Marine Ply Backing Pads

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yachtorion

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I'm replacing all the thru-hulls and valves on my Hunter with new marelon ones. The existing fittings are unknown age and quality thru-hulls of unknown material with equally unknown valves - so I figured safest just to ditch them. The existing fittings don't have backing pads, but the forespar replacements recommend fitting them. I'm planning to fit 18mm ply backing pads glassed in with epoxy and epoxy compatible CSM.

I've got 18mm timber lying around. eBay special "marine" ply to specification "BS1088, EN13986 CE2+, Meeting Requirements of EN314-2 Class 3, EN636-3S, EN335-3 Classes 1, 2+3, E1." which as I understand it means that not only does it meet the ubiqutious British Standard but is also boasts high bonding and rot resistance specifications.

So my question is... can I use this stuff well epoxied and happily leave it for the next 20 years, or do I really need to put in an order for some Robbin's elite?
 
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Forespar recommend you use something like white oak, I used 20mm HDPE, now the valves, hull and backing plates are all plastic and I can forget rot and corrosion, well at least in that area.
 
David2452 said:
I used 20mm HDPE, now the valves, hull and backing plates are all plastic and I can forget rot and corrosion, well at least in that area.
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Plastic chopping boards are probably the most accessible source of HDPE, although they're rarely found in 20mm thickness. Many are LDPE (low density polyethylene, although the actual density of both is quite similar). HDPE has the number '2' inside the recycling logo, LDPE the number '4' , although often neither will be present. HDPE is somewhat stronger and rather more heat-resistant (110C compared to 80C).

If buying from a specialist plastics supplier, these people have competitive prices: http://www.directplasticsonline.co.uk/
 
I use HDPE a little for repairs etc on farm tanks.


It might be worth looking at the compressibility figures for HDPE / LDPE.

https://duckduckgo.com/l/?kh=-1&udd...que_For_Polyethylene_Flanged_Joints.sflb.ashx

I'd be inclined to check the torque on the flange used for through-hulls after the plastic has had a chance to creep to its new shape.

Whilst you can get away with no sealant, does anyone use some anyway ?
 
Same here in that I used 12-18mm odd bits of exterior ply as it's totally encapsulated and is there to provide belt and braces strength only.

I may be wrong but as far as I know the only real difference with marine ply is that there are no voids in the layers.... And the price!
 
If you decide you need to spend money to buy material for backing pads, worth considering acetal. Similar to hdpe but tougher and heavier meaning that once you tighten up the fitting the material will creep less. I've bought some as backing material for my bow roller - although I haven't fitted it yet.
 
PaulMcC said:
If you decide you need to spend money to buy material for backing pads, worth considering acetal. Similar to hdpe but tougher and heavier meaning that once you tighten up the fitting the material will creep less. I've bought some as backing material for my bow roller - although I haven't fitted it yet.
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Good stuff but about three times the price of HDPE.
 
Lakeland are a good source of HDPE chopping boards, but only about 8mm thick. I also use PTFE sheet as an insulator when bolting SS to gel- fibreglass - many use butyl tape for the same job. PTFE sheet seems to creep less, is white and lasts longer.
I always have some 12mm marine ply for backing pads - 22mm sounds a trifle generous even for winch backing pads.
 
My own preference is to use aluminium plate if I can't find a good lump of stainless steel.

If you have some suitable ply laying around the following may help:
It is very important that no moisture can get into the timber so full encapsulation is a must.
If the area is curved you may have to remove some material from the sides, this is common for through hull fittings.
However, you can save a bob or two by first applying you epoxy resin and at least one layer of cloth to your ply. The using standaerd resins to retain it. The stress is all pulling to the hull or deck so epoxy is only required to keep the timber dry and not required to hold in place.

First step is to drill any holes over size and and trim any edges ready for curved areas. Then fill the holes with an epoxy resin mixed with Micro-fibres to a toothpaste consistency. This drill and fill also applies to the deck area holes.

Before you stick the board to the hull or deck cover the area with a layer of glad wrap or Cling wrap; then mix an add a layer of micro fibres mixed with an epoxy rein od the hull side of the board and and press into position, you can bolt it at this stage but only finder tight or tight enough to get a flush fitting on both sides.
Let this fully cure before the next step.

After curing remove the bolts and apply a suitable sealant on outside only to prevent any moisture getting trapped. Let the sealant cure before fully tightening the bolts.

You should now have a fully sealed backing plate that will not be a major operation to remove 20 years down the track because the plastic film behind it will make removal a simple and mess less task.

Good luck and fair winds. :)
 
I have used "marine" ply epoxied/filled in place on a very curved hull for my Marelon seacocks.
Seems fine after over ten years. But I now use HDPE chopping boards to back all above water line holes.
Doesn't bond to anything of course but doesn't rot either and esay to replace.
I see Direct Plastics sell sheet PVC. This does bond to resin and may be hard enough!
After all my foam sandwich boat has PVC foam.
 
No doubt there are various versions of BS 1088! However, unless the appearance is important, in this case not, or the structural integrity under tension, ditto, it is probably not going to make a lot of difference. Under the compressive loads that you will be applying the presence of a few voids in the laminate is unimportant. I would use what you have, with plenty of epoxy as you suggest, and all will be well.

For reference, I am currently rebuilding a 1972 boat that has been bodged by a succession of owners. Many fittings have been backed by a wide variety of pads including solid timber, poor quality plywood and chipboard. Surprisingly, all of them have worked to an acceptable extent, without leaking.
 
Whatever material you use make sure that inside and outside are parallel so that the valve and nut pull down square and evenly all round. Hulls can be surprisingly uneven.

Yoda
 
Isn't the point of a backing pad to be slightly compressible so that it will compensate for irregularities in the hull? That's why I wouldn't use metal, surely it will impose point loads on the hull which is what you want to avoid.
 
Thanks folks. eBay ply and plenty of epoxy it is then. It has the advantage of already being in the cupboard.
 
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