Spectra lashing instead of moused shackle?

[whimsy]

I've got lines I want to fasten at my masthead using existing fittings. My mast is a carbon fiber pole with a thick aluminum plate on top. The fittings are padeyes through bolted to the bottom of the plate, and ended up with limited clearance between the padeye and the mast. The lines have spliced eyes with SS thimbles.

I found that the shackles I intended to use were too big to fit through the gap between the mast and the padeye. Now I'm looking at smaller shackles, but they don't fit very well through the eyes I spliced on the line, and probably aren't as strong either.

I'm wondering if I could do a lashing with 1/8" spectra cord and get similar results as I would with a moused shackle. It doesn't need to be something quick to undo like the soft shackles I've been reading about--I hope to leave them up with no cause to remove them for a decade.

I don't want this to be the weakest link here, and don't have a compete set of specs, but here's what I can find:

I believe I can buy the same schaefer padeye today, with a 750 lb SWL.
The line is rated at 7,700 or 5,100 lb breaking strength. (two different sizes)

(Yes, I know I'm mixing working load with breaking strength...and don't know what safety factor was used...but they are equivalent around strength if the factor is 7~10. Dunno, but I put them in the same ballpark, with the rope probably being stronger)

If I get 1/8" amsteel it has a 2300 lb (min) breaking strength.

I figure that a lashing with two turns (4 segments) should have 4x2300 lb breaking strength, reduced a bit by any knots and tight turns, but still likely as strong as the other parts. I'm not quite sure how to do that analysis on a lashing, but I figure that three turns (6x2300) is pretty safe.

I've not tied a lashing quite like this before, so I could use suggestions here as well.

[whimsy]

Thinking about it and looking for answers, perhaps a soft shackle would be just the thing to use.

Any other recommendations?

[Stumble]

I am not sure what you are trying to describe. If it is just a normal mast head crane, there is no need to run the shackle through the mast. Just run a chase line, then pull the tail of the halyard. If not, can you post some pictures or a drawing? I am having a hard time visualizing what you are trying to do.

[teknocholer]

I did something similar with the topping lift on my Nonsuch 26. The padeye at the end of the wishbone (boom) is on the outside of the end casting, so that the shackle was levered sideways against the casting. To get a better lead, I used a Dyneema link.

I made a loop of 3mm (1/8") Dyneema, about 36'' in circumference, joining the ends with an end-to-end splice. This was doubled and put through the padeye, then the end loops were brought together and attached to the shackle. This gave 8 parallel turns of cord with a breaking strength of 2200 lb/turn. I wrapped the whole bundle in rigging tape to reduce chafe.

This method means you would have to use shackles to attach the lines to the Dyneema link, but you have the shackles already, and it's neater than knots, with no chance of working loose.

The setup is in place for about 5 months of the year, then disassembled and inspected before being put away for the winter. The cord is going into its fourth season with minimal chafe, but I don't know if a lifespan of a decade is realistic. I certainly recommend regular inspections, although that's harder in your case than mine. At any event, the cost and effort of making a replacement are minor.

[Brion Toss]

Hello,
First, the standard safety factor in recreational sailing gear is a mere 2:1. It is absurdly easy to exceed the strength of steel shackles, either with lashings or soft shackles. Cheaper, too, as a rule. The only downside, if you can call it that, is that lashings and soft shackles are skill-intensive; you have to know how to distribute the load among turns of a lashing, and how to start and finish securely, for instance.
Fair leads,
Brion Toss