How to make new ply board for under berth cushion.

[maej]

I need to make a new board to go under the starboard saloon berth cushion of my boat but I don't know what I need. It will be covering the flexible water tank and I intent to screw it down to help secure the tank in place.

• What grade of ply is necessary (Marine, exterior WBP)?
• What (if anything) should I use to coat or prepare the new board once cut to size?
• Is 12mm thickness enough to secure the 150L ish water tank if I were to ever get rolled.

I've been looking at Robbins elite, but with £40 delivery on top it's just too much. Are there any suppliers of good marine ply (assuming I need marine ply) in the Warwick area so I can collect?

A local supplier sells far east BS1008 marine ply (NOT BS1088). Is that any good?

 

[tillergirl]

Exterior WBP, 10mm (assuming there are cross braces?) primed and painted with household paints.

 

[prv]

Proper marine ply is for building boat hulls with. Since cabin woodwork should never be immersed in water long-term like a hull, full-on marine ply is overkill.

Pete

 

[Tranona]

Depends on the size of the hole and the cross braces if any. What is the rest of the bunk top? and what are you going to screw it onto? WBP is fine and bunk tops are normally minimum 12mm, but if there is a large unsupported area that may bow. My bunk tops are 15mm and the removeable bits are supported by batten all the way round the edges about 50mm in. suggest you also strap the tank down. It might have eyes at the ends for lashings to straps attached to bulkheads.

 

[kevink 6771]

fasten the tank down, use 12mm exterior ply, not marine, paint it with normal gloss then it will be fine, use cross braces to support the ply.
simples

 

[maej]

Depends on the size of the hole and the cross braces if any. What is the rest of the bunk top? and what are you going to screw it onto? ...

The other boards on board are 9mm unpainted ply. I'm not sure if they have been varnished or coated with something 30 years ago but whatever it is they aren't that shiny now.

This new board, which is replacing a grp panel of about 7-9mm thickness is covering an area that is 454mm at it's widest point and 1785mm long. It is supported around the entire outside edge by resting in a recess in the interior moulding. There are no cross beams in place. The screws go in to existing holes in the recess which is the inner moulding then the sadler foam. The tank lid that this replaces had around 50 screws holding it down.

 

[LadyInBed]

This new board, which is replacing a grp panel of about 7-9mm thick

Why do you want to replace a grp panel with wood?

 

[vyv_cox]

The other boards on board are 9mm unpainted ply. I'm not sure if they have been varnished or coated with something 30 years ago but whatever it is they aren't that shiny now.

This new board, which is replacing a grp panel of about 7-9mm thickness is covering an area that is 454mm at it's widest point and 1785mm long.

I replaced all the original flimsy, warped boards on my 34 with 12 mm exterior ply. I varnished them all, something like 10 years ago and all remain in perfect condition.

I assume you have put a flexible tank into the original tank? Without trying to calculate it I would have thought that 12 mm ply would be sufficient to support the weight of a full tank of water as the depth of the berth is not great. You are only trying to support the weight of two people, which the timber on the seat will easily do. I think you will need to retain all of the original screws to ensure that the plywood remains in place in an inversion.

 

[William_H]

Why do you want to replace a grp panel with wood?

My thoughts exactly.
What I would suggest is that you get some polyurethane foam or polyester foam? of about 10mm thickness. You could go for honey comb or end grain balsa.
Cut it out to fit in the hole. Chamfer the edges on one side. (bottom when fitted. Then get a large flat surface big enough to lay up the whole piece and wax it generously. Mark out the size of the finished board plus a bit. Lay up fibreglass or even better carbon fibre cloth fully wetted out to a depth of about 3mm of glass or 1mm of carbon fibre. in multiple layers of cloth.
Then lay the foam on top in place and lay up more layers of fibreglass or carbon over the top. The chamfer was to make the bend less dramatic for the glass so avoiding air gaps. Lay the fibreglass or carbon cloth so that it intersects the ridge of the foam at 45 degrees in this cover piece. This makes the glass more inclined to follow the contour. You might need filler in the corners if the chamfer is not enough. lay up a similar thickness in the cover. Do it all in one go.
Use polyester or epoxy resin. When it has hardened and you can get it off the mold board you are done except to drilling the holes and perhaps trimming the edges.

You might like to put some pigment in the first layer and lay down a coat of resin to make a gel coat.
You might find it cheaper than plywood and certainly will last longer and look more professional.
Just a thought olewill

 

[Malabar]

If practicable take into account the need to allow air to circulate beneath the mattress. If you don't you are likely to suffer condensation and hence damp. Ideally the base should have air vents cut in. Otherwise you are in the business of using some sort of underlay and/or lifting the mattress to dry out every time you use it.

 

[maej]

Why do you want to replace a grp panel with wood?

The grp panel is the lid of the moulded in water tank and it has a deep recess built in to it where the pick up pipes are connected. This recess means I can't use it over the new flexible tank that is fitted inside the moulded in one.

My first thought was to cut off the end with the recess so I could use most of the grp panel and then I'd only need a short section of ply, but that would mean it would be very difficult to go back to the moulded in tank in the future and I'd like to keep that option.

... I assume you have put a flexible tank into the original tank? Without trying to calculate it I would have thought that 12 mm ply would be sufficient to support the weight of a full tank of water as the depth of the berth is not great. You are only trying to support the weight of two people, which the timber on the seat will easily do...

Yes, I think the tank is about 150L so similar to the weight of 2 adults.

 

[Tranona]

From your description, 12mm would be fine, screwed down. However you might consider bolting a removable beam across the opening about midway along. Bolt it through the flange and make it so that the top is level with the new board. This will retain the tank and provide support for the board. I have just fitted a similar size, but metal, tank in my boat. It is bolted down to a platform and through a bulkhead, then a beam as I described, then 15mm boards screwed down - boards were the original bunk bases. If i had not been able to repair the tank, I would have used a flexible tank which has eyelets to lash it down and the same beam and board arrangement.