Hello Bill Kinney,
Thank you for your detailed and informative post.
My broken line is 16mm polyester double braid. Alpha Ropes states its breaking strength is 12,125 lbs.
This line broke at 39 knots apparent on a 54 sq m sail, so, per Harken's:
581 sq ft * 39kn ^2 * .00431 = 3,809 lbs
So my Alpha Ropes broke at 1/3 of its rated breaking load.
Possible causes you mention:
Tight against sharp edge: it broke at the turn of the block on top of the genoa car track. This is an unmodified factory original Amel track car and block, used in several hundred Super Maramus.
Chafed or damaged: the cover on the broken line looks smooth and good as new. Its twin on the other side looks smooth and good as new. No detectable chafe.
Bowline: the failed line broke 1.6 meters away from the tension point on its knot at the sail clew.
I am afraid this leaves "manufacturing defect" as the only other choice from your e-mail.
Oh well! I guess no more Alpha Ropes for this boat!
SM2K Nr. 350 (2002)
At anchor, Kea
---In amelyachtowners@..., <greatketch@...> wrote :
This is WAY more information on jib sheets than any sane person could want... but we are sailors, so by definition our sanity is suspect!
Let's refresh our memory about what the original factory spec for Super Maramu Jib sheets was from Amel:
16mm (~5/8 inch) polyester double braid. Nothing fancy. Amel used low-stretch exotics on their boats where it mattered, and avoided them where it did not.
Line of this type, at best, has a typical breaking strength of 16,000 lbs, and 41 meters of it will cost about $350. There is no need for the strength (and cost!) of dyneema in this application. The reason to spend money on exotics like dyneema is they allow you to downsize the line, keep the strength and lose the weight, OR keep the size and reduce stretch to a bare minimum. Neither is something we really care about in this application.
Just to put us in the ballpark for what we need, Harken has published a formula for estimating the genoa sheet load based on sail size and wind strength.
SL = SA x V2 x 0.00431
SL = Sheet load in pounds
SA = Sail area in square feet
V = Wind speed in knots.
For a 700 square foot genoa, fully out, in 40 knots of wind, the predicted sheet loads are about 4800 pounds. 50 knots bumps this up to 7500 pounds, and I very much doubt the sail itself is strong enough for much past that.
It is important to understand that jib sheets are not (usually) selected based on breaking strength. They are selected for ease in handling, and for most boats, anything that is big enough to handle comfortably is way, way, more than strong enough.
Certainly if you want to use something bigger than this because it is comfortable to handle, go ahead--it's not a huge deal. But understand that bigger lines do have a cost other than just money. The extra weight means the sail take more wind to "fly" and it will cut the boat's light air performance.
Anytime a line breaks in the middle, we can assume there was a reason other than simple load. It might have chaffed or damaged in some way, there might be a manufacturing defect, it might have come tight against a sharp edge. I can say this with confidence, because even the best tied bowline will reduce the strength of the line by at least 40%, and will ALWAYS be the weak point. With exotics (dyneema, spectra, etc) the knots are an even tougher problem. The lines are "slippery" and don't take to the crushing loads in a knot well at all. To keep anywhere near the full strength, they need to be spliced. If you tie a knot, it's likely you don't have any more real world strength than you would have had with a polyester double braid!
For what it is worth, I do not use a bowline on my jib sheets, rather I use a single length, and larks head the line to the clew of the sail. It is not any stronger, but it never hangs up on the shrouds on a tack. You can't always do this with exotic lines because they can be slippery enough they slide under heavy load.
Bar Harbor, Maine.