Spectra Lashing

[Bob Pingel]

I am working on a boat with running backs, the boat is a Nelson Marek 36. The actual backs are commerical covered Kevlar, and terminated into stainless eyes. There is then a short strop the connects a runner block. The block is tensioned with a 2:1 tackle to a winch.

My question involves the strop. It is 3' long and is currently made of 3/8" bare Spectra. There is an eye on each end. The eyes are formed with 3 brummels and then a 10" bury.

I need to replace these strops.

I see three options:

Duplicate the existing. This makes me nervous because of the short bury. I know the three brummels pick up a lot of load, but the bury seems way too short. Without a long overlap, I can't get even close to 96 diameters. Am I being too conservative on the bury?

Create a grommet of the right length. I think I can do this without breaking any rules.

Do a spectra lashing with say 1/8" 12-strand spectra. I would make a small eye splice and ring hitch to the top eye. I would then take maybe 8 turns. The finish is my question. I don't like the idea of half hitches in bare spectra. I could splice another eye and slip it onto the block the bottom of the lashing (the head pin is removeable) - after it is in place, I would balance up the turns to equalize load on all the parts. Regardless it seems right to place a seizing on each end of the lashing to hold things together.


Any ideas?

Thanks,

Bob

[NickfromWI]

I say stick with what has worked in the past. Sounds like it'd be the easiest to recreate, as well.

(this is my first real post here!)

love
nick

[Brion Toss]

Hi Bob,
Sorry to have been so long in replying --- way too much rigging here lately.
The first option is to stay with what you have, or something like it. Multiple Brummels, properly done, will hold close to 80% of rope strength, without any bury, so given a good diameter of rope what you have, though a bit rough-looking, is probably fine.
A true grommet, with a locked Brummel to start, would be significantly stronger of course, and would look much neater.
The lashing might be strongest of all, even with the same total rope mass, as it could have no significant splice distortion, given that a splice would only be at one end, and the multiple turns would share the load. It is also possible to make the lashing quite handsome. As for the effects of knots on strength, the turns and friction mean that you can have very, very little load on the bitter end, so you can pretty much secure it as you see fit.
To make the lashing I'd put a couple of round turns on to set the length, then apply a bit of load, and make the rest of the turns, keeping them neat, and finish with a French-whipping-like series of half-hitches.
No matter which option you settle on, see if the current strop length is really what you want. Of the three options, I like the grommet the best.
Fair leads,
Brion

[Bob Pingel]

How would the long cordage length effect elasticity. Each part is lightly loaded, but would the whole thing stretch or just the length of the lashing?

Say we have a material that stretches 1%, the lashing length is 30", and the total material in the lashing is 300". Would I see 1% of 30" or 300" as stretch? I would guess that the stretch would be closer to 1% of 30", but I can't quite see the physics involved.

thanks,

Bob

[Bob Pingel]

Since lashing is tightly bound and from the friction of the pins, the theoretical 1% stretch would only occur over the lashing length.

Did I miss anything?

Bob