What thickness of backing plate should I use?

[daviddb]

snipping<< The rule of thump I have used is that a...>>snipped. Ish.

I have no views worth a light over and above the views already expressed - but I do habitualy go by the Rule of Thump for she is an excellent rule.

 

[JumbleDuck]

I have no views worth a light over and above the views already expressed - but I do habitualy go by the Rule of Thump for she is an excellent rule.

A happy typo, I think. Give it a thump, and if it doesn't fall off you're good to go. I shall use that again!

 

[Otter]

I don't like the idea of the hardwood plug and would suggest that closing the vent hole with laminates of fibreglass, adding thickness if you think that this is necessary, would be much better.

Thanks for this and the other replies. Why no hardwood plug as some have suggested?

 

[pvb]

A bolted-on stainless disc isn't going to be very attractive. If you're able, a better solution would be to taper the edges of the hole underneath, fill the hole with fibreglass laminate, spreading the area over the taper underneath, then perhaps epoxy a ply backing plate underneath, and bolt the winch on. The stresses are primarily in shear, so it will be strong enough.

 

[PuffTheMagicDragon]

Thanks for this and the other replies. Why no hardwood plug as some have suggested?

(1) You will be relying on the bond between the hardwood and the existing laminate. Unlike a proper homogeneous lay-up, this is largely unpredictable.
(2) Hardwood reaction to varying levels of humidity, mainly expansion and contraction, will be different from the existing structure.
(3) A re-build of the hole, using the same material as the matrix, is the proper way. Putting in a plug instead is, with respect, a botch-up.

Of course, all the above is IMHO.

 

[bedouin]

(1) You will be relying on the bond between the hardwood and the existing laminate. Unlike a proper homogeneous lay-up, this is largely unpredictable.
(2) Hardwood reaction to varying levels of humidity, mainly expansion and contraction, will be different from the existing structure.
(3) A re-build of the hole, using the same material as the matrix, is the proper way. Putting in a plug instead is, with respect, a botch-up.

Of course, all the above is IMHO.

I don't think you can rely on the bond between any plug and the laminate unless you do a major grind out and replace job. The only requirement for the plug is to provide a pad to allow the compression forces from the bolts to be transfered. The strength is being provided by the backing pad bonded to the existing laminate.

BTW on reflection I would use ply rather than hardwood for it - more stable and less prone to rot.

 

[charles_reed]

A bolted-on stainless disc isn't going to be very attractive. If you're able, a better solution would be to taper the edges of the hole underneath, fill the hole with fibreglass laminate, spreading the area over the taper underneath, then perhaps epoxy a ply backing plate underneath, and bolt the winch on. The stresses are primarily in shear, so it will be strong enough.

When filling holes in GRP, I've found the most useful product you can use is polyester putty with glass-fibre filler. Far easier to use in small areas, bonds as well as epoxy and takes 1/3rd the time to harden as taken by epoxy.
Not seeing the particular site, it's difficult to imagine what I'd do, except use a generous plywood backing pad and, having fixed that in position under the hole, then fill the old vent and finally make good and drill the holes for the winch after every thing had set. It might be advisable, to avoid voids, to drill and bolt through one or even two of the future winch fixing holes.
I certainly wouldn't use stainless steel anywhere near an aluminium or bronze winch-base, and would use Duralac and buyl tape (not silicone sealant) when I did fix it down.

 

[Otter]

Its a stainless steel winch.

 

[Daydream believer]

Post 15 would be my preferred method except i would put the stainless beneath the ply so there was no fear of the bolts pulling through the ply. As i see it the harwood infill plug is just that, an infill of no structural consequence. A steel plate on top would take the tipping edge of the winch & spread the downward push across the plate. this plate would look ok if polished & was round to match shape ( but bigger) of the winch base. A square one would look odd i expect

 

[oldsaltoz]

If you are even considering using any timber, be it plug or ply, remember that we replaced wooden boats (and many other timber items) with fibreglass, why anyone would want to timber in the structure of a fibreglass boat is beyond me. Timber will rot.

If you MUST use timber backing plates you MUST drill all holes for fixings oversize so they can be filled with a mixture of an Epoxy resin and Micro Fibres, then re-drilled to required size.

This not only prevents moisture getting the timber, but also prevents the fixing bolts from crushing the ply when tightening them or when they are loaded up.

You need to spread the load over a wide area with the centre being the most solid and tapering to prevent gel coat flexing and cracking. so are looking at 1.5 times (minimum) the size of the winch fittings width in total.

Then adding a suitable backing plate, 6mm 316 stainless would be my choice. extending out at a minimum of the width of the fixings each side, so the plate in total would end up 3 times the distance between the bolts measured fore and aft.

