Back to mounting an under-deck backing pad for sheet winches: hard-setting epoxy, or a soft-setting adhesive?

[NealB]

Differing views around the boatyard this afternoon .......

I'm going to mount a 12mm marine ply backing pad under the cockpit coamings, to mount larger sheet winches.

Is it better to fix the ply to the underside of the coaming with a hard epoxy (and, if so, is plastic padding epoxy filler suitable?), or, as some suggest, would a soft setting adhesive, like butyl, be more sensible (in that it would allow for a bit of flexing in the grp)?

 

[thinwater]

A hard no on butyl from an engineering perspective. It will creep under load and not transmit the force evenly to the deck. A good sealant, but not for load transfer.

A softer adhesive, such as Sika 291 would be fine (it will not flex under the range of force it will see). Or epoxy. But there is no benefit to flex. You want the force to be shared equally, and any flex reduces the force transfer.

The best backing plate would be to layup more layers of glass, taper into the hull. No actual plate, no stress riser, less weight, a permanent part of the boat. You see this a lot on quality race boats for high load items, like winches. Next best is pre-laminated fiberglass plate, with rounded corners, tapered edges, and bonded with epoxy. Part of the hull.

If you are using plywood, Sika 291 or equivalent is as good as anything. Thickened epoxy is fine, including Plastic Padding Epoxy Filler.

 

[RunAgroundHard]

Download this manual and read uo about mounting hardware: -

Fiberglass Boat Maintenance | WEST SYSTEM Epoxy

Soft mounting a winch is not a good idea in my view.

 

[Neeves]

We did as Thinwater suggested but, 2 layers of heavy weight glass then stainless or aluminium plates bonded to the new glass. If you are using stainless its worth the effort to polish them first - they look better. Pre drill the stainless in advance. Use polished, or shiny, dome nuts or you could use stainless inter screws for a really professional finish.

If you do it carefully it looks 'original' and does not look like a lash up.

Polishing the plates is something to do in you long winter's nights.

Jonathan

 

[Channel Sailor]

Plywood I would say is rather soft, even with penny washers. The washers could dent the plywood over time, even just when tightening the bolts. If the ply was epoxy lay-up coated to give it a rock hard surface, then the washers would not dig in. How about a stainless steel plate instead. Between the backing plate and the deck I would use a thin layer Sikaflex 291i or similar, just to spread the load nicely across what could be a rough surface under the coatings. It can be a thin layer because the force is compression not shear, Butyl I would say is far too soft. how about epoxy coat the plywood pad and a wrap or two of a weave fabric to get the hard surface, the use a then layer of epoxy resin stiffened with micro balls to bond the pad to the underside of the coaming.

If the coamings can flex then maybe the layup in them is not strong enough for the winches anyway.

useful info would be the winch size and is the coaming layup cored. I think my winches are designed for 600kgs working load, but visualise if they were accidentally used for a mooring line with snatch loads or a towing bridle in an emergency.

On my retrofit winches I have custom built epoxy layup backing pads shaped to suit the angles under the deck, stuck on with Sikaflex 291i. Bolted down with penny washers as well.

 

[thinwater]

A note on penny washers. At least in the US standard weight (what we call fender washers) penny washers ALWAYS bend under load, over time. What you want are double thickness washers, which curiously, no chandlery carries, even through they are the ONLY ones that should be used for backing on boats. Odd.

 

[Concerto]

Neal, your Westerly Falcon has solid glassfibre on the cockpit coamings. They were laminated to accept winches without the need for a backing plate/plywood. On my Fulmar I moved the genoa winches to the cockpit coaming from the aft end of the coachroof 9 years ago. Also I retain maximum sail area for any conditions and love heavy weather sailing. The winches were secured with just penny washers to spread the load and they are rock solid. They were set on butyl rubber and after the initial tightening, I have only needed to do one minor clean up as they bedded in and never leaked. Do not use any sealant that is an adhesive as this will cause problems in the future.

 

[NealB]

Thanks for all the replies: a similar range to those in the boatyard.

As Concerto says, the decks are absolutely solid: there's no sign of even tiny flexing, for example, if you jump on them.

The old sheet winches had what looked like butyl sealant on the base, and were therefore fairly easy to remove, but there were no signs of leaks or movement. They were just fixed by six through bolts, washers and nuts.

I was only thinking of backing pads as I'm no engineer of any sort, so it feels like belt and braces.

All good food for thought: thanks again.

 

[Channel Sailor]

Penny washers, yes I too have noticed there appears to be often for sale rather thin penny washers. I found a specialist supplier somewhere that sold me thicker ones, I don‘t recall who it was.

