Bonding stringers and bulkheads when filling voids with poured foam

I'm starting a complete refit on a 15' GRP, displacement hulled open boat. It was originally designed for use with an inboard and so has a deeply moulded, narrow, full length keel. Apart from that there are no moulded stiffeners so I have a very flexi hull right now!

I'm planning to install full length timber stringers and then notch in timber or maybe ply transverse members to the same level to provide a flat base for a plywood sole. The timbers will be bonded using either thickened epoxy or maybe Gorilla glue original with an epoxy fillet.

To the question.... I'm then going to pour buoyancy foam to fully fill all the voids between the members and level to the top. Should I still tab the stringers and cross members in the traditional way to avoid local stress concentrations or will the presence of the foam provide sufficient rigidity that this is not now required?

Thanks,
Bernie

 

From experience, I would fully encapsulate as much timber, including tabbing, with epoxy, and glass enough for strength before any expanding foam..
Poured foam has a nasty habit of leaving small and large voids, which collect moisture and will incur rot of any non epoxied timber.

I would recommend solid foam board, glued together in preference to expanding foam, every time, if possible,

 

Will you be installing an inboard engine again?

Can you post a few photos of your project please?

 

Sailboat, powerboat?
A 15’ displacement hull typically will show a lot of curvature everywhere, and should not require a great deal of internal structure, and definitely not heavy Timbers.
Many small boats are adequately stiffened with crossbeam thwarts and a solid cap rail.
The full length keel should be an effective longitudinal stiffener.

 

I would add to the topic that if you fill the entire bottom of the boat with foam,the bilge water will have nowhere to pool and will slosh around in the cockpit area.With one or two carefully located bearers-with limber holes-the boat should be rigid enough and will drain.I would also add that encapsulating wooden structure with pourable foam almost guarantees decomposition of the wood at some inconvenient time.

 

Kapn nails it quite well. The best way to approach the issue is to make it so the sole can be set easily. Build the sole, tab it down with 1708, fill with expanding foam using hole cuts. This will stiffen it well. Fix the holes..done

 

Many thanks for all your thoughts - I have quite a few options to consider! I've attached some pics of the hull as it is now.
I've screwed some temporary battens to hold the shape of the sheer line and fix the beam as it was quite saggy before. Once I'm certain all is 'square' I can start with the stiffening proper.
I'll be using full lengths of sapele for inrails and outrails plus a sheet of ply bonded to the inside of the transom. I think some kind of stringer bonded back to the lower part of the transom board would be advisable to transmit the force from the engine through to the hull structure.

I was originally going to fit an inboard - I have a couple of old Brit two-strokes that I've recently restored that would fit - but then changed my mind as I already have another similar project lined up for one of those.

I want to use this as an open fishing boat and have it easily trailable for launch and retrieve so have decided to go with an outboard instead for lightness and simplicity. (I also plan to give the boat to my son and he's from a generation that isn't as used to priming, tickling, coaxing and wielding Whitworth spanners as a regular part of the boating experience!).

 

That boat is quite small. Just buy a piece of coosa or equivalent that'll drop into the groove.

I'd be inclined to put the spanners on top of the gunwhales versus inside. Inside, they can fall and open the boat and the hullsides may cant unless that is screwed on seat cleats..

While a small challenge to put spanners above; it is best.

 

Just some random thoughts....
I would not be too keen on filling the void space under the sole / deck with foam - one argument against this would be that if you take a wave on board and partially fill the hull, the weight of all that water is much higher, and hence the stability is worse.
It should be possible to get enough reserve buoyancy if you build an anchor locker forward, a transverse box seat approx amidships, and a stern seat?
If you have watertight access hatches in these compartments then you can use them for stowage.
Are you planning on adding a centre console for the steering and engine controls, or will it be a simple tiller steered outboard (perhaps with an extension if you want to move your weight forward)?

 

Martin ~ the foam would stiffen the hull and make it unsinkable. If he has concerns about broaching seas; he can scupper it.

 

Excellent advice.Where would you put the spanners?

 

Looks like a consensus against foam filling and also avoiding timber - it would make things lighter and simpler - and I take the point about stability if any water that is shipped can't drain to the bilge. Well, it's a blank canvas at the moment and I have all winter to work on it so will have a rethink.

I hadn't previosuly heard of coosa board but it looks like a good option for using as formers - can't find an equivalent in UK though, just plain PU foam board which I guess would work well with maybe an extra layer of glass?

I suspect going with the outboard option will require a significantly smaller toolset than if I'd installed either of my Stuart Turner or Excelsior engines - in which case the spanners would look more like this

 

I think you are heading in the right direction.If the boat is being trailed regularly there is a case for some foam cored longitudinals.Before putting them in place it could be worth adding some limber holes.A simple way is to get hold of some plastic water pipe-32mm or so-and split it in half.Fix a generous length across the location where the foam will be located and it needs to protrude a good 20mm either side of the foam.Glass over the split pipes and then add the foam.Glass over the lot and then trim the plastic pipe back to about 8mm from the face of the foam and you will have a limber hole that doesn't cause weakness or retain dregs of bilge water.Low density polyurethane foam is adequate for forming stiffeners over but for structural considerations you really need something a bit better.One of the PVC foams is much better and I would guess that 65Kg/cubic metre would be a starting point.Easy composites might be worth a look or East Coast Fibreglass.I have used the UK equivalent of coosa board and it has it's uses but I find foam a bit easier to form and with edges that need less work if you chamfer a panel back to the lower surface.

 

You would love foam. The boat will run quieter, stiffen, and be unsinkable.

What's not to love?