Thickness of plywood for flooring

Try searching: calculate plywood deflection Also, might be worth contacting these people:

The thicker you go with the plywood the heavier it becomes and start to get awkward to manipulate within the confines of a boat. Have you considered removable supports that span either the width or length of the hatchway? This would enable you to use thinner ply yet removing the supporting spars would give you unrestricted access to the engine compartment.

Richard

Assuming you mean 3/4" ply, then I'd would say so, it's slightly better than 400mm centres, I'm quite a fat bastard and I *just* *about* sense some deflection on my garage roof, so if you want to feel no deflection at all maybe 1" ply?

But I'd struggle to justify these prices

More expensive but for that job I would use phenolic ply sometimes known as buffalo board. Often used as flooring in trailers and horse boxes. Has high resistance to moisture brought in on footwear , and many varieties have a non slip pattern already formed on the surface.

GH

As noted above there is already one support lengthwise. I guess it would be fairly easy to add some more, some tabs fixed to the existing surround and put some lengths of U angle on them so easy to lift out.

In fact a total redesign removing the existing (fixed) lengthwise piece of angle and replacing it with two or three removable ones might well be the way to go.

However the weight of the hatch panels does make it more soundproof so I don't want to make them too thin.

Quite! Plywood prices do seem to be all over the place with some sellers charging ridiculous amounts. Fortunately we have a local sawmill who sell ply at sensible prices. An 8' x 4' 18mm exterior hardwood ply sheet from them is £30.72 including VAT.

It turns out that the existing panels are thicker than I thought, I've found a measurement I made a while ago that says that they are 29mm or so thick. Thus I can make the panels of 18mm + 9mm and simply cut holes in the 9mm to provide the recesses for handles.

I have used this in an outside situation and I recommend it's use. It also provides a convenient anti-slip surface and is of course waterproof. It's not cheap though/

I also feel the spans the OP requires can be accomplished with 19mm /

OP here. How wide are your pieces of ply running across a 700mm span? Remember that I'm aiming for strips that will only be 385mm wide, not whole 8' x 4' chunks.

However, as noted earlier, I've discovered that the thickness can be up to almost 30mm so that should help.

Beware, there is 'external' ply which won't last long and good quality stuff.

Normal house flooring is (or was) 18mm chipboard on joists at 450mm centres, or 22mm on joists at 600mm centres, so your dimensions should allow 18mm, but a better way might be to just get a sheet of

Two different suppliers quote £160 and just over £60 for the same

Unless you lie on Teesside, not very flexible for delivery, eBay sellers seem to be better for that and not horribly expensive, I've got a trailer that could do with a new bed ...

Nobody has yet mentioned Marine ply which I believe is made without voids and uses waterproof resin. Cost will escalate!

But Marine Ply isn't necessarily **stronger**, it's just guaranteed not to have voids and to have waterproof glue, etc.

If I wanted guaranteed strength I'd surely need to go for Structural Ply.

I have used the 'external hardwood ply' from my inexpensive supplier and it seems fine so far. None of what I'm making is going to be outdoors, nor would failure be catastrophic in any major way.

This is a narrowboat application where I would say the width of the hatch is >600mm. I haven't measured it.

One hatch is possibly 1m long, the other 2 probably 500mm each. I found it to be substantive, doesn't (perceptively) bow and replaced the existing buffalo of the same thickness that was in a poor state.

Some heavyweights have walked on it without drama!

A 30mm sheet sounds pretty unmanageable as well as expensive. If you are concerned then ribs can be added after.

You have to be careful. Some marine ply can deal with moisture but not necessarily waterproof. You really need ply that is quoted as WBP, some marine ply is.

Marine ply is void free whereas normal WBP can have inner layers that are not void free.

Personally I would use Buffalo board. Marine ply looks filthy after a short with any foot traffic unless painted. Even then the paint would likely lift or get worn in short time.

OK, thanks, it sounds as if I should be OK then. I think I have some scrap pieces of 18mm ply around, I will try out with one of them if I can find one big enough.

This is why the existing engine covers are so unmanageable, they are made of 30mm material of some sort, probably ply or similar. Lifting the larger one (1540mm x 740mm) with its single finger pull is, shall we say, difficult.

I reckon my proposed single pieces of ply 1335mm x 385mm should be manageable, even if 28 to 30mm thick. I'm putting proper hand width lifting handles in, not just a finger pull. Making it the same thickness as the old covers means it can sit on the same supports.

That's a good plan, it will give you confidence. Mine are support on each of the 4 edges.

My first section is 500 x 600mm x 19mm and has a single finger pull. Apart from the pull being stiff, it does come up quite easily and easy to put to one side for access. The larger one ~900 x ~600mm is more cumbersome and I wouldn't want it to be twice the weight whilst perched on thin supports and performing a near tightrope balance!

Once the first one is up the others can be slid and removed equally as easy.

You could dispense with the single finger pull and have two