Stringer question, glassing over 4lb foam?

[theFunsmith]

Made some serious headway on my console and overhead electronics box over the winter. I am currently planning out my moves for when the weather improves enough to return to the hull. My next step is getting the stringers into shape. I was wondering if anybody here had tried laying glass over the top of foam. I am using vinylester, and was wondering if the resin would melt the foam, or if I would be safe forming stringers (with 4lb 2 part foam in waxed mdf molds) and laying my glass over the top.

[RickK]

I think we've seen this done here. I think someone was buying pre-made foam stringers and glassing over them.
I did the opposite and made forms (the inverse of what you're doing) and glassed over them.  I then scribed the hardened stringers to the hull, tabbed them in, drilled holes in the tops and poured foam into them.
You could also do the same with your forms. I used cheap plastic drop cloths to line the forms, then I glassed over the plastic.  When the glass hardened, I peeled the plastic off the glass and scuffed the glass with a grinder to smooth.
There is many ways to tackle this.

[Levi]

If you're going the foam over glass route it's best to make a mold as Rick described. If you try pouring foam into a mold your results will be less than satisfactory because there's no constricted space to keep the cell structure of the foam small and therefore "closed cell". In an open mold you'll find you have big voids inside the foam and it won't be long until those voids fill with water.
For my flatback I'll be using a pair of prisma hull beams that already have a layer of triaxle stitched on them but I will reinforce them with at least two more layers of glass. I'll probably go with vinylester resin too.
Depending on your hull shape and your location you may also want to look into the hull beams but shipping may be cost prohibitive. Mine will be about $175 each for a 12' length and bought locally but having something that length shipped could get expensive quickly.

[theFunsmith]

Thank you for the replies gents. Maybe I can shed a little more light on my thought process. My stringers currently are solid. I am just going to be raising them 3" and extending them back to the transom where they were cut out to glass the new transom core in. I was considering cutting the tops off of the existing stringers, then temporarily mounting mdf to the existing stringer sides as a mold for new foam. If constriction to that foam is essential, I could theoretically cap the mdf mold.

[RickK]

Aaron, one of our members, raised his sole by 3 or 4 inches by taking a plastic fence post and wrapping it with glass. Then he glass that to the stringer tops.  Gave him a place to run wires too. 

[mshugg]

I think that rick is onto something.  If your stringers are sound, why cut the tops off?  Just add a spacer, whether the glass wrapped tube, or layers of foam.  Or create your mold and pour with 4lb foam over the uncut (and uncompromised) stringer Then you can simply glass over your spacers.  It would be less work, and the result would be just as strong.

[theFunsmith]

I like both of those ideas. I suppose there is no reason not to cut the tops off. I was considering it as a way to cut unnecessary weight, and a way to quickly remove the leftover globs of previous sole adhesive. Leaving the current stringers intact and adding some kind of tubular or foam spacer does sound pretty quick and easy though. Thank you guys for the quick replies, damn I love this site.