 

[bignick]

A note on penny washers. At least in the US standard weight (what we call fender washers) penny washers ALWAYS bend under load, over time. What you want are double thickness washers, which curiously, no chandlery carries, even through they are the ONLY ones that should be used for backing on boats. Odd.

View attachment 166713

While I agree with everything you have said, your photo isn’t showing a cupped washer, it is showing that the load bearing area isn’t big enough and the grp (or more likely the core) has distorted and the entire washer has sunk.

 

[bignick]

If you are going to use a plywood pad, rather than a piece of grp as recommended above, make sure you coat the plywood first with normal unthickened epoxy. Plywood can soak up more epoxy than you expect and if you don’t coat the wood first then you can end up with a poor bond between the plywood and the grp laminate.
Theres a good video on it on YouTube by Andy at boatworkstoda:
How To Fiberglass Over Plywood | Boatworks Today

 

[bluerm166]

Sikaflex 292

 

[Refueler]

Persona;lly ? Alloy plate with tapered shape ... such as an elongated oval ... pre-drilled ...... epoxied in place ....

 

[thinwater]

While I agree with everything you have said, your photo isn’t showing a cupped washer, it is showing that the load bearing area isn’t big enough and the grp (or more likely the core) has distorted and the entire washer has sunk.

Actually, look again, it's both. But I agree with you.

I also have many other pictures of bent washers and many bent washers in my tool box.

It is always some of both. If the surface was strong enough it would not dent, and the washer would not bend, and you wouldn't need the penny washer. If the washer were strong enough, it would pull into the surface without bending. Standard penny washers are a waste of money that lead to problems later. They should not be sold for load distribution on boats, only double thick washers. And it takes 4 standard washers to replace a double thick washer.

See below.

• Double thick fender washer not bending at same tension as bent washer below it (balsa core, 3 x 1708 skin).
• Bend washer on a brand new boat show boat.

 

[MisterBaxter]

Given the above input on the construction of the boat and on bendy washers, I might be inclined to go for a stainless steel plate with just a thin smear of epoxy thickened to a heavy syrup consistency under it.

 

[Refueler]

Given the above input on the construction of the boat and on bendy washers, I might be inclined to go for a stainless steel plate with just a thin smear of epoxy thickened to a heavy syrup consistency under it.

It does not need to be a laminating epoxy .... Araldite or similar is fine ... its just really to fix the plate in position.

I use 5 minute two part epoxy for similar jobs ... quick so it holds plate in position etc.

 

[NealB]

Given the above input on the construction of the boat and on bendy washers, I might be inclined to go for a stainless steel plate with just a thin smear of epoxy thickened to a heavy syrup consistency under it.

Thanks, MisterBaxter!

I'm slightly confused though (easily done, I must admit) ........... the only post in this thread, that refers to the construction of my boat, is Concerto's #7, where he says, "Neal, your Westerly Falcon has solid glassfibre on the cockpit coamings. They were laminated to accept winches without the need for a backing plate/plywood".

I can confirm that my cockpit coamings are solid grp, and also that they are very ...... errrr..... solid.

 

[Refueler]

Thanks, MisterBaxter!

I'm slightly confused though (easily done, I must admit) ........... the only post in this thread, that refers to the construction of my boat, is Concerto's #7, where he says, "Neal, your Westerly Falcon has solid glassfibre on the cockpit coamings. They were laminated to accept winches without the need for a backing plate/plywood".

I can confirm that my cockpit coamings are solid grp, and also that they are very ...... errrr..... solid.

Fine ... but I would always have that nagging thought - should I have put a plate ....

 

[NealB]

Fine ... but I would always have that nagging thought - should I have put a plate ....

Yes .... good point: you must know me rather well!

PS Interestingly, though, one of the views expressed at the boatyard, yesterday, was by one of their two very experienced shipwrights. His advice was along exactly the same lines as Concerto's.

 

[doug748]

Thanks, MisterBaxter!

I'm slightly confused though (easily done, I must admit) ........... the only post in this thread, that refers to the construction of my boat, is Concerto's #7, where he says, "Neal, your Westerly Falcon has solid glassfibre on the cockpit coamings. They were laminated to accept winches without the need for a backing plate/plywood".

I can confirm that my cockpit coamings are solid grp, and also that they are very ...... errrr..... solid.

I would agree with Concerto. The bolts are mainly in shear, they won't let go from a sound, solid GRP base. I once saw a video of a whole section of combing ripped off with the winch still bolted to it

Sometimes I make up stainless plates where it is more convenient or makes a neater job, but when I replaced my winches I just reused the bolts and washers.

.