mast (and have a 2:1 snotter), the Outrigger Junior spars are connected with a simple line, similar to what Beat describes.

CLC Outrigger Junior tack

Other inspirations?

Here’s a novel, simple way to rig a 1:1 snotter:

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Other evolutions?  List them in the comments and I’ll revise this post accordingly!

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