[Update] In the end we decided not to build chainplates that go through the hull. This is what we have ended up with Final Dyneema Chainplate design[End Update]
This is a rewrite/reorganisation of this post (no diagrams yet) to, hopefully, make it clearer.
I think this is our 3rd generation of Dyneema Chainplate design. We think it is a significant improvement on our previous plans, yet realistically it might demonstrate some “Lockdown overthinking” so we are not ruling out variations on our original Synthetic chainplate idea in: Chainplates. We are going for a radical dyneema option.
All these designs and the process is for us to work out what we are going to do on our boat. We are happy to share to give you ideas for your own boat, but you need to check your plans with appropriate professionals as we can take no responsibility for whether they will work for you.
Although our previous design was significantly stronger than what our Rival 38 has had for the last 43 years, we have come up with big improvements (with a lot of conversations, especially with Simon T who has a Rival 32). Specifically these are:
- More flexibility in deck position so they can be closer to the original chainplate position
- More flexibility in the positioning on the outside of the hull to avoid rubbing strakes, coving strips etc
- Simpler to build
- Ability to have tie connections down into the hull (as Rivals should really have had)
- Reduced possibility of chainplate lashing line chafe.
- Reduced friction when tensioning the rig, making it simpler to tune
The previous design already met these requirements, but they are worth repeating:
- A chainplate design ideally suited to synthetic shrouds, that eliminates the need for deadeyes and toggles to reduce the number of potential points of failure.
- A chainplate that is much stronger than the original Rival implementation. See Deck repair question and note that Cherry Ripe had one chainplate fail on a recent transatlantic trip (their YouTube videos haven’t caught up with that point yet).
- A chainplate that cannot leak, fully sealed with epoxy from the inside of the boat
- A chainplate that could be fully repaired with parts that can be carried on board
- A chainplate with minimal chance of hidden problems causing a sudden failure
We really need to define our own terminology so
- Chainplate structure: the permanently bonded structure between the hull and deck that the Chainplate Lashing will thread
- Chainplate Low Friction Ring: A standard Low Friction Ring that is the point to which the shroud is attached by a tensioning system.
- Chainplate lashing: a light (we are going to use 4mm) dyneema line used to hold the Chainplate Low Friction Ring by being routed through the Chainplate structure. At a minimum the strength of the total lashing needs to exceed the strength of the original stainless steel shroud.
- Tensioning system: we will be using a simple, thin dyneema line to lash the bottom of the shroud to the Chainplate Low Friction Ring. It will loop around multiple times to give a mechanical advantage and the end will be attached to more mechanical advantage (can be done with a winch or other means) to get enough tension into the shroud.
- Knee: a shaped piece of material (we are going to use 10mm G10 or FR4) that ties the underside of the deck down the side of the hull. It helps stop the tension of the shroud on the chainplate structure breaking or distorting the boat shape.
The greatest inspiration for me was this video from Free Range Sailing “Our sailboat REBUILD begins ⛵💪 – Episode 157“
They have an unusual situation for their backstays, when drilling the holes for the lashing they don’t go into the interior of the boat. A key design goal was to find a way to have a similar lashing for the low friction ring in places where it isn’t possible for a hole to go from the desk to the outside of the hull without going into interior. Also I didn’t want to have to add GRP matting to the outside of the hull. Finally, I was unhappy with the lashing as it relied on knots which is a problem with slippery dyneema.
Key Design elements
First we have a permanent structure to be built
For each shroud two plastic pipes will go through the deck (approx 40mm gap between them – scale as appropriate). On a Rival they can be in approximately the same place as the existing chainplate. They should line up with the shroud (following both the fore/aft and athwartships angles). Below decks they will curve to exit through holes in the side of the hull (still the same distance apart and level with each other). The pipes end approx 50mm above the deck.
A “backing” plate on top of the deck will provide reinforcement. It will make it easier to keep water out of the holes in the deck.
A “backing” plate on the outside of the hull will spread the loads and can be shaped to allow a very smooth curve for the chainplate lashing between the two pipes.
Inside the boat a knee will be fitted between the pipes. It will tie the deck to the hull and will extend down far enough to spread the loads over a large area of the hull.
Inside the boat the pipes will be encased in thickened epoxy. The will prevent any water intrusion. It will also create a single solid structure of deck, hull, pipes and knee to ensure loads are widely and evenly spread.
Second we have the connection for the shroud tensioning system
The permanent structure allows a lashing to attach a low friction ring above the deck. The shroud can be directly tensioned to the low friction ring using a lashing. As the low friction ring is lashed directly to the chainplate structure we eliminate a deadeye (with two thimbles) and a toggle – so removing several of single points of failure.
The chainplate lashing line starts at the low friction ring. Then it loops several times going through one pipe, across the outside of the hull, back through the other pipe and around the low friction ring. When there are enough turns for the maximum load the lashing terminates at the low friction ring.