The plate should have well rounded edges top and bottom, Before fitting the plate mix up some epoxy resin and add a layer of resin mixed with Micro fibres to the consistency of Toothpaste and tighten the fixings till the gap between deck and plate is filled. Then leave it to fully cure for a day at least, then tighten to required amount. This ensures the load on the deck and backing plate is spread evenly over the point of contact and fills any curves.

Good luck and fair winds.

 

[prv]

Post 15 would be my preferred method except i would put the stainless beneath the ply so there was no fear of the bolts pulling through the ply.

With a 30mm washer on the end of each bolt, and the load nearly all shear anyway, I wouldn't be worried about them pulling through.

Pete

 

[Gwylan]

I'm fitting a new winch to the aft cabin roof. Size 42 winch, genoa sheets but not a huge genoa, about 300 square feet and a 700 square feet chute.

It's going through solid laminate with no moulded reinforcement. The bad news is that it's replacing a vent so I'll have to put a hard wood plug into that and epoxy it in place. My plan was to then use a sandwich of 316 stainless disc above and below about 50mm wider than the diameter of the winch, sikaflexed in place, bolt through six holes at the edges and then bolt the winch through the middle. But the bolts will go through the plug so the load will all be transmitted and held by the top and backing plate. Is 3mm plate thick enough? There's a very slight curve in the glass fibre so I need to have plates that can take a tiny amount of curve. If I go up to 5mm will then bend very slightly? What thickness do you think is the minimum?

Thanks for the advice.

Seeing other posts I would go for a much larger ply rectangle glassed above and below the old vent hole. Then maybe put the S/S plates on either side. Certainly the ply should be bigger than the 50mm overlap you have mentioned. Properly glassed in the ply will give you all the strength that you ought to need and the S/s plates should mean that if it does pull out then you have a whole other set of problems anyway.

The West System publications have all sorts of hints, tips and techniques for doing this sort of thing.

Not a drop of Sikalfex to be seen

 

[PuffTheMagicDragon]

With a 30mm washer on the end of each bolt, and the load nearly all shear anyway, I wouldn't be worried about them pulling through.

Pete

+1

 

[Otter]

OK, summary I think of the way to do it:

1. Glass in hole from vent using micro balloons and polyester resin - easier to mix and less critical volumes
2. Use oversized polished 316 discs above and below to match round shape of winch
3. Use resin to fix these in place so that all the disc is in contact with the GRP
4. Bolt through these

No wood to rot. Overkill engineering. Load spread over a 300mm disc top and bottom.

Thanks everyone.

 

[sailorman]

OK, summary I think of the way to do it:

1. Glass in hole from vent using micro balloons and polyester resin - easier to mix and less critical volumes
2. Use oversized polished 316 discs above and below to match round shape of winch
3. Use resin to fix these in place so that all the disc is in contact with the GRP
4. Bolt through these

No wood to rot. Overkill engineering. Load spread over a 300mm disc top and bottom.

Thanks everyone.

be aware of hexotherm, dont fill in one hit if its thick / deep

 

[Scotty_Tradewind]

OK, summary I think of the way to do it:

1. Glass in hole from vent using micro balloons and polyester resin - easier to mix and less critical volumes
2. Use oversized polished 316 discs above and below to match round shape of winch
3. Use resin to fix these in place so that all the disc is in contact with the GRP
4. Bolt through these

No wood to rot. Overkill engineering. Load spread over a 300mm disc top and bottom.

Thanks everyone.

Yes a lot to be said for those advising glassing in the hole. Sounds like it will be very strong. Cannot see.. is it a cored deck? how thick?

 

[Otter]

be aware of hexotherm, dont fill in one hit if its thick / deep

Yes, good point, about 4 inches diameter and I think about half an inch thick. Maybe best done in stages?

Yes a lot to be said for those advising glassing in the hole. Sounds like it will be very strong. Cannot see.. is it a cored deck? how thick?

Solid but not very thick, not like the main cabin top or the hull, about 3 inches thick solid laminate in places, 1.5 inches at its thin-ist! She's a big girl is our Kipper.

 

[bedouin]

If you are even considering using any timber, be it plug or ply, remember that we replaced wooden boats (and many other timber items) with fibreglass, why anyone would want to timber in the structure of a fibreglass boat is beyond me. Timber will rot.

Do they really build boats without using any wood? I thought many GRP boats used wood to provide stiffness in key areas, I know mine does.

 

[prv]

Do they really build boats without using any wood? I thought many GRP boats used wood to provide stiffness in key areas, I know mine does.

No wood in the hull or deck of mine, just closed-cell foam where previously balsa might have been used.

Loads of it in the fit-out, of course, and not just for visible aesthetics.

Pete

 

[bedouin]

No wood in the hull or deck of mine, just closed-cell foam where previously balsa might have been used.

Loads of it in the fit-out, of course, and not just for visible aesthetics.

Pete

Perhaps yours is different - many AWB boats I have been on seem to use plywood a lot to provide lateral strength and rigidity (if that is the right word when applied to a bendytoy )