making plywood backing plates waterproof

sarabande

sarabande

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I have about 20 backing plates to put under winches, blocks, stanchions, etc., which are presently bolted simply through the ply deck with largish (not very large, though) washers underneath.

I want to use 1/4 proper marine ply as backing plates to spread the load, and as these plates live in places difficult to access, to make sure that the job is done properly and just once.

With very limited experience of using thin epoxy or polyurethane varnish to make ply waterproof, what recommendations do more experienced owners have about techniques please ?

The ply is VG quality and quite dry, after being stored for many years. Not only treating the plates but bedding them so that no moisture can lie between the plate and the deck.

(small boat 24 ft OA, not a biggy :)

TIA
 
sarabande said:
I have about 20 backing plates to put under winches, blocks, stanchions, etc., which are presently bolted simply through the ply deck with largish (not very large, though) washers underneath.

I want to use 1/4 proper marine ply as backing plates to spread the load, and as these plates live in places difficult to access, to make sure that the job is done properly and just once.

With very limited experience of using thin epoxy or polyurethane varnish to make ply waterproof, what recommendations do more experienced owners have about techniques please ?

The ply is VG quality and quite dry, after being stored for many years. Not only treating the plates but bedding them so that no moisture can lie between the plate and the deck.

(small boat 24 ft OA, not a biggy :)

TIA
Click to expand...

Why not use ali plate Tim
 
If it is marine ply it is by definition "waterproof". Any coating is belt and braces unless it is in an area that gets wet regularly with fresh water. The most obvious source of water is through the fastenings for the fittings so ensure a good seal down the holes and between the plate and the plank. Exposed end grain is the weakest part and some value in sealing the end grain with epoxy, but varnish or paint is fine on the faces.
 
Carefully sand down all the edges.
Round all the corners,
Scratch the backs with a stanley knife to aid adhesion.
Stick them in position with premix chopped strand and resin.
When cured, paint them over with resin.
Bore through holes,
Bolt down fitting using quality sealant..

Oh what a load of faff.. I am going with sailorman.
.+1 for Alloy
 
As the deck is ply I would clean off paint from under the deck and epoxy the pads into place, mix the epoxy using a set of scales, add silica for strength, then paint with epoxy resin, then bilge/locker paint as req'd.

I would also wiggle some runny epoxy into the bolt holes.

Small countersink at the deck level to allow a donut of selaant around the bolt. I've had great success with Arbo BR non setting mastic.
 
By the time you've epoxied the ply and bedded it down, etc, etc - I wouldn't bother!

1/4 marine ply is not going to achieve you very much load spreading. And ply is not good at crushing loads and 1/4 is way too flimsy to offer any 'stiffness' either. And when it's all in place and you drill the holes, you've then got to return to seal them with epoxy or any leaks will get into the 'core' of your construction.

Either use the aluminium as suggested bedded on polyurethane, or (bread board bedded on the same - varying levels of support for this idea), or if you want to use epoxy (or vinylester), carefully cut yourself a couple of neat concentric circles of heavy biax and laminate them directly to the underside of the deck. Or get a sheet of window glass, polish it and laminate yourself up a 10mm thick of GRP and cut your backing plates out of that. Every boatowner should have a good supply of this GRP 'plate' available.

Your large penny washers will be sufficient to stop the nuts sinking in to the glass and the glass will spread the load over the deck.

One of the American magasine did some testing a long time ago of backing plates under the nuts of U-bolts used for jackstays used for securing leashes on harnesses. The benefits of plywood backing plates of all thickness were delusional.
 
I was trying to avoid aluminium as I have little experience in cutting small plates from sheet, and though I now have a decent bandsaw, I don't have a metal cutting blade.

However, ali looks like the way to go. Thanks,.

One last question,: something about 6mm thick or beefier ? And what about corrosion problems with stainless bolts, washers and nuts ?

That's two Qs for the price of one. :)
 
sarabande said:
I was trying to avoid aluminium as I have little experience in cutting small plates from sheet, and though I now have a decent bandsaw, I don't have a metal cutting blade.

However, ali looks like the way to go. Thanks,.

One last question,: something about 6mm thick or beefier ? And what about corrosion problems with stainless bolts, washers and nuts ?

That's two Qs for the price of one. :)
Click to expand...

6mm is fine, but it must be marine grade. And you need to triple check this with your supplier. Then don't go too big with your backing plate - enough is enough and any larger increases the risk of it not sitting uniformly and having it create point loading. Make sure you round the corners and upper edges for the same reason. (80 grt pad on an angle grinder).