Rather than use knots to tie the lashing at each end (which lose a lot of strength), terminate each end with an eye splice. These both loop over the low friction ring. Eye splices retain approximately 80% of the line strength. As all the loops of the lashing go over the “rim” of the low fiction ring the shroud tensioning lashing is held captive by the chainplate lashing. Therefore if the low friction ring breaks the shroud is still held captive. We can use eye splices rather than lashing knots as there is considerable flexibility as to how high the low friction ring ends up above the deck.
Third we have chafe and UV protection
The pipes extend approx 50 above the deck, their ends should be slightly flared. As they are slightly flexible they will automatically align (in a gentle curve) with the tensioned lashing so that chafe is minimised. The lashing can be easily inspected for chafe as it enters the pipes.
Extending the pipes above the deck also prevents dirt, particularly gravel, being washed into the pipe as this could quickly cut through the lashing.
The up-stand of the pipes allows a fabric sleeve to be secured at the deck so that everything from the shroud to the deck can be protected from dirt, chafe and UV. If the sleeve is a basic rectangle, with Velcro along it’s length, it can be easily removed to inspect both lashings and the low friction rings.
On the outside of the hull the backing plate can be filed and sanded to provide a smooth rounded route for the lashing to go between the two pipes.
Rather than rounding/smoothing the backing plate on the outside of the hull a plastic pad could be added to provide a lower friction, smoother route for the lashing.
A pop-on plastic cover for the hull backing plate would protect the lashing as it goes between the holes. This would protect it from being damaged by docks, dinghies and the sun. It could be removed to inspect the lashing.
Instead of a single low friction ring for the chainplate lashing it would be possible to use 2. One for each pipe. The two rings would not be directly connected together above the deck but only by the lashing going down through the pipes. The advantages are a) alignment with pipes would be slightly improved as the lines from the pipes only come together at the bottom of the shroud rather than at the chainplate low friction ring. b) two rings so each has half the load c) each ring will only have half the number of turns of the shroud tensioning lashing, so a little less binding and friction.
Rather than a single chainplate lashing line, for each shroud, it would be possible to use several, each would act in parallel. The first eye splice on the low friction ring, through one pipe and back through the other before the other eye splice goes onto the low friction ring. Each “turn” of the lashing would be a separate line. If one line chafes through, it will be very visible but the shroud won’t suddenly become slack. This method would require very consistent splicing so that the lines are very equal in length (although even the small amount of elasticity and creep in dyneema will tend to equalise small differences over time).
All the key potential chafe points for the lashing are easy to inspect as it is highly unlikely that the lashing will chafe first in the hidden but smooth run inside the plastic pipe. Instead chafe will come first a) where it exits the hull, b) where it exits the pipes at the deck, c) where it loops round the low friction ring, or d) where something rubs against it.
Replacing either the chainplate lashing or the shroud tensioning lashing should be straightforward, even potentially possible at sea on the appropriate tack.
The most difficult task will be replacing a pipe when it wears through (although plastics such as hdpe should be very wear resistant). There are a few options
- start with an oversized pipe so that a smaller pipe could be inserted through it later as a replacement (or have an fixed outer pipe and a floating inner pipe from the beginning)
- coat the pipes in a mould release agent during construction so that they can be removed (some ingenuity may be required to ensure that they don’t move during use)
- if the pipe fails then use a dremel with a flexible attachment to sand the route through the thickened epoxy so a pipe isn’t needed (a short length of pipe could be inserted at the top to provide the gravel protection).
Our construction details
We are hoping to use HDPE pipes, they should be low friction and hard wearing. However, the smallest I have found them is a 20mm external diameter. Maybe inserting a smaller sacrificial tube inside them would be a good solution or a different type of plastic?
I’ll use a heat gun to flare the top of the pipe to make sure the lashing doesn’t get damaged by the edge.
We will use the same dyneema line for both the shroud tensioning and the chainplate lashing to reduce the number of items we need to buy and carry.
Our main mast cap shrouds are the only ones with a chainplate that has a connection to a bulkhead. So some detailed thought will be needed (one pipe each side of the bulkhead?)
Our chainplates are in the deck and are close to the bulwark so the internal intrusion will be small. This solution may not be the right one if you have very inboard chainplates. In that case look at my original “padeye” design.
I’m going to use 10mm G10 for external chainplates and 10mm FR4 for the knees (I want first resistance inside the boat).
All holes in the deck and hull should be sealed with thickened epoxy (drill oversize hole, fill with thickened epoxy, when cured drill correct hole through the epoxy).
When drilling the final holes angle the drill to reduce the curvature of the pipes.
Our holes and backing plate in the hull will be a bit lower so that they are below the rubbing strake. You might want to miss things like cove lines.
