Re: some thoughts on tendons

From: Ed Scott (
Date: Tue Apr 14 1998 - 14:04:47 PDT

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Subject: Re: some thoughts on tendons
Date: Tue, 14 Apr 98 14:04:47 -0700
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From: Ed Scott <>
To: "Wind Talk" <>
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>1) When the tendon gets old, it begins to crack horizontally around the
>small bolts that are holding it in the cups. The problem is that unless
>you take the universal apart you CANNOT SEE THE CRACK by inspection.
>The crack is hidden in the cup. I saw one that was cracked 2/3 of
>the way through, but still looked new from the outside, and had been
>used by the owner that day. It was due to fall apart in: a) one day
>b) one month c) who knows? It was replaced that minute, so we never
>found out how many more bends it had.

Mine split right across the bolt in the upper cup (that inserts into the
extension) I think. I still have the bottom half, so we can confirm this.

>2) There has been some talk that the little piece of safety rope that
>is supposed to keep the board and rig together is not strong enough.
>I looked it up, and it turns out that it is pretty strong. From the
>Marlow catalog, Marlow 8-plait pre-stretched polyester 4mm (that is
>the white line with the orange and black markers) has an average
>breaking load of 840 pounds (380 kgs). The 5mm is 1325 pounds (600 kg).
>>From the Samson catalog, the LST Yacht braid (various colors) 3/16
>(5mm) breaking strength is 1200 pounds. The Samson Trophy braid 3/16
>is 1150 pounds. 'Average breaking strength' is a bit vague, and they
>always test a new unworn piece of line, but it looks like the line
>is strong enough.
>I think the problem is in the knots. They never are under a load so
>they never get tight, unlike the knot at the end of a downhaul or
>outhaul. When the tendon breaks, the wet knot tied in slippery
>polyester just slips right out. I have used 2 vicegrips on a wet line
>on my universal and pulled the knots as tight as I could, and I bet
>I could walk out there now and just untie them with my fingers.

I think the Windsurfing Hawaii U-joints use black rope with red (?)
braid. I think my knots were secure. I checked them a few times, and
even, re-tied them a couple times.

Your figures might be correct for new rope, but for old rope that's been
exposed to the elements (UV and salt water) over a year or two (mine was
2 years old, I think), and friction from the insert holes probably wears
them thin. I replace downhaul lines which get worn, no reason not to
replace this line as well. Also, I'm not sure what the instantaneous
force is in a nasty wipeout from 8' with 190 lbs plus my rig moving at 30
mph coming to a complete halt. That's probably a lot of instantaneous
force, as opposed to constant force that are reflected in the specs. you


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