Simpler Dyneema Chainplates

[Update] I have written a lot about Dyneema standing rigging so I now have a guide to it all in: Dyneema / Synthetic Rigging Summary[End Update]

When I was coming up with the design of through deck dyneema chainstays and the update I had referred to this video from Free Range Sailing.

The key bits are are at 1m30s and 16m30s.

Since then I have had another idea, this should work for any boat that has strong bulwarks

I had ruled out exactly copying Free Range Sailing Sailing solution as it relied on being able to drill holes that went through the transom without going inside the boat. Therefore they didn’t need to worry about a waterproof solution. However, I have already adapted their solution for a drogue attachment. Now I am bringing the two ideas together.

I had another smaller concern, they have lashed the Low Friction Ring on with 4mm Dyneema and used knots to secure the lashing. Knots are not a good option for Dyneema, they are weak and can slip.

So I have a new design. Rather then keep the existing chainstay positions I am going to move them all slightly outboard to the bulwark. In this photo you can see a couple of shrouds attached to chainplates (circled in red).

Note that the bulwark here does not have the toerail cap fitted (and it still isn’t fitted). The bulwark is part of the joint between the deck and the hull. It is built really strongly and part of the problem we have at present is that the loads from the shrouds are not transferred into the hull but instead can lift the deck which is what has caused cracks (only in one place). Here is the a snippet from an original drawing showing the chainplates but we don’t have a drawing for the ketch rig and the mizzen chainplates are further inward away from the hull. Note that only the main mast cap shrouds (one per side to the top of the main mast) have the stainless steel strip bolted to a bulkhead for much greater strength.

So my new idea is to:

Preparation:

  • Drill 2 holes through the bulwark for each chainstay. They will slope down as they go from the inside and they will not go through to the inside of the boat. I’m thinking 25mm diameter at the moment.
  • On the outside of the hull for each chainstay I will fit a 10mm G10 (outside so no need to use the more expensive for fire-resistant FR4 version) backing plate. This will be attached to the hull with thickened epoxy.
  • Smaller holes will be drilled in the backing plate in line with the centre of the larger holes through the bulwark (large enough for 3 strands of 5mm Dyneema line).
  • I’ll plug the holes in the G10 and fill the holes in the bulwark with thickened epoxy.
  • Then I’ll drill the smaller holes from the outside through the middle of the thickened epoxy.
  • Next I fit a G10 backing plate to the inside of the bulwark with thickened epoxy (this is so the shrouds will clear the edge of the toe rail cap).
  • The holes are drilled from the outside through the inner sheet of G10.
  • The holes are very carefully smoothed, especially the entry/exit points which will be very rounded off (in the direction the load will pull the line)

The Chainplate

To avoid knots and to make for a quicker installation I will have a length of 5mm Dyneema with a locked eye splice at each end (4mm plenty for the Mizzen). Also one generously sized Low Friction Ring (suitable for a 5mm line to go 3x around the ring).

  • On the inside of the deck one eye splice is looped over the low friction ring (a reasonably tight fit but not very critical).
  • The other loop is passed through a hole to the outside, along the backing plate and back in through the other hole.
  • It goes around the ring and back out through the bulkhead then back through the second hole
  • The other eye splice is now looped over the low friction ring which is now held in place by more 3x the strength of an eye spliced 5mm Dyneema Line. That is approximately the same strength as a 12mm Dyneema line (but as our Dyneema Shrouds are sized for stretch rather than strength this is plenty to spare).
  • This Low Friction Ring is now lashed to the bottom of the appropriate shroud to hold the mast up.

If we feel that we can’t get the G10 and thickened epoxy smooth enough to avoid chafe on the lashing we could either line the holes with HDPE as Free Range Sailing did, or fit chafe sleeves to the Dyneema.

Conclusion.

Compared to the previous design this solution has a number of advantages:

  • No holes in the deck
  • No waterproofing challenge
  • Even easier to inspect and replace
  • Moves the shroud mounting points slightly outboard which
    • reduces loads as a more favourable angle
    • makes walking past on the side deck easier
    • moves them further from the sails reducing the potential for chafe
  • Less work to fabricate
  • Stronger and no need for any knees to connect the side deck chainstays to the hull.
  • With G10 backing plates epoxied to the hull on the inside and outside (so connected to both hull and deck) with the loads spread widely, the chainstays should be massively stronger.

Plus for anyone needing to re-rig the boat without taking the mast down then the new chainstays can be fully prepared and fitted with the original shrouds in situ.

This is one of the plans I said I was working on the other day.

4 thoughts on “Simpler Dyneema Chainplates

  1. Jacob February 20, 2021 / 1:20 pm

    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.

    Liked by 1 person

  2. dave42w February 20, 2021 / 2:46 pm

    Thanks. Really good questions Jacob. Think I’ll do a full post to reply to them properly.

    Like

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