Shetland F4 18ft - Below deck construction questions - stringers, bulkheads, foam

[Franknstein]

Hi all

I have a 1987 GRP hull made by Shetland which I've had since 2016 and been rebuilding.

In 2017 I ripped out the sodden deck, and layed a new 19mm marine ply deck and glassed it to the hull without removing the original foam. This boat originally had no bulkheads, it has three stringers - one on keel and one on each of the chines, made from PVC pipe and glassed in. The deck was supported by 3 lengths of hardwood planks loose-laid across the upper two stringers and then poly foam poured in. In my opinion the under deck construction originally leaves much to be desired.

Anyways, my boat was listing a little last summer, so I ripped out the newly laid deck and decided to remove all the foam. Interestingly this boat had no drainage design, so I've used PVC pipe cut lengthways in half and running them from bow to stern alongside all the stringers to facilitate drainage to the rear bilge and made a hole with a bung. I've since sorted the source of the water getting in the hull.

I've now made custom bulkheads and extended the heights of the stringers and tabbed them all into each other and to the hull.

I was hoping to drop the old deck (still perfect condition) back down in place once I've re-foamed, and screw it down to the bulkheads - but I was hoping to not glass the deck back in place but just screw it back down, then sikaflex the gap all round. Planning on adding some plywood lips beneath the original deck-lip all round so it will be screwed down all the way around with screws about 4 inches apart. Additionally screws down into the three bulkheads. Reason for wanting to just screw it down is in-case I get some issue down there that needs repair, it will be a lot easier to access if I just need to unscrew the deck instead of cutting it out!

I also made a mould for a below deck cooler box - see pics.

My questions really would be:
1. Will screwing the deck down be strong enough? Or do I have to glass the deck back in?
2. I removed all the original foam, but was considering laying cellophane all over the construction work, pouring the foam, and when set, pull the foam out and remove the cellophane. This would mean I'd have perfectly formed foam blocks in each compartment but be totally removable for access. My concern is in the original design the foam was used as a medium to create strength and rigidity to the hull, but if the foam isn't adhered it may be a step in the wrong direction? Although surely my bulkheads and stringer mods should be ample?

See what you think
thanks

 

[Portofino]

Avoid re foaming .
You must have cut the new middle bulkhead to fit the tray ?
Add some more bulkheads and cross members ,make a denser grid over the flat hull sections . the additional can be boomerang shape to fit under the cool box tray and reduce weight .
Yes screw down the cabin sole , Sole is a better description than “ deck “ btw .

After a bit of bashing about in waves you can lift the sole and check the integrity of the work for piece of mind .

 

[Franknstein]

Avoid re foaming .
You must have cut the new middle bulkhead to fit the tray ?
Add some more bulkheads and cross members ,make a denser grid over the flat hull sections . the additional can be boomerang shape to fit under the cool box tray and reduce weight .
Yes screw down the cabin sole , Sole is a better description than “ deck “ btw .

After a bit of bashing about in waves you can lift the sole and check the integrity of the work for piece of mind .

Thanks for your reply. Yes I did cut into the middle bulkhead for the tray.

I guess even some battens glassed to the hull parallel between the existing bulkheads may suffice, instead of lofting out another two new boomerang bulkheads? What do you think.

If the foam I intend to pour in will be completely removable what would be the harm in having them in there - was also thinking of painting/sealing the foam inserts with something - I dont know, plasti-dip, stonechip or something like that.... I like the idea of having foam in there, could buy me some time if there is a hull breach.

Glad to know I can screw the sole down without glassing it in!

 

[Jamie Dundee]

I like the idea of having foam in there, could buy me some time if there is a hull breach.

You’d be better off filling the void with 2 litre plastic drink bottles or plastic play balls if you want extra buoyancy, foam is not the best idea.

 

[gordmac]

If you use small bottles or balls put them in something, doesn't take a big hole to let them escape otherwise!

 

[Durley245]

I manufacture the shetland under my Brand. I never use foam either but What on earth you using all the wood for under the Floor for. It looks like you have the later model, but the floor pan provides the additional strength needed. You don't need to use any wood, and certainly not plastic bottles. All additional weight. Raving mad. I the mould for that tray.