Stringer foam
 

Hey guys, I've got the stringers on my 74' 23sf cleaned out and I'm about ready to fill them back up and continue on with the rebuild. I was wondering what type of closed cell foam to use, like specific brands and densities? How much foam am I going to need? Also, is there anything I should know before doing this? I was thinking of maybe bracing the stringers shell so the foam doesn't make the fiberglass expand or anything like that. All input is welcome. And if anybody could explain how I upload images to this post that would be great also.

I would use no less than 8 lb density myself but that is because I only believe in doing a job once. The 8 lb is nice and hard. You may be able to get away with 4lb if you layer up the glass cap good. Quantities should.be calculated using dimensions inside cavities to be filled and a good online volume calculator. Figure 10-15% extra for waste and learning curve..... with the foam you're damned to get it all out of the container you mixed it in so you lose some there too. Buy lots of mixing containers. Best but most expensive way is to make sure your pour expands over the top, then cut/ belt sand down to finished dimension. As far as bracing, you can use a 2x4 or straight edge braced off hull sides/ other stringers. Word to the wise: do one at a time and be patient!

I have to ask why re-foam? Is there any structural benefit of adding the foam back other than floatation? I seriously doubt the amount of foam in the stringers would do much to keep one afloat offsetting the engine or other heavy objects.

Now you are referring to the filled stringer boxes and not foam poured loosely between, correct?

Cap the stringers w glass first.

Drill holes in the top of the cap.

Go in small batches low to high.

Each sq ft of foam is equal to roughly 60lbs of flotation. Submerged weight of fiberglass (.33 k factor) and aluminum (.63 k factor) is very different from the weight on land. This manual explains how to calculate your flotation needs. Page 20 (or page 24 of the pdf)

http://www.uscgboating.org/regulatio.../FLOTATION.pdf

On a 23ft seacraft each stringer is roughly average of 15 inches deep x 2 stringers x 16ft +/- . . . that's a LOT of flotation just in the stringer foam. If you round it to 1 cubic ft for each foot of stringer . . . that's approx 32 cubic ft x 60lbs = 1920lbs of flotation.

If the hull is solid glass (which it isn't) but just for the explanation let's say the hull weight 2600lb was solid glass . . . the submerged weight would be 2600 x .33 = 858lbs of flotation would be needed to float the hull. When you factor in plywood cores and balsa cored decks . . . it'll weigh even less. Even the motor (assuming it's a 50/50 mix of steel (.88) and aluminum (.63) = 1.51/2 = .755 k ) so a motor underwater only weighs 75% of what it does when it's not submerged. A 600lbs motor would only weigh 450lbs underwater. My point is that the stringers full of DRY foam is PLENTY of flotation for 23 with motor and people.

This video from Metan Marine shows you how they foam stringers on their builds. See 10:14 - 11:00. You can thank Mike B. for contributing technical information to the site :) Looks like 2 or 3lb foam to me . . . that's just a guess.

https://www.youtube.com/watch?v=ZPj4rBlZyPo

When lost, the hulls are found.

Yes hooligan, I was referring to the stringer boxes. I figured that they put foam in it for a reason so I was going to replace all of it, even the loose foam on the outer side of those boxes. All of the foam in the back half of the boat was wet and holding small amounts of water. I appreciate the info and input from everyone so far

I think I figured out how to post pics also. Here's a few to show y'all what's going on at the moment..

Notice the holes cut into the tops of the stringer boxes. They were never glassed back in.

Wouldn't a watertight hollow stringers be more buoyant than a watertight foam filled stringer of the same size?? Same would apply to stringer boxes. I suppose foam is just there in case hull is damaged not just swamping.

Foam has a surprising amount of compressive strength - it also keeps the stringers from flexing and folding/fracturing. Kinda like the core in a deck. You could get the same strength by adding glass but that would weigh more so there is a weight/structural aspect too. Also foam is way cheaper than the glass needed to get the same rigidity.

I kinda think the foam has strength and supports the bottom on both sides of the keel.it is boxed in and when the hollow stringers become full and solid it adds density and toughness,along with flotation.these boats were meant to have it for a reason

Be carful if you pour to much, If the tops are on and you pour threw a hole it can expand at a rate so fast it is easy to over do it and crack or swell the stinger case apart. When I did the ones on my 74 I cut the tops out then formed up the sides with wood a few inches higher than the stingers (mainly because I was raising them 2") and poured it in batch by batch. The foam rose up like bread in a bread pain crowing at the top higher than the forms. I simply took a 20" sawzall blade and shaped them to my liking on top then wrapped the tops with glass over lapping to the sides tabbing to the hull. If you are not raising them pour them like they sit then once the foam rises out of the cavities trim it flat to the top then glass the tops closed to get you box back enclosed for strength. The foam density may be effected if it doesn't have room to fully expand in an enclosed compartment or worse it splits something apart. I was filling pontoons on a work barge and found out the hard way splitting welds open as the foam had no were to go.

I know this post is a little old, but does anyone have any suggestions for brands/densities for the foam? I will be re foaming my stringers soon and would like some input. Thank you

Are those black truffles there?

US composites or ping member Shine here. He has deals on stuff like that.