Help with plywood grades

[stephen low]

Bit off topic chasps but am rebuilding a powerboat and I need some feedback on plywood species.

I have been getting prices on the plywood I need to do the job as I am reflooring and replacing the transom. So am looking at 12mm for the floor and 15-16mm for the transom.

However prices are all over the shop which I was hoping might provide me with some sort of quality guide - seems not! For example, in 12mm thick structural grade I can get ply sheets from about $50 up to $100 or if I go the Aust Standards stuff even more expensive again.

I am less worried by cost and most focussed on the functional need from the ply. Clearly the transom timber needs to be dense and tough to hold nearly 200kgs of motor but the flooring I can get away with something less.

Am trying to get timber species names for the plys and know hoop pine is one good variant and have also been quoted for Birch, Phillipine Mahogany, Maple and F14 rated plywoods.

These I believe are all structural grades and as waterproof glues are used in these plys and they will be sheathed both sides in fibreglass, there is no immersion issue again just strength.

So what should I be looking for in terms of ply timber species for strength and density, can anyone help???

Looking to some good feedback.

Cheers

Steve

[Rog246]

Steve have you asked specifically for Marine ply ? It is very different to ordinary plywood obviously for its application. Hope that helps a bit. failing that ask a boat builder what he would use

Rog

[renovationinnov]

steve as above is a good start but you can get structural marine graded ply
just remember that there is different structural grades in each ply thickness with different tensile strenghts as well
so ring a marine boat surveyer for better advice on you load needs
rob

[Three Peaks]

As stated above, not all structural grade plywoods are marine grade. Marine grades are made much differently and use different resins in the plys and also more solid plys with basically no voids in the sheets.

Agree with the sentiment to call a marine engineer or boat builder to find out what they use in your neck of the woods. Up here we would use fir or sapele in a marine grade. Don't forget to use proper fasteners and techniques and for areas that need to be sealed, 3M brand 5200 caulk is the best marine sealer/adhesive I've ever seen. Once it is set up you would have to cut the plywood with a power saw to get it apart.

Bob

[stephen low]

Thanks guys, yes do realise I need waterproof glued ply and this is available in structural grade plys. Marine plys are far more expensive and given the fibreglass coating that will be applied, unecessary in this application.

I have a good mate that is a small dinghy builder and his was the suggestion on hoop pine but he can not shed any better species options.

However I do also know a marine surveyor so that is possibly the best place to start thanks for the reminder Rob.

Cheers guys and thanks

Steve

[Legless]

Hi Stephen,
I grew up in a boat shed where my father and grandfather built clinker and laminated ply launches for hire on the NSW Central Coast. So my information is old and probably out of date and could also be specific to NSW. The Marine ply we used for floor boards was ½” and for the transom would have been at least ¾” depending on bracing and this was for putt putts not outboards, though these days any of the surviving boats would have outboards. Unfortunately the days of the Tasmans, Simplex, Chapman and even Oldes have long gone. Always use Marine ply in boating, ‘normal’ grades of ply rot extremely quickly even when glassed due to the humidity. I don’t know what timber the marine ply consisted of except the veneer, and that was Coachwood (Ceratopetalum apetalum) this is a widespread rainforest tree of SE Queensland to SE NSW. Coachwood was good as it varnished quite nicely, as I said this may have been specific to NSW, I’m not sure.

I was down at our old boatshed the other day and noticed the owner was repairing one of the half cabin boats with what looked like 1” ply but it was very pale with straight grain, it certainly wasn’t Coachwood. Hope the above info helps.

Cheers

Legless

[Dimi]

Hi Stephen

Under NO< NO< NO<, circumstances use anything other than certified MARINE PLY.
Even though it will be glassed in there will be penetrations for bolts and fittings that will allow moisture in and eventually cause the structure to fail.
The transom will need to be between 30mm to 40mm thick usually by laminating smaller thicknesses together with waterproof glue or resin especially if using a larger motor.
The amount of flex and strain the motor will put on the transom will need to be well supported.
Dimi