Plywood to fibreglass hull

[Mickster1969]

Hi there,

We bought a 17' Magregor Venture a short time ago. It is a fibreglass hull with wooden seating and deck inside. I pulled up some punky panels in the bow today and found the wood underneath was completely gone and rotted away entirely in places so I have to replace it all. I have a few questions which I would greatly appreciate some help with as I can't seem to find good answers on the net.

1. The bulkhead spaces were all filled with polystyrene (some completely sodden) but I don't believe there would have been enough flotation from these to keep the boat above water (if so it would be bow up I believe as there are no blocks in the stern space). Is it worth putting fresh polystyrene in place?

2. I need to replace three ribs/bulkheads that support the forward berth type area (think that is what the semi-circular port to starboard supports are called?) and I can see that they will need to be cut to shape and then glassed to the hull. Would I glass the whole rib/bulkhead or just the section next to the hull? How many layers are generally advised for this? Also is it not worth leaving a hole between each bulkhead at the centreline to allow any ingress of water to run aft?

Any help you can give or even link to a good website or utube video would be greatly appreciated!

 

[macd]

Is it polystyrene or some sort of injected foam, like builders' foam? The latter is often open-celled and takes up water: not good stuff. Closed-cell foam is often used to give strength to a structure. It often comes as a 2-pack and expands in situ. Commonly used in canoes. Probably better all-round, though, to fabricate new bulkheads which do the job on their own. Might even liberate some storage space.

Bulkheads in a GRP boat are usually bonded only at their edges, typically with several layers of chopped-strand mat or cloth. If this isn't evident elsewhere on your boat, maybe on a neighbouring boat with the owner's permission?
Ribs in GRP boats are often built from GRP on a simple core of closed-cell foam, which is more flexible than wood, although wood can certainly be used. The strength comes not from the core, but from the heavy lay-up of GRP over it and to the hull.

The holes you postulate are called limber holes and are indeed worthwhile. Best practice would be to bond a short plastic pipe in the rib/bulhead with epoxy, rather than leave bare timber at the mercy of bilge water.

 

[TQA]

NB if glassing in new bulkheads the ply wood should not be in direct contact with the hull. I am told that a thin strip of high density foam should be fitted around the edges of the bulkhead to prevent the direct contact.

NB lots of good info here. http://www.westsystem.com/ss/

 

[Poignard]

NB if glassing in new bulkheads the ply wood should not be in direct contact with the hull. I am told that a thin strip of high density foam should be fitted around the edges of the bulkhead to prevent the direct contact.

NB lots of good info here. http://www.westsystem.com/ss/

The WEST manuals and videos are first-class

http://www.westsystem.com/ss/how-to-publications-2/

 

[Alan ashore]

NB if glassing in new bulkheads the ply wood should not be in direct contact with the hull. I am told that a thin strip of high density foam should be fitted around the edges of the bulkhead to prevent the direct contact.

Although this is certainly best practice, to reduce the extent to which the bulkhead creates a "hard spot", I'm not convinced the effort is worthwhile when you are repairing a smallish oldish boat that will not, I'm sure, have been built with this refinement.

 

[Mickster1969]

Hi Mac,

It is polystyrene - not any kind of builders foam. This was the stuff that tv's come packed with. It was cut to shape in mny places and densely packed in the spaces between the deck, hull and bulkheads.

 

[Mickster1969]

Thanks TQA,

I read somewhere else that the plywood edges hould be sealed with some form of resin which I guess would achieve the same aim.

 

[Mickster1969]

Thanks all for the replies - I'll check out the Western site later!

 

[TQA]

Thanks TQA,

I read somewhere else that the plywood edges hould be sealed with some form of resin which I guess would achieve the same aim.

Nope there are two totally different things involved.

Sealing the edges with multiple coats of thin epoxy prevents the plywood absorbing water through the end grain.

Using a strip of foam to avoid direct contact with the hull prevents the bulkheads pushing directly on the hull and giving the effect of ribs looking from the outside. Like ribs on a skinny dog.

 

[PCUK]

Lots of good advice here. Also read the West Systems book and you will quickly become your own expert.

 

[PuffTheMagicDragon]

Here are some methods of fixing a bulkhead to a hull. In y opinion, the best solution in your case would be (a) but using high density closed cell foam where there is a space marked. using A strip that is wider than the bulkhead you can cut the top corners so that you will have the base for a fillet when applying the fibreglass.

 

[William_H]

Regarding the question of cutting holes in the bulkhead to allow any water to escape into the main bilge. This much depends on the chances of water getting into the area divided by the bulkhead. If the area can be sealed by a top cover ie typically bunk areas then these can make an excellent buoyancy tank. Fit a water proof inspection port in the top and you can even use the area for storage of seldom used stuff.
On this subject buoyancy tanks on each side of the hull at the turn of the bilge (chine area) can provide not only buoyancy but roll stability if the boat is swamped. So on a sail boat the whole bunk area down each side can be made buoyancy tank so any water ingress is limited to the centre of the cabin. Unless of course you get a hole in the side of the hull. Likewise any buoyancy foam you fit is better under the gunwhale or hull sides than bow and stern. good luck olewill

 

[Mickster1969]

Good tips - thanks Puff and will.