Broken bed

 Mickysf, Marine Plywood has nothing to do with strength. The glue used to repel water and used to bond the laminates makes it Marine quality. Strength will still be determined by thickness.

"Marine plywood is stronger and more durable than regular plywood, which has only three layers. ... As a result, you can use a thinner sheet that will have the strength of a thicker, regular sheet of plywood. In addition, it doesn't have as many voids as regular plywood, which makes it structurally stronger."

So my 53 years in the industry has been wasted..............I know nothing it appears. I wonder why it is called Marine and comes in various thicknesses.

I was quoting from the industry itself and that bank of understanding contains many, many accumulated decades of experience and knowledge. Marine ply is inherently stronger by construction and useful in situations like that described by the OP.

Getting hold of marine ply BS1088 can involve some chasing around. DIY stores like B&Q etc. often sell WBP and call it 'marine'.

That's right CY. Coming from a town with a fishing heritage 'proper' BS 1088 marine ply was easier to find maybe. But we could often pick up off cuts reasonably cheaply for jobs which called for quality and strength.

Resorcinol glue and Gaboon timber is what makes proper Marine Plywood.  WBP rated plywood is not Marine Grade. This is what I was taught and have only known. However my son calls me a Dinosaur so perhaps I need to move with the times and dismiss fact.

Yes, WPB is not marine ply, despite some caling it so. However it is better than the 'regular' ply that some caravan manufacturers use in their construction. If you can find the pukka stuff I would for a weight baring bed base.There again may be I'm the old school belt and braces type. I do like to see quality jobs being done even in those repair ones.

You can buy it on line eg Plydirect etc

Well agree with you there mickysf, I too like to see a proper job done. However I am rubbish at D.I.Y and usually end up paying to have it done properly. I have the know how but not the ability.