The halyard tackle, fixed as usual to the middle of the yard, must have enough drift such that when the yard goes to +/- 90 degrees, the rope doesn't make too great an angle with the sheaves of the blocks, otherwise damage will result. When a span is used at the yard end of the tackle, some extra freedom is allowed the lower block, as the span itself can twist a bit, as well as whatever rotational freedom there may be in the shackle and thimble. This means that the block to block distance when a span is used can be somewhat shorter than when a direct yard connection is used.

When making the design, I would first mark the points at the one third and two thirds points of the yard, and also at the centre. I would then draw a line from the two thirds point to intersect with a line from the masthead block to the centre of the yard, such that the  angle between the yard and this first line is 30 degrees. Then draw another line from the one thirds point to this intersection, and this will be the shape of the span that you are aiming for. Then allow let's say five times the length of the halyard block for the tackle - two blocks plus the length of three blocks between them. I have found this to be enough drift.

I tie an alpine butterfly knot (which can take a load in two directions) in the 6mm Dyneema at the intersection point, large enough that a full turn can be taken around the bow of the shackle on the lower halyard block. I tried a stainless steel thimble here, but it soon failed due to fatigue. The 6mm Dyneema should be tied right around the yard at the one third and two thirds points, with a strong eye just to keep them from sliding, not to carry the load.

I use Dyneema for my lifts as well. I would use 6mm for yours. I tie two forward lift lines halfway down the topping lifts (to an alpine butterfly again) one on each side and lead the line on the mast side to the bottom batten/boom at the after end of the batten parrel, and the line line on the other side to a point level with the mast line. Actually, a span at the bottom of this latter line will help with gathering the bunt of cloth here.

The reason for doing the forward lifts in this way is twofold. It stops the annoying clatter from a lift that is parallel to and close to the mast; and more importantly, it brings the line of the topping lifts further forward. With the weaverbird planform of sail, I have found that a gybe with three panels up is safe enough without this form of forward lift, but a gybe with two panels up risks getting the yard trapped behind the topping lifts. This planform of sail seems to be much less prone to misbehaving during a wild weather gybe than the HM kind; but there is still a risk, even when the yard has a light extension to bring it to the same length as the lower battens (essential). Moving the topping lifts forward is the cure.

How traumatic is a yard breakage? Very. You can break a batten, mid-ocean, and lash it to the next one and carry on. If you break a yard, you have to fix it.

Does a bridle or span help to spread the load? Yes. I've done it, using 6mm Dyneema.

If I remember correctly, Footprints has had a yard of 100mm dia, and this wasn't enough - but the length was much greater than your proposed yards, and it was probably T5 temper. My guess is that 4" dia T6 with a span to spread the load will be more than adequate.

Arne and Paul - thanks so much for the input.  My yards are at 45 deg and the sails are high aspect with a typical ketch (steep) sheeting angle.  The sail on Edmund Dante is similarly sized, but the yard at 70 deg. and at 6 meter length about twice that of mine - 3.2m on the main and 2.7m on the foresail.  I don't know conceptually how the loads will differ in these two plan forms.  

Some numbers comparing sections (cross sectional area would indicate relative weight, Iy and Ix stiffness/strength in the vertical and horizontal).

	Cross Sec. A  mm^2	Iy mm^4	Ix  mm^4
A. 150mm x 3 mm	1,385	3,738,888	3,738,888
B. 80mm x 5mm + 50mm x 1.5mm	1,406	1,751,154	569,347
C. 127mm x 3.175mm	1,244	2,382,815	2,382,815
D. 100mm x 3.175mm	965	1,131,518	1,131,518

(A) is the workhorse and can take the largest loads.  (B), the combination of two tubes, is the most narrow and would have the least windage.  (C) and (D) are 5" x 1/8" and 4" x 1/8" (yes, this is the US) and are easily available.   

If one were to assume that (B) has adequate properties, then (C) would be plenty strong and lighter.  The gamble is whether (D) would work.  Would using a bridal to spread the halyard loads help.  

So the question is - How traumatic is a yard brakeage?  :)

Cloth is coming on the 15th.

Erik

Erik and Evi Menzel Ivey wrote:

Some questions regarding battens...

I think previously folks have recommended that the boom and battens could be 50mm x 1.6mm.  When counted from the bottoms, the transition battens for Panels 5 and 6 would be same diameter but 3mm wall.  What about the yard - single tube, 100mmx3mm wall?

e

150 x 3 would be safer, and please note all tubes Battens and yards must be T6 temper else you'll have bending problems.

Arne Kverneland wrote:

Erik...

  -  it's about time now, don't you think?

Cheers, 
Arne

Progress maybe....    The foresail design is (somewhat) finished (till I change my mind) in 3D and laid out flat in QCad.  Actually this has been enough of an effort that I am not going back.  :)

Originally I kept the same profile at the battens along each panel - max draft at the second seam, flat section from the 3rd seam to the luff, but reducing the amount of camber from 8% to 4 and then 2 in Panel 5, 6, 7.  This resulted an adequate shape for Panels 1-4 (as counted from the bottom).  But for Panel 5-7, when looking at a horizontal cross-section, the effective draft was too far back.  The solution was to have max camber at the first broad seam for these panel, which got the draft - looking in a horizontal cross-section - forward of 50%.  Some pictures, to make sense of my less than adequate description have been uploaded in this album  .  

e