Making yourself some GRP plate (with rounded extremities) eliminates all the hassles with getting good marine aluminium.

Ideally, especially with larger boats and higher loads, the fastner holes through the plate want to be drilled undersized and then tapped for the fixing bolts. This takes so much of the sheering loads off the bedding compound and helps stop leaks in the long term.
 
I hope the OP won't mind if I seek clarification of a couple of the answers given.

TimBennet said:
Either use the aluminium as suggested bedded on polyurethane, or (bread board bedded on the same - varying levels of support for this idea), or if you want to use epoxy (or vinylester), carefully cut yourself a couple of neat concentric circles of heavy biax and laminate them directly to the underside of the deck.
Click to expand...

What are the supposed disadvantages of bread board (or other similar sheet plastic material)?

What weight of glass cloth is best suited to this sort of job?


TimBennet said:
Then don't go too big with your backing plate - enough is enough and any larger increases the risk of it not sitting uniformly and having it create point loading. Make sure you round the corners and upper edges for the same reason. (80 grt pad on an angle grinder).
Click to expand...

Any suggestions for a rule of thumb for backing plate size? (I'm about to make backing plates for new skin fittings 32mm to 54mm o/d in a GRP hull c8.5mm thick, also need to do a couple for harness clip-on points.)
 
Chopping boards are typically polyethylene. It has a tendency to creep under load (Span a gap with one and put a weight in the middle. Slowly it will become more and more bent). So washers under the nuts will tend to dig in and things loosen. It was touted as the miracle material (cheap, waterproof, readily available) and some people have claimed to use it with success. But there are also many people who have had problems.

GRP 'plate' is ideal. You can buy it, or laminate your own on a sheet of glass or piece of formica covered ply. A half sheet (4' x 4') is a convenient as you can reach in from all sides. It will give you more than you need but pre-cut backing plates sell well to other boat owners. Alternate layers of 3 to 600grm woven rovings and 300 grm CSM until you get a 8mm thickness in simple polyester resin.

If you're sure the backing plates are sitting 'flat', then you can bed them in any sticky sealant of your choice. If you think they may be bridging a hollow, then a polyester 'mud' might be better.

Size of backing pieces is interesting. You are countering two factors: One is the tendency for fasters to rip through the structure when loaded and the other is for the fitting to locally distort the area to which it is attached. For example, when installing the lower fitting for an inner storm jib forestay, a 100mm by 100mm square of say aluminium might stop the nuts ripping out, but wouldn't stop you then ripping the whole fore-deck off unless the 'span' of the area could take the load, or the fitting was 'tied down' to the stem. In contrast, stanchions rarely pull their fasteners through the deck, but frequently crack the gelcoat and go 'floppy' due to the area of the deck in the vicinity not being 'stiff' enough. A small backing plate might not solve this problem where as a CSM/Woven Roving/CSM 'patch' applied to the under surface might help enormously.

Try and visualise what problem you are trying to solve and then try and 'estimate' the possible loads. Is it in tension or is it in sheer? Is the base area solid or a sandwich? Work in 'units' you can understand. For instance, all the components of a modern harness and leash are rated at 22kN. That's about two the weight of two old minis. How much backing plate do you want to happily suspend two minis from your U-bolt? I'm happy with a 120mm square piece of 8mm GRP plate with a strip of 30mm x 4mm stainless steel flat bar under the nuts on a piece of the cockpit side close to the stiffening corner and a secure structural half bulkhead.
 
Thanks, Tim, that's really useful.

Seems that polyethylene would be suitable for those situations like skin fittings and harness U-bolts that are normally not under load.

I've previously tried laying up additional layers of GRP directly onto the hull to reinforce around skin fittings, but found that, particularly when done at arms length in an awkward space, it's easy to end up with a top surface that's not sufficiently flat or parallel to the outer surface of the hull to get the best clamping of the fitting. So pre-fabricating some sheet and bedding that on the hull might well be advantageous.
 
sarabande said:
I was trying to avoid aluminium as I have little experience in cutting small plates from sheet, and though I now have a decent bandsaw, I don't have a metal cutting blade.

However, ali looks like the way to go. Thanks,.

One last question,: something about 6mm thick or beefier ? And what about corrosion problems with stainless bolts, washers and nuts ?

That's two Qs for the price of one. :)
Click to expand...

6m/m ok cut with a hack saw or jigsaw, use duralac to inhibit corrosion its cheap a chips
 
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