Arne Kverneland wrote:

                                                                Stavanger, Thursday

                                        Loads

...

As for increase in load because of the camber; this I have tried to describe since NL 24, back in 1991. It doesn’t take a rocket scientist to understand that with increased power comes increased load. Luckily a modest upsizing of 26% of a batten (or mast) section will double its breaking strength. Also, with cambered panels comes the importance of making the edges of the sail strong. A rip that starts with a torn leech is much worse than a hole or rip in the middle of a panel.

Arne

　

First, to David:
Yes, someone has to be first. We all owe you a debt of gratitude, not mainly for breaking things, but for solving the unforeseens in new developments of the rig. You dare to invent, and then sail farther and harder than most of us. I wish you a quick and satisfying repair session.

And responding to Arne:
I agree with your analysis of loads and camber. Flat sails distribute strains.
As a horse-and-carriage non-scientist cruiser, I'm willing to sacrifice some power for robustness. In racing, things should break from time to time, to ensure they're not over-built. But for our happiest cruising, things should break not at all. (A bend now and then is acceptable.)

Many of my posts on this site have tried to influence other non-rocket-scientist
sailors to stick to what is proven to be hardy, rather than to take up the promise of speed or windward wizardry.

I can cite for my authority only the pleasant fact that mehitabel is a very robust and satisfactory cruising sailboat with flat-as junk sails. And not only downwind in the Trades.

Cheers,
Kurt

I've now taken out the two battens, and there is evidence of dryness and delamination. Photos in a new profile album, CFRP battens. That was the hardest thing, to be sure that I'd wetted out fully.  Also, these two battens got 3 layers plus a half length layer, the top batten got 4 plus a half, the batten below got 4 plus a half length alloy tube. It was hard to decide exactly what the layup should be, with the long time taken until full cure, so not all battens are the same. 

                                                                Stavanger, Thursday

                                        Loads

Counting the battens and panels from top is not my idea; I just adopted the PJR practice. David’s batten 5 and 6 are thus what I would have called batten 2 and 3; the two that take the hardest sheet load.

The first year I sailed my Johanna with the new JR (2003), I bent my top sheeted batten (no 2). This batten takes a much higher sheet load than the lowest batten and boom. In fact, to get the twist right, the sheetlets and sheet are arranged so that batten no 2 takes 4 times the sheet load of what the boom and lowest batten take. In addition batten 2 is extended about 50cm aft of the sail to avoid sheet tangle, and that put even more load on it. Luckily, the aluminium, being quite flexible, gives more feedback than carbon so even in a normal good breeze (F4+) I could see it was panting. I bent it in a bad downwind squall (F6+?) at the same time as an inferior weld broke in my new yard, so that bent as well. Limping home was just the right description of what happened next - luckily downwind.

By replacing the bent batten with one of the same diameter (50mm) but with 3.2mm walls instead of 1.5mm, it has held up well and I never see any signs of it panting any more.

Such squalls with full sail set are obviously a challenge, in particular if one is down below for a second with the auto pilot on. I find that when the sail is reefed well down, the lee topping lift supports the sail, yard and battens so they see much less load then. That’s why my new topping lift is 12mm.

As for increase in load because of the camber; this I have tried to describe since NL 24, back in 1991. It doesn’t take a rocket scientist to understand that with increased power comes increased load. Luckily a modest upsizing of 26% of a batten (or mast) section will double its breaking strength. Also, with cambered panels comes the importance of making the edges of the sail strong. A rip that starts with a torn leech is much worse than a hole or rip in the middle of a panel.

Arne

　

Uh-oh. Battens 5 and 6 broke suddenly, close reaching in a 35 knot rain squall. They weren't bending, so gave no prior warning. Limping back to Mahurangi Harbour to think how to mend them.

David, do you think low aspect cambered sail stresses the battens more than a high aspect sail? Given same sail area.