Our cap shrouds have a piece of stainless steel bolted to the bulkhead that has a bent over top that sits under the backing plate. It has a hole fitted over the chainplate bolt and so when the nut is on the chainplate bolt is connected to the bulkhead. This will be replaced (on all our chainplates) by the FR4 knee. The top edge of this shaped piece of sheet material will be fitted to the underside of the deck and the long edge will fit vertically down the inside of the hull. In our case it will go down far enough to “hook” over the first horizontal stringer. The inner edge of the knee doesn’t have to be a straight line but can be cut away as a nice organic curve. The best place for the knee is between the two pipes. It should be glued in with thickened epoxy with good fillets along all the edges that touch the boat.
It is going to be tricky to fill around the pipes and knee with thickened epoxy so that there are no air pockets. My current plan is to create an enclosed space that I can fill (using thin plywood held in place and “sealed” with epoxy fillets). So the plywood is creating a kind of mould covering the pipes and part of the knee. Before I fit the deck backing plate, I will drill some extra holes in the deck and inject into them slightly runny thickened epoxy, until it is full to deck level. Once they are filled these holes will be covered by the backing plate. I can remove the plywood to confirm that the space has been properly filled.
The strands of the chainplate lashing are going to be under a lot of tension between the two holes on the outside of the hull. So it is vital that the route out of one hole and into the other is very rounded and very smooth. That transition from pipe to backing plate is going to be the key load point of the lashing, so it is vital that it does not chafe through here. We are going to carve a solid plate of hdpe (we will make ours as part of our plastic recycling work) that will sit on the G10 plate and be a very low friction, smooth, curved surface for the line. We will also fit a removable hdpe cover plate to protect the chainplate lashing from being damaged by docks or anything else.
Fitting the lashing
When you are ready to lash the Chainplate Low Friction Ring into place you have a choice. You can use a single ring per chainplate structure. Or for slightly higher cost you can use two. Having two improves the alignment of the chainplate lashing slightly and makes tensioning a little easier. If you use one then you need to size it so that the outer sheave can fit 3 turns of the lashing line rather than 2 (the number of turns depends on your calculation of loads and the line you are using – I’m planning to have 6 of 4mm, 3 per pipe, which is quite a lot stronger than my shroud).
Whether you use one ring or two your chainplate lashing needs a eye splice at each end designed to loop over the exterior of the low friction ring.
If you are using One Low Friction Ring then:
With the low friction ring above the pipes fit the eye splice from one end of the lashing. The other end goes down one pipe to outside the hull. Then in the other hull hole and back to the deck. Now loop it over the low friction ring and go down the first pipe again. From the hull outside return as before. Repeat for another loop through the chainplate structure. At this point the low friction ring should have one eye splice and two loops. Each pipe will have 3 lines through it. The outside of the hull will have 3 lines between the holes. Now slip the eye splice from the loose end onto the low friction ring (4 lines in total on the top of the low friction ring). While holding up the low friction ring up, tidy all the lines so that they don’t cross over outside the hull and as little as possible in the pipes. You can now use the tensioning system to connect the chainplate low friction ring to the shroud.
If you are using Two Low Friction Rings then:
With the first low friction ring above the pipes fit the eye splice from one end of the lashing. The other end goes down one pipe to outside the hull. Then in the other hull hole and back to the deck. Now loop it over the second low friction ring and return down the same pipe again. From the hull outside return up the first pipe and over the first low friction ring. Back down the first pipe, outside the hull and up the second pipe. Now slip the eye splice from the loose end onto the second low friction ring. While holding up the low friction rings up, tidy all the lines so that they don’t cross over outside the hull and as little as possible in the pipes. Each low friction ring should have 1 eye splice and one loop of lashing line. Each pipe should have 3 lines. Each low friction ring should have 3 lines all going into the same pipe. The outside of the hull should have 3 lines. To tension the shroud the tensioning lashing should start from one of the chainplate low friction rings, go up to the shroud and down to the other chainplate low friction ring. Continue to add more turns, alternating between the two chainplate low friction rings.
Tensioning the Shroud.
There are videos from Rigging Doctor, Tula’s Endless Summer and Sailing Zingaro showing how to tension using a lashing. Although, of course, we have our own ideas which we hope to test before too long.
We believe this design will create
- a chainplate structure that you can easily build yourself.
- something that you can visually inspect for wear and damage.
- something with no metal parts that need to be fabricated, that can corrode or fail due to fatigue.
- where you can carry all the parts to replace everything that works with the chainplate structure (lashing line and low friction rings, shroud tensioning line, shroud) even at sea.
- a significant weight saving
- a significant increase in strength on boats like Rivals where the chainplates are not very well connected to the hull
- huge cost savings if your chainplates or turnbuckles need replacing
- a very simple, strong, neat, light way of connecting dyneema shrouds to an older boat