Seacock backing pads, WBP plywood?

timmygobang

timmygobang

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I've read a few post regarding backing pads and the consensus seems to be to use 18mm marine plywood.

I've just called Travis Perkins who've quoted £60 for a sheet of 8x4

I only need a small piece so seems a bit OTT to get a big sheet.

Question is would WBP Plywood painted with bilge paint be sufficient?
http://m.wickes.co.uk/mt/www.wickes.co.uk/hardwood-plywood-18x1220x2440mm/invt/110037/

I don't want to skimp on safety over a few quid but thought id throw it out there.
 
timmygobang said:
I've read a few post regarding backing pads and the consensus seems to be to use 18mm marine plywood.

I've just called Travis Perkins who've quoted £60 for a sheet of 8x4

I only need a small piece so seems a bit OTT to get a big sheet.

Question is would WBP Plywood painted with bilge paint be sufficient?
http://m.wickes.co.uk/mt/www.wickes.co.uk/hardwood-plywood-18x1220x2440mm/invt/110037/

I don't want to skimp on safety over a few quid but thought id throw it out there.
Click to expand...

WPB & the old BS 1088 both use the same glue.
the mirror dinghy was make from WPB ply.
i have only ever used 12m/m ply for backing pads, you must be able to find an off-cut without spending your hard earned
 
I certainly wouldn't use genuine marine ply. That's top quality stuff for building plywood boats out of, not just a bit of a backing pad. WBP is plenty. I would hope you could avoid buying a whole sheet of anything.

Pete
 
One think to consider when using any type of ply that is when it gets wet it expands so using it as a backing pads for sea cocks is that it will put extra stress on the skin fitting. It may be OK is the skin fitting is metal but if its plastic it could brake the skin fitting and thus sink your boat.

I would use plastic in the form of kitchen cutting boards as others has said

Just a note I don't think most of the cutting boards are nylon but if they are nylon also expands when exposed to water as some have found when nylon rudder bearings have been fitted to their boat.
 
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Rogershaw said:
One think to consider when using any type of ply that is when it gets wet it expands so using it as a backing pads for sea cocks is that it will put extra stress on the skin fitting. It may be OK is the skin fitting is metal but if its plastic it could brake the skin fitting and thus sink your boat.

I would use plastic in the form of kitchen cutting boards as others has said

Just a note I don't think most of the cutting boards are nylon but if they are nylon also expands when exposed to water as some have found when nylon rudder bearings have been fitted to their boat.
Click to expand...



Strictly speaking a properly installed backing pad will be glassed in, so should stay dry.
 
Rogershaw said:
Just a note I don't think most of the cutting boards are nylon but if they are nylon also expands when exposed to water as some have found when nylon rudder bearings have been fitted to their boat.
Click to expand...

You're right: I think many are some other form of high-density plastic.
But if nylon, I doubt that expansion is an issue (unlike with rudder bearings). Tests have shown expansion due to water absorption can be as much as 1%, or 0.01mm in a 10mm thick board. That's 4 thou in old money. I very much doubt that this would be of any consequence.
 
must be a workshop/maintenance outfit near your boat...ask to go through their wood waste bin or skip.....you are only looking for a tablemat sized bit of decent stuff 1/2" thick
 
capsro

Nice in theory. A friend vice commodore of our club has just replaced the glassed in plywood on the deck of his boat after water ingress past the stanchion mounting holes.

If you are glassing the ply wood in why not replace the plywood with just a thicker layer of flbreglass.

macd

Yes the expansion of nylon is small and may bot be signification in this application. The point I was trying to make is that I don't think cutting boards are nylon as other plastics like HDPE are much cheaper than true nylon
 
Rogershaw said:
capsro

Yes the expansion of nylon is small and may bot be signification in this application. The point I was trying to make is that I don't think cutting boards are nylon as other plastics like HDPE are much cheaper than true nylon
Click to expand...
I just bought 2 new white plastic chopping boards this week It doesnt say on the packaging what they are, HDPE I'd think or possibly PP.
 
VicS said:
I just bought 2 new white plastic chopping boards this week It doesnt say on the packaging what they are, HDPE I'd think or possibly PP.
Click to expand...

No recycling symbols?

I think plastic is fine if you just want to spread the load from the backing nut, but if the hull is flexible and/or the seacock will have heavy plumbing on it, I would be looking to actually reinforce the area around the hole with grp.
Plywood is not ideal for the job, as any water that creeps into it will rot it in time. If it's all there is, then bore the hole oversize and give the whole thing plenty of epoxy coating.

Maybe I'm overdoing things, but I met the new owners of my first boat a while back, and it's nice to feel there's nothing I skimped on too badly 18 years ago.
 
Rogershaw said:
One think to consider when using any type of ply that is when it gets wet it expands so using it as a backing pads for sea cocks is that it will put extra stress on the skin fitting. It may be OK is the skin fitting is metal but if its plastic it could brake the skin fitting and thus sink your boat.

I would use plastic in the form of kitchen cutting boards as others has said

Just a note I don't think most of the cutting boards are nylon but if they are nylon also expands when exposed to water as some have found when nylon rudder bearings have been fitted to their boat.
Click to expand...

I saw these threads after coming back from Wickes with a sheet of exterior plywood.

I am actually using plastic seacocks (Marelon seacocks), I would have thought that they wouldn't be exposed to too much water as long as there's plenty of sealant around the hull fittings

Wes Gary uses plywood and would have assumed he wouldn't want boats sinking after watching his vid :)
http://www.youtube.com/watch?v=bHNNVrGPeew
 
Installation instructions for an airmar speed/temperature sensor plastic housing

"Never install a plastic thru-hull sensor in a wood hull, since swelling of the wood may overstress the plastic causing fracture"

IMHO ply in this application will swell over time. I wouldn't risk it.
 
Rogershaw said:
Installation instructions for an airmar speed/temperature sensor plastic housing

"Never install a plastic thru-hull sensor in a wood hull, since swelling of the wood may overstress the plastic causing fracture"

IMHO ply in this application will swell over time. I wouldn't risk it.
Click to expand...
The answer is to do what many modern boat builders do; epoxy coat the plywood backing pad to make it waterproof. Then fit it and paint it as well.

How many instances of seacock failure through backing pad failure has anyone actually heard of.

That'll be a figure close to zero I suspect..
 
Rogershaw said:
Installation instructions for an airmar speed/temperature sensor plastic housing

"Never install a plastic thru-hull sensor in a wood hull, since swelling of the wood may overstress the plastic causing fracture"
Click to expand...

That's interesting. I have a Nasa Clipper log in my wooden boat. Ive never seen any problems but I think I will go and have another look before we launch for the season.
 
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