In my last post (Deck repair question) I was writing about the inadequacies of our chainplate and particularly of the backing plate that have caused the only cracks in our deck.
I shared it on the Rival Association’s private FaceBook group and got some really helpful responses. It seems that chainplates are generally seen as being a bit feeble on Rivals, although I have not heard major tales of woe, more a feeling that they are out of sync with the quality and robustness of everything else.
Having looked at some of the suggestions and had a long discussion at home. That being one of the discussions where I get into trouble for using “vague” words like strong, pull and push – comes of being married to someone who trained as a Civil Engineer.
So as we look to ensuring we get no more deck cracks, definitely no falling masts and no holes in the deck this is where we are now at.
- We will remove, clean and inspect the bronze chainplates (really just a bolt with an eye on the top and a flange that sits on top of the deck, while the bolt goes through and has two nuts to lock together). From others who have done this and one person who destructively tested one by cutting it through in multiple places – we expect them to be sound.
- The hole in the deck will be drilled larger, the core checked, any damp bits removed and then filled with thickened epoxy. A replacement hole the right size will be drilled through the middle of the epoxy.
- We are then going to build in situ a backing plate with knees out of 10mm FR-4 (see very professional model below)
We read an excellent article on backing plates at PracticalSailor and are completely sold on using Precast Fiberglass, frequently known at G10 although the fire resistant version FR-4 seems to be more easily available for us. This is standard glassfibre cloth with an epoxy resin but is made at high pressure so is very dense. Especially when bonded to a surface with thickened epoxy (which makes it a very even joint, smoothing out any irregularities to spread loads evenly) they say it makes an excellent backing plate. Moreover they also noted that “A fiberglass-reinforced backing plate bonded to the laminate provides considerable sheer strength; if not bonded, backing plates should be seen primarily as reinforcement against tension or compression-i.e. loads that are in-line with the bolt.”
Our understanding is that a common way to have a chainplate tied to the hull (so that the deck doesn’t lift) would be a custom length of stainless steel bolted to a bulkhead (or knee) that is “tabbed” to the hull. By tabbed we typically mean first butt jointed with thickened epoxy and then layers of fibreglass with epoxy resin creating a wide bond to the hull. That is because the epoxy fillet used for the butt joint is far stronger than the small area of fibreglass hull. So the failure point would be to for it to come away along with the outermost layer of fibreglass cloth.
We don’t want to spend money on custom stainless steel to connect our bronze chainplate to a new knee (and anyway think that mixing metals is a bad idea due to potential galvanic corrosion). We also want solutions we can work with ourselves and preferably that are not too labour intensive (we want to be on the water sailing).
What we figured is that we can take advantage of the fact that FR-4 (or G10) provides good sheer, tension and compression strength if bonded to a laminate AND that you can make strong epoxy fillets to join FR-4/G10 as the material won’t delaminate.
So we can save ourselves the mess and work of using fibreglass cloth this way:
- Drill FR-4 backing plate for chainplate bolt.
- Bond backing plate to underside of deck with thickened epoxy.
- Hold tightly in place with chainplate bolt (coated in vaseline so epoxy does not stick to it).
- Use thickened epoxy to bond a similar “backing plate” to the hull just below the backing plate (if there are lumps and bumps or bolts for the hull deck joint it does not matter, choose a spot that avoids them, the two plates do not need to touch each other). Use enough epoxy to ensure an even bond despite any hull curvature. This is going to spread the load over the hull just as would normally be achieved using layers of fibreglass cloth. But with much less labour, less mess and needing less space.
- When it is all cured, remove the chainplate bolt and refit with sealant (either butyl tape if you want to be able to remove it or sikaflex sealant if not). Leave to set before tightening fully.
- Trim a couple of FR-4 triangles to act as “knees” connecting the hull plate and the backing plate. By doing this last you can ensure a good fit despite the fact that in boat nothing is level, flat or parallel. Then use epoxy to butt joint these in place, one at each end of the backing plate. Once held firmly you can apply neat epoxy fillets to both sides of each triangle butt joint.
For the cost of one extra FR-4 plate and some thickened epoxy for it, you should now have a the hull and deck tied together so that the chainplate bolt cannot lift the deck causing it to crack. Plus there are other advantages
- you have avoided any possible galvanic corrosion,
- you have avoided needing to have any custom stainless steel parts made
- you have a technique that you can do yourself even at sea with the normal repair materials and tools you will have to hand (spare backing plates, epoxy resin and thickener, butyl tape, hand saw, sandpaper)
- you have saved the mess and time of fibreglass work
- the solution is compact and adaptable to tricky spaces and difficult access.
So far we think this is a great idea. Anyone want to puncture our ego’s?