Marine Ply .. or not

[Tafflad]

I fitted a Tower to my boat this year, or in fact the dealer did.
I'm very pleased with it.

What I intend doing out of season is laminate in some supports behind the deck fittings, to cut down on flex, and strengthen the support point.

Intent is to glue in some 3mm ply - about 60cm x 20cm been advised, with fixing bolt through the middle. ... that is 1/8" of 24" x 8" for those not yet in 21st Century...

Once glue has set, glue in a second layer over the first.

Advised to use 3M 5200 as the adhesive.

The Q ... should I be using Marine ply for this, or as this is well above the water line, is WBP ply good enough.
I could always put a coat of yacht varnish over the ply & edges once fitted.

 

[Bob_VT]

Re: Marine Ply .. or not

Marine plywood does not have voids or knot holes. Plain exterior grade plywood sealed with polyester ot epoxy resin will outlive it's needs and serve you fine.

 

[allpoints360]

Re: Marine Ply .. or not

It sound like you're describing backing plates. Why not use steel?

 

[haulnazz15]

Re: Marine Ply .. or not

Also not reason to glue it really as it won't go anywhere if it's thru-bolted.

 

[sean64]

Re: Marine Ply .. or not

hi i would not use ply as if it gets wet it will not dry out but will rot and spread. marine ply is 2 the price but it will not take in water. thats y its called marine ply.

 

[coolbikeguy]

Re: Marine Ply .. or not

hi i would not use ply as if it gets wet it will not dry out but will rot and spread. marine ply is 2 the price but it will not take in water. thats y its called marine ply.

but as bob said ... a good ext. grade ply and use a good coat of an epoxy resin .... will last longer than plan marine grade as the epoxy has the ply sealed and waterproof

 

[Capt'n Chris]

Re: Marine Ply .. or not

It sound like you're describing backing plates. Why not use steel?

Steel is a poor metal to use in any boat...even powder coated, unless maybe a stainless steel of some sort. I would use 1/8 aluminum for something like that.

Captain Chris
SternMate?

 

[haulnazz15]

Re: Marine Ply .. or not

Steel is a poor metal to use in any boat...even powder coated, unless maybe a stainless steel of some sort. I would use 1/8 aluminum for something like that.

Captain Chris
SternMate?

Who cares if it rusts? It's going to be away from water (likely under the cap rail) and clamped onto fiberglass, which doesn't care about rust. A piece of flat stock from Lowe's would be just fine.

 

[allpoints360]

Re: Marine Ply .. or not

Steel is a poor metal to use in any boat...even powder coated, unless maybe a stainless steel of some sort. I would use 1/8 aluminum for something like that.

Steel would provide superior strength for a backing plate.

I've always considered it a degree of quality when a boatbuilder uses stainless steel backing plates. You see it in better sailboats.

 

[marine4003]

Re: Marine Ply .. or not

Who cares if it rusts? It's going to be away from water (likely under the cap rail) and clamped onto fiberglass, which doesn't care about rust. A piece of flat stock from Lowe's would be just fine.

first of all, who wants flakes of rust dropping down onto the deck, then if you ever wanted to remove the tower..the bolts will have to be cut off with a grinder,if it gets wet and drip's..rust running down onto the deck isnt fun to remove...1/4 aluminum backer w/SS fasteners is the way to do it right.

 

[Tafflad]

Re: Marine Ply .. or not

first of all, who wants flakes of rust dropping down onto the deck, then if you ever wanted to remove the tower..the bolts will have to be cut off with a grinder,if it gets wet and drip's..rust running down onto the deck isn't fun to remove...1/4 aluminum backer w/SS fasteners is the way to do it right.

The deck curves in 2 planes where the 5/8" fixing bolt is bolted through, (the fixing bolt has 3" x 5" stainless backing plate ) ....it may not be easy to get metal to fit neatly ... but I guess I could try.

The ides of using marine ply was, that at 3mm thick it would flex to deck contours, and if laminated in place would set to become 'one' with the deck, making a large piece of the deck stronger.

My thought were prep up 2 pieces, clean underdeck with acetone, and put a reasonably thick buttering of resin (thickened with filler or angel hair) use thru fixing to hold it in place until it's starting to set, then laminate second piece onto first (no thickening)

Painting a layer of resin all over the wood face & edges.

If you think this is not the best way ... I can re-plan my pre-season project

 

[Texasmark]

Re: Marine Ply .. or not

Marine ply is as stated, better quality "pine", or if you get into the pretty wood, "Philippine Mahogany", is laminated with waterproof glue, and usually has more plies per unit thickness (like cabinet grade plywood). This all makes for a strong, lightweight, material that resists moisture........somewhat, especially the mahogany. Pine is wood and wood can rot if not protected; mahogany also, but at a much slower rate. For a backing plate which is under the gunwale where there is little occurrence of moisture contamination I wouldn't worry about contamination, nor would I go the price of marine, especially mahogany.

Steel is heavy besides the oxidation, if carbon. SS is hard to work with (for me anyway) plus finding a suppler.

Alum is a great choice if you can find a supplier also. But if you are in a place that caters to the marine industry/consumer, then any material you choose should be readily available.

If I were securing the stays/guys for the mast on a sailboat then I would back with metal, probably aluminum. If I were securing a deck cleat, half inch of plywood (for a smaller boat, twice that for a large one) with a big thick flat washer is what I'd use, pre drilled and coated with linseed oil. Hardware would be SS regardless of fresh or salt water. Don't think I'd even go with a marine glue product because as also stated, what difference does it make if the plies come unglued in this application, but they probably won't. But this my opinion and I don't usually go the cadillac approach..Dodge trucks are in my driveway.

Mark

 

[lowkee]

Re: Marine Ply .. or not

As a backer plate, I wouldn't bother with marine grade unless you have the extra cash to drop. I think normal ply is fine as long as you seal it well. I would lean towards glassing it as well. I'd def go thicker than 3mm, as 3mm is nearly nonexistant. You could likely snap 3mm by hand, a tower won't even feel it. Go 1/2" if you have room, 3/8" otherwise. The longer and wider the backer, the better, also, as it will ensure stress points are spread over a larger area, lessening the risk of gelcoat stress cracks.