Bow shackles at chainplates for dyneema lashings

[zachduckworth]

Hi,

I’m designing a DIY dyneema re-rig of my Islander Bahama 30. Looking at using bow shackles as the lower end of the lashing system. The highest quality forged 1/2” pin bow shackles have a 3000 lbs working rate. This is far below the rating of the dyneema. My boat has 7/32 standing rigging per original specs, which has a min breaking limit of about 5550. So at 20% about 1200 lets say. And I keep my shrouds closer to 10 to 15 percent breaking. That’s to say that the baseline load on the shackles will be a little over 1/3 of their breaking strength. My current turnbuckles (16 yo old Nactec 500s) are on 3/8 pins in a 1/2 “ hole in the chainplate and have been fine so far. In other words the pin is the weak point in this design from what I can see and would actually be stronger than what I have now.

Does this sounds like a good plan?

[Benz]

The published working load limit on shackles varies with intended use: between ones for industrial lifting, overhead use, and marine use such as anchors, the WLL may be 20% to 50% of break load.
It makes engineering a bit of a nightmare. Everything should have the break load posted, and let people engineer from there.
But your shackles should be fine for the purpose.