Foam for stringers

Wildman · #1 08-26-2010, 09:36 AM

I am looking to refill my stringers with foam. The boat is a 1972 23CC.

The P.O. tore the tops off them and now they are wet. I have started to dig them out and clean the outside to prep for layup of new glass over top.

I am assuming 8LB foam is what I should be using? How much (how many kits) should I plan on buying? I dont want to be short but I also dont want to be buying 2X as much as I need!

JohnB · #2 08-26-2010, 10:27 AM

I am not sure I would put the foam back in. I would drill some drains near the bottoms of the stringers, and then use epoxy to stitch and strenghin it. Someone else might want to chime in, but I don't think the foam in these was structural, but I could be wrong. The foam just created a convienant mold to make the boxes. The glass box of the stringer is it's strength, and epoxy will give you a lighter stronger material and a chemical bond in addition to the mechanical bond.

Wildman · #3 08-26-2010, 11:11 AM

Here is the problem there are LARGE holes cut in the boxes on top. He did cut some holes in the bottom of the stringers so they drained but the foam is now soaking wet anyhow. I don't mind re-foaming as I don't like the thought of leaving them empty. I will use the very top of the foam as a mold to lay over top of the strings and seal them back up. I think the whole stringer system wil get a coat of glass to re-seal it.

(I have also heard the wood in stringers is just a 'mold' for the glass, which I proved wrong when my mako sank.)

Bushwacker · #4 08-26-2010, 12:17 PM

JohnB is right . . . foam provides NO significant structural benefit, other than to provide a male mold for the stringer. It's only benefit is flotation (and additional weight when it gets wet!

) Stringers function like a beam, and most of the stress in any beam occurs in the outer fibers so that's where you want the most glass. I-beams put most of the material at top and bottom for that reason, but a box section section stringer is used because it's light and also has more torsional stiffness than an I-beam.

Wildman · #5 08-26-2010, 01:18 PM

Let me re-phrase it then.

I need to re-foam the stringers to re-glass the top of them because the PO cut so much out of the top of the stringers its not allowing for enough 'torsional' strength.

Right now the boat will rack back and forth as you move across it!

Cheg · #6 08-26-2010, 04:45 PM

I used 2 lb foam. Mertons has it for $72 for a 2 gal. kit. Used most of two of these kits (4 gals.) The temp. was around 65 to 70 degrees when I did it so it may expand more for you in the warmer weather.

Bushwacker · #7 08-26-2010, 04:57 PM

Reglassing the top of the stringers will certainly help, but once you install the deck/cockpit sole and bond it to the top of your reglassed stringers, that'll tie all the stringers together, creating a BIG increase in torsional stiffness! I think Potter used a polyester cabosil/putty for that, but if you use epoxy putty, it'll cost a little more but will have much higher bond strength. That will definitely eliminate the racking you have now with no deck in there!

Wildman · #8 08-27-2010, 08:38 AM

Quote:

I used 2 lb foam. Mertons has it for $72 for a 2 gal. kit. Used most of two of these kits (4 gals.) The temp. was around 65 to 70 degrees when I did it so it may expand more for you in the warmer weather.

Thanks that was exactly what I was looking for.