bertram31.com

Down to the bare hull in the 25 and the 2 main fiberglass box stringers look like a good candidate for pouring with a structural type foam. Looks like a good way to add some beef with very little weight gain. I know most of the boats built today use a maze of glass boxes injected with a structural foam.

Can one of you guys recommend a structural foam for this application?

Scot,

4 LB Density Urethane Foam is what you'll want... The source I use is US Composites in West Palm Beach.

Da' Judge did his B25 with 4 lb.

Br,

Patrick

Thanks Capt Patrick,

Is the "4 LB" a reference to the cured weight p/cu ft?

That'd be it...

scott,

what are your plans for letting water move through the hull?? I've been thinking of doing the same thing with the foam but trying to plan out where I can and cannot put foam has me confused.

My thoughts so far are to put it in outside the main stringers and leave the stringers alone.

what are your thoughts?

My 69 drains the outboard section of the main box stringers via a gap at the stern. I will be foaming the interior of the box stringers only. I have fuel tanks going into the voids outboard of the stringers. I will be retaining and glassing the small openings (drains) at the stern end of the stringers to allow any water outboard of the stringer to find it's way to the bilge.

As far as I can tell my hull does not drain via the interior of the box stringer. If your wanting to foam the voids outboard of the stringers....that would be a different matter.[/b]

scot wrote:My 69 drains water, via a gap at the stern, both in and out. If your wanting to foam the voids outboard of the stringers.,. and keep the water out...that would be a different matter.[/b]

hummmmmmmmmmmmm, Bertram enginering at its finest

You are correct Thuddster....Truely Bertram engineering at it's finest indeed. Mark II 25 Bertrams came with a fiberglass box stringer grid system 35 years before the rest of the boating industry. Thanks for pointing that out!

The small gap (drain) is in the stringers, sorry for not making that clear!

With all of the Arm Chair Engineers on this site there is not a problem that we cannot solve.........

In fact I can remember only one problem that we really never solved

Do ROCKER STOPPERS really work????

Harry

Scot, having removed 1600 lbs of wet foam from a 2,500 lb (supposedly) b20 my advice is ensure that there is NO WAY water can get anywhere near the foam. The foam is a sponge it will retain water like Rosie O'Donnell. The foam encapsulation must be watertight. It does make the hull quieter and more rigid though, done right it is great, I'm not sure from a risk/reward perspective. I know that you like to keep things simple...

Harry...DON"T BRING UP THE ROCK STOPPERS!!!!!!!

Rocket,

The urethane foam used today is closed cell and will not absorb water. I have pulled the old, water-logged stuff out of boats as well....Your right it's nasty!

Thanks

Closed cell foam wont absorb water.

SCOT- Are you talking about using 2 part pourable foam--opening stringers and pouring it in and then encapsulating stringers?

Capt Patrick--are you talking about supporting existing stringers with structural foam sheets cut and glassed outside of the existing stringers ?

I am waiting for the thaw here in Michigan to access my Stringers on my 25 that I picked up in December.(also a 1969)

Thanks for the clarification in advance.
Capt Dana

Capt Patrick--are you talking about supporting existing stringers with structural foam sheets cut and glassed outside of the existing stringers?

My only reference here is to the 2 part expandable urethane foam...

Closed cell foam wont absorb water.

In theory that's true. However, in actual application on a boat the rigid foam is subjected to the flexing of the boat which ruptures some of the cells & if water is present, as is the case in the bilge, it will be absorbed into the ruptured areas.

80% of that absorbed water will not drain out. Expansion & contraction of both the hull & the trapped water will rupture more cells, allowing more absorption. There is probably an equilibrium point where no more cell crushing occurs, but by that time, a very significant amount of the foam is water logged. As Rod & others who have "been there, done that", can testify, the only way to get rid of wet foam is to dig it out...

The only closed cell foam that truly will not absorb water, under any condition, is closed cell rubber foam. Most closed cell rubber foam is also a urethane, but is formulated to be soft & flexible.



Br,

Patrick

The stringers are hollow glass boxes and the intent of foaming would be increased strength and sound deading in the engine bay area. I intended on drilling half a dozen 1" dia holes on the top of the stringers and pouring the foam down the holes, then glassing over the holes before the decks are installed. I don't expect much water to get inside the sealed box stringers, but as we all know water WILL find it's way into every area on a boat. But I certainly would NOT want anything that "may" end up taking on water?

So what's the verdict? To foam, or not to foam?

Capt Dana, mine is getting a center located in-line, inboard. I would not go to the trouble with any other 25 drive line configuration.

I cast my vote for NO FOAM. ( allthough the rubber foam sounds interesting ...and pricey)

Them stringers are not broke Scot, don't try to fix what aint broke....

I cast my vote for NO FOAM. ( allthough the rubber foam sounds interesting ...and pricey)

Buju,

I'm with you... I would never foam in the bilges myself. The soft rubber foam should only be used for cushions & bolsters...

Br,

Patrick

Scott--

I foamed my 25 back in about 1980. Give me a call and I'll talk you through the process. Yours will be much easier since you are only doing the stringers and the sole of your boat is removed. 1" holes are not large enough. Call me.

Da Judge

Trey Dibrell
Galveston
Cell--409 739 0415

I did the outer cavities (outboard of the stringers) of my 25 with 4 lb foam to quiet things down and soften the ride. After my experience with the 20's new foam, I think it'sa good idea. That said, I would not fill the stringers-you really don't gain anything at all, IMO...

CMP

waste of time in my opinion.

and whoever thinks closed cell foam does not absorb water can come down and take the garbage pails of the stuff out to the dumpster next time we dig some out :-D

Having only seen the grid of 2 of those 25 Bertrams I can tell you the main issue with them was the grid not properly bonded to the hull to begin with, and the lack of bulkheading properly supporting the boat.
Thats why there is usually a abnormal amount of cracking going on the gunwale area, like nasty narly looking cracks- because first the stringer grid starts to let go, then the lack of bulkheads kick in so your riding around on a wet noodle so to speak.

The good news is a bulkhead just forward of the helm area that runs to the hull bottom, and some grinding and re-glassing of the existing stringer grid and your good to go.

Thanks for the inputs guys. Trey I'll get in touch with you in the next few days.

Raybo Marine...

lack of bulkheading properly supporting the boat.

I would disagree based on the service record of these very successful hulls. Basically there are still hundreds of them in service after 40 years. I've ripped into more than a few boats and every boat has a some areas that can be improved on, but I would say for the most part Bertram got it right on the 25's.

Scot,
I beleive all the original WET foam that I removed from my B20 & B25 was good ol closed cell foam. Same with the spongy Whalers. Good in theory, but when them cells open up...

I think the areas that CMP foamed in might have some merrit. Allthough them areas would great for hatches and additional storage, or fuel tanks in your case.

But to cut holes in, and compromise the original layup of the stringers to fill with foam...why? The biggest concern I'd have would be pouring a little too much in there, and when it expands it could actually delaminate the stringer from the hull. I'm not certain this could happen, but it would sure as hell be a concern of mine. Capt. Pat, Trey, CMP, anyone else... is this a possibility?

a few observations...surfboards are generally made with closed cell foam...trust me...it absorbs water. it just does it slower than open cell. my B25 because it is an inboard is rock solid. (ask patrick and bruce) with no bonding or cracking issues because the stringers are very overbuilt and over bonded to the hull.

scot...if you leave the pour holes open till after cure i would imagine they would act like freeze plugs on an engine block giving the expanded foam a place to go if there was too much

Buju,

No question about it, expandable foam, not properly vented, and/or pouring too much volume into a void too quickly can cause catastrophic failure to the structure. Trapped foam that's still expanding can create several thousand pounds per square inch of pressure...

I agree with Mark & Robbie in that foaming the inside of the stringers doesn't return a lot of benefit & that reinforcing the tabbing to all the stringers and the bulkhead would be a better idea.

The majority of the load carrying capacity of a beam or stringer is in it's vertical dimension. The second factor to yield a gain in loaded capacity is the compression/tension resistance between the top & bottom surfaces of the beam. A hollow box beam, (which is essentially what the stringers are), is just as strong as a foam filled beam of the same height & width.

Adding thickness to the vertical dimensions of a beam will yield significantly more load carrying capacity. Beefing up the hull to stringer tabbing, then sistering both sides of the stringer with something like 3/8" Coosa board & glassing over that & down onto the hull would be the way I'd probably go.

Br,

Patrick

Randle said:

.if you leave the pour holes open till after cure i would imagine they would act like freeze plugs on an engine block giving the expanding foam a place to go if there was too much

Randle that was my idea exactly, assuming the foam would expand out of the holes and I would trim off the excess once cured and glass over the 1" or 2" holes.

Note: My project is an extreme exercise in engine noise control and foaming in the stringers would help with structural born sound, while also strenghtening the hull. I am concerned that the hollow stringers could act like giant organ pipes with a loud engine 8" away from each of them.

HOWEVER, It's looking like the risk of getting a bunch of wet foam out weights any benefit....hey this is why we have the board and ask the questions!! Thanks to all. At the very least I can add "Soundown" to the engine bay side of the stringers.

Capt Patrick:

Beefing up the hull to stringer tabbing, then sistering both sides of the stringer with something like 3/8" Coosa board & glassing over that & down onto the hull would be the way I'd probably go.

Sounds like a much better plan to me.

scot wrote:Thanks for the inputs guys. Trey I'll get in touch with you in the next few days.

Raybo Marine... I would disagree based on the service record of these very successful hulls. Basically there are still hundreds of them in service after 40 years. I've ripped into more than a few boats and every boat has a some areas that can be improved on, but I would say for the most part Bertram got it right on the 25's.

I stand by my statement having seen 2 with the same exact problem.
However- if the stringers had not come loose to begin with then there would not be a problem, BUT, factor in lack of bulkheading and then there is a problem.
When the decking is being supported by the inner liner, which is supported by the stringers and it comes loose there is a problem- but when you have bulkheads tying it all in there is a back up.

The years of the boats slip my memory, maybe it was a production method that changed at some point- but im 2 for 2 on the same boat with the same problem.

Actually I have some pictures.

First few are the original bulkhead that supports the helm and windshield area- in other words most of the weight.
That fabric lined pywood bulkhead that is not tied into the stringers does not cut it, the one measly piece of tabbing against the hull is useless as well.
Remove that junk and install a true bulkhead, glassed to the hull sides and hull bottom and you have something.
The last picture shows the new bulkhead in place.

Forgive the first sideways picture









check out that cut-away, no support for the cabin or helm at all unless you count that fabric lined piece of plywood that was not properly glassed in to begin with.





in house fabricated glass over plywood made on a table to eliminate bodywork in the boat, then ground and glassed in place.



[/img]

they really did build the inboards different....my stringers arent hollow.