rotten stringers and floor - foam reinforcement?

Hello,
I have a 40 year old 21' fiberglass boat with an end grain balsa bottom. I was working on removing 3 bad stringers and I noticed the fiberglass was delaminated on the bottom of the hull. I was told it was "sandwich construction" and the delamination was poor construction of the boat and it may be junk. The layer under the delamination which I believe to be the balsa is still solid although I am sure it is wet. Someone suggested using a foam (like boston whalers) and leveling it off and putting in a new floor to be able to safely use it. Can I do that? Would that offer structural support? What if I put in sister stringers and and then filled it with foam. I don't want to just junk the boat so even if I could do something that would get 5 years out of it would be better than nothing. Any input or ideas would be helpful. Thanks!

 

I don't know if there is a cut and dry answer here but I'll give you a little input. I'm of the opinion that foam adds significant structural benefit. The seaworthyness and longevity of Boston Whaler is a good example and some of the toughest boats built are foam filled. I don't think it is the best idea to rely on it alone for structural integrity however. The other potential problem is that it will trap and absorb water below deck without some thoroughly planned drainage. Trapped water leads to rot in wood cores as you already know (and water log issues). That's not really an issue for you at this point, so I think you are on track with sistering up the stringers and filling with foam. I'm not keen on short cuts and patch ups but in some instances it is the practical option. Maybe a good quick and dirty method of getting a boat sound again. One potential issue you do need to consider is that the bottom could continue to flex because of the delamination. It could actually seperate and shear off due to excessive flexing. In an ideal situation the foam bonds everything together to form one piece, but the existing delamination you have will still not be fixed. You still may be o.k. but that is the main thing I would consider. Also, I usually opt for 4lb density foam when looking to add structural integrity. That will double your cost though.

 

I do not get a very clear view of the situation, but I imagine a cored bottom, delaminated, and stringers to keep panel size down.

You are replacing stringers, so I guess you can reach the area safely. I also presume you have removed the delaminated inner skin.

Wet balsa: As long as it is not dark coloured, it is salvagable. Find out where the water came from (inside or perhaps a cracked laminate outside?) and if the balsa still is adhered to the outer laminate. If not, replace the balsa, and do a proper installation (hotcoating and bedding compound, preferably vacuum).

Install a new inner laminate, nicely scarfed into the existing laminate, and install new stringers. With this construction the boat survived 40 years, so if done well, you are good for another 40 at least.

Personally I am not fond of foam filled boats. About any example I have seen had water logged foam, not helping in buoyancy anymore, and making for a lot of weight. I prefer hollow, inspectable and drainable areas in that respect.

 

You need to remove the core and replace it. Structural foam or balsa would work. Anyone that states that a boat that lasted forty years or more is of poor construction is a fool.

 

Some people get a little confused between construcion foams used for cores and buoyancy 2 pot exspanding foams that also have a slight stiffening effect for a while till they get wet and keep on drawing moiture till they are saturated . .

 

OK, to put it differently:

I do not like 1K or 2K polyurethane foam (PU, PUR, PIR), whether in sheet form, or 2K mix and pour, anywhere near a boat, perhaps except for DIY refrigerator construction. (Its thermal properties are good).
These foams have a long term water absorption of potentially 100% in volume. Boats using these foams should be constructed such that no water can ever enter the foam. Tough job...

I do like rigid, constructional foams which basicly come in 3 flavours now: PVC, SAN and PET foams. Brand names are Airex, Divinycell, Klegecell, 3Cell, Corecell, Armacell, and I certainly did forget to mention a couple.
These foams have a potention water absorption of some 2% in weight at max, which means that a general 40ft boat will absorp some 1,6 kilograms (3 lbs) of water. At most.

 

Foam Testimonial

I'll chime in from my current project. I have a 19ft center console that had all kinds of rotting wood problems. Transom, stringers, and most recently the deck. I separated the inner liner from the hull and am working on them independently.

The 2 part pu foam was full of water, probable 400lbs of extra weight. The gunwale rod holders and other openings let water seep into the foam. I cut pried and scraped all the old foam, fixed the stringers, and put way too much thought into drainage scheme to make sure no water gets into the new foam I put back.

I am adding tubes for the horizontal gunwale rod holers, piping drainage from the vertical (trolling) rod holders, glassing in the sides and the tops of some compartments, and directing all drainage to the bilge area. I agree from some of the statements that it would be best to have all open and drainable spaces, but with a small child, I would not want to sink the boat and be free swimming offshore.