Strengthening with marine ply ?

[Tafflad]

(originally posted in motor cruiser forum - advised to post here)

I fitted a Wakeboard 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.

Advice by dealer is to glue in some 3mm ply - about 60cm x 20cm, with fixing bolt through the middle.
Once glue has set, laminate 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.

Wood is used to curve to match deck shape, is 2M 5200 the right stuff to use?

 

[vyv_cox]

It sounds as though the ply and adhesive will be completely encapsulated by the new layup. I would assume therefore that the ply does not need to be marine grade, particularly since I have various bits of WBP far less well protected on my boat with no serious deterioration. Similarly the adhesive, which only holds the wood in place prior to layup.

This is quite a neat piece of advice, as the wood is only acting as a spacer, the strength is all in the additional GRP. My transom is made in a similar way, but the sandwich layer is polyurethane foam, which has virtually no strength at all. The double skin adds considerably to overall rigidity.

 

[tillergirl]

The relevant difference between marine ply and WBP in this context is the absence (or reduction) in the permissible voids inside the laminates in marine ply. It is therefore a 'better' product in that sense. Is it going to be vital to use marine ply? Probably not if well done. The crucial thing is avoiding dampness coming down through the fixing bolt. That's a much greater risk than the general pervading dampness ever present on boats. How pronounced is the curve? I would think the thicker the better, 3mm's pretty light for backing - I'm assuming this Tower thing is quite substantial and therefore I would be tempted to do three laminates.

Not used 3M 5200 personally. I think I would be tempted not to use something that is permanent but others may have different ideas. The backing pad will be held in place by the fixing bolts so it only really needs sealant not an adhesive; the fit will be important - nice and flush to the underside of the deck so that it gives a uniform bracing effect. All this is assuming that the general lay-up of the deck in this area is of sufficient construction for this Tower. Otherwise a more extensive reinforcement of the underneath of the deck with glass mat and epoxy might be called for.

 

[tillergirl]

PS I didn't read 'laminate' in your post in the same way as VYV. I assume you meant just gluing the ply to the underneath of the deck.

 

[Tafflad]

Maybe my explanation was not very good.

The through deck fittings are in the vertical face of the gunwale, and above the deck/hull join.
The deck is pretty thick .. and have used it all season as is, just want to make sure it stays this way.
The purpose being to stiffen and prevent deck flex (not that I have noticed any) and to strengthen the fixing point.

Dealer who fitted this .. advises that for thinner decks what he does is on the underside glues in a strip of 3mm ply fully smothered with adhesives, bolts it back up and lets it set .... then sandwiches another layer on top the next day.
There is no grp layup.

I am open to suggestions over which adhesive to use .... he advises that he does the job with 3M 5200.

 

[TQA]

5200 will do the job BUT you need to do something with the raw plywood. 2 or 3 coats of wood primer with an extra coat on the end grain would do. A couple of coats of epoxy would be better.

 

[Tafflad]

5200 will do the job BUT you need to do something with the raw plywood. 2 or 3 coats of wood primer with an extra coat on the end grain would do. A couple of coats of epoxy would be better.

not sure what is meant by epoxy ... to me epoxy is 2 tubes of araldite.

or you talking about a paint, sealer, adhesive ? ...can you give me a name or similar ?

 

[DownWest]

Araldite is epoxy, true. But a pretty expensive way of using it. Google WEST epoxy and do some reading. If you are reinforcing a grp structure by laminating thin ply, then epoxy would be a very good way to go. As said, Marine or WPB is less important than keeping the water out, esp from the bolt holes. One way is to drill them oversize, then use some thickened epoxy to form a bush for the bolt. Might be overkill for what you are doing, but is considered good practice for long lasting fixes.
A

Ply is commonly used to reinforce GRP structures, but needs protecting from damp. Epoxy is the best way.