Today I have had a couple of really helpful and thoughtful comments from Jacob. So much that I think it worth responding with a post.
Having extras in the case of the DIY tangs, is a good idea. I do not mean to discourage your idea, but the tensile strength of UHMWPE will surely make the tang the weak link. On another note, if stainless steel sailmakers thimbles will work you, USStainless.com has a 12 mm (M12)
So things have moved on a little. Currently we are looking at using recycled HDPE (see Transforming waste with DIY Plastic recycling) so we should be able to make new tangs anywhere in the world from waste plastic. I think HDPE sounds better for this application than UHMWPE eg creep under load. They can be made by melting the plastic in a grill or oven or using a hob (use a pyrex bowl in a bath of oil to get the temperature high enough) and then a simple home made mould (eg a drainage tube with a clamp to provide pressure to a a plug sliding inside the tube).
My idea to make the tang out of plastic is intended to have a number of advantages over my first idea of stainless steel tube and washers( see last but one paragraph of Termination of Dyneema Shrouds. The most contentious issue?):
- Cost (especially if we make them from recycled HDPE)
- Reduced chafe of the Dyneema Eye splice due to smoother transition to ears which will stop the eye splice sliding sideways off the tang.
- Increased strength of the eye splice due to the larger bend radius
I’m now thinking of a further refinement. We could put a 25mm Stainless Tube onto the bolt. Then a 25mm hole in the HDPE tang which fits onto the tube. This way the HDPE can’t be “sawn” through by the bolt thread and there is a much larger bearing surface for the HDPE. If the HDPE does wear through then the dyneema will still be around a smooth 25mm tube.
I really like your idea and have thought of doing a similar set-up on my boat, too, a Southern Cross 31. I was thinking of using 10 mm Dyneema with a stopper knot through the bulwark, with an aluminum bronze or G10 backing plate running the length of the bulwark that would have my shrouds holed through. This is for distributing the load a bit better. Then eye splicing the other end around a frictionless ring or sailmakers thimble. I am worried mostly about the load on this part of the boat that previous had a different/lesser load on it. Are you worried about this? That is the bulwark maybe not having the structural integrity to deal with the greatest loads your boat might need to withstand.
This is really helpful. I do think the application is going to need to be customised for every boat as there are such wide variations in the positioning of the chainplates and the structure of the hull/deck joint and bulwark, if there is one.
The chances of a stopper knot slipping worries me, also how much weaker a knot is than an eye splice. Yachting Monthly found the Dyneema strength reduce to 35% of the original by an overhand knot.
So if using a knot I’d put it through an eye splice to stop the slipping, as in this video https://www.youtube.com/watch?v=jU_mmdbQeCQ
However, I think my solution is easier and stronger. A length of dyneema with an eye splice at each end. First eye over the low friction ring. Then out of the bulwark through one hole, back out through another and put the second eye over the low friction ring (I’ve thought I’d go around the low friction ring and out/in the bulwark one extra time). The ability to get a larger low friction ring that can take two eye splices is a key reason for me moving from a Stainless Steel thimble.
As for the bulwark backing plate, this is where construction varies such as lot as does the height of the bulwark and the position of the chainplates. I included a sketch of our thinking in my Dyneema Rigging Summary post.
My understanding is that bonding the plate with thickened epoxy is going to distribute loads much more evenly than bolts ever will (providing the material behind it is well bonded together). Also that having two holes for the lashing to spread the load is better. Also a horizontal spread shares load better than vertical, however, I would have thought that the load will reduce very quickly with distance from the hole, so a lot of the full length backing plate won’t be helping at all.
We have a “decorative” rubbing strip quite close to the hull/deck joint. This means a full length backing plate would be very thin and a bit high. So we will cut sections out of this for more triangular backing plates.
I’m pretty happy that due to the construction our bulwark is strong. However, between internal and external backing plates I think it will often be possible to get the strength you need. If in any doubt about the quality of bonding to the hull a bolt or three should make a very strong connection between inner and outer backing plates (but at a cost for inspection and potential waterproofing and corrosion issues.
If you are concerned about moving away from a chainplate bolted to a bulkhead transferring the load down to the hull then maybe look at internal G10 knees bonded to the hull and bulkhead.
As for the material of the backing plates. My preference after reading a Practical Sailor test of backing plates if for G10 epoxied into place (also easier to process to get a really smooth path for the dyneema).
Hope this helps 🙂