What adhesive?

Hi,
I'm helping a friend to repair the double hull cabin roof on a Shetland 570 grp motor boat.
The middle layer (I think it's called a core?) is 10mm sodden wood.
The roof has been cut out on the inside of the cabin and we intend to use 10 mm marine ply, resined for waterproofing and refit it.
The roof was built with rectangular pieces of wood and glued to the upper surface.
What type of glue should I be using to restore strength to the roof.
It will be glassed over, once the plywood is installed.
Absolute newbie to boat repairs.
Thank you in advance for any advice.

 

Welcome to the Forum Diyguy.

For general reference for other readers, here is a link to a sister ship for sale.
https://iow.boatshed.com/shetland_570-boat-154723.html

Your original core was most probably balsa.
If there is camber (ie some curvature) in your roof, then you would probably have to score the plywood with multiple longitudinal cuts to allow it to bend to the shape.
Two part epoxy would probably be your best bet for gluing the plywood to the existing outer fibreglass skin of the roof.
You will be doing everything overhead though, which will make things a bit more difficult.
And overhead fibreglass laminating is no fun at all!
If you can manage to glue in your new 10 mm plywood core, you could probably get away without glassing over it, so long as you seal it on the inside as well with epoxy.

Do you have any photos that could be posted of your boat, and of the inside of the roof after you have stripped away the rotten core?
You probably can't post any yourself until you have a few more posts to your name, but if need be I could post them for you if you email them to me.

 

Depending on availability and expense, you could use scored foam to replace the balsa.


And re- fibreglass over that.
Trying to bend marine ply in a compound curve, without leaving voids, is quite tricky. That Plywood will probably get saturated as well.

There is also a good case for thoroughly waterproofing the underside of the cabin top with Epoxy, and replacing the Balsa squares.
These were only saturated because the upper Glass had holes or was badly laid.

 

Diyguy Allan has emailed me some photos of his roof to post here, which I will attach below.

Allan, re the inner skin of fibreglass that was cut out, you might be able to re-use it if you clean it up, and then glue it in place with epoxy after re-fitting the new core?
If you can get hold of scored foam, that would be easier than using plywood.

Do you still have to remove the inner skin from around the hatch, and replace the core underneath in this area as well?
I presume that the two round holes are for the ventilators? It looks like water might have got into the core via the fasteners for these vents, and then seeped through the core gradually (?)
Does the remaining area of inner skin that is outboard of and aft of the 'U' shaped cut out appear to still be ok (re an intact core) when you tap it?

 

The badly protected holes that let the water in are pretty obvious.
No wonder the wood rotted.

 

You better make sure you do the holes different.

Does anyone walk on that roof?

 

It might be worth a quick experiment with XPS extruded polystyrene instead of plywood. It does not absorb water, won't rot, can be glued with epoxy and is easy to cut with a sharp craft knife without creating those horrible granules that you get with EPS expanded polystyrene. It is sometimes used as a core material in boats but I've just been using some for DIY home insulation. Whatever material you use you will probably need lots of thickening filler in the epoxy to make it non-runny and to reduce weight and cost.

 

no no no

It has no ability to support the laminate.

Shear strength is horrible for a ceiling!