Flotation Foam, Structural or not?

[Boataudio]

Hi, this is a great source for help. I am in the process of replacing my deck on a 1980's 17.5 foot Larsen silverline. I have read hundreds of posts and have gotten tons on useful info. One post mentioned removing the wet flotation foam and replacing it with something other than pour in foam like the pink insulation sold at Lowes or Home Depot. One of the concerns was whether the foam was structural or not. My question is how do I know if the foam is structural? Thanks for any help and thanks for this forum.

 

[erikgreen]

Re: Flotation Foam, Structural or not?

Whether foam is structural or not depends on the design of the boat it's in.

Just knowing what I do about Larson boats I've seen, I'd guess probably not. Boats where foam is structural are very much in the minority, partly because it makes them harder to manufacture and partly because foam dense enough to really be structural is expensive compared to floatation foam.

Erik

 

[Boataudio]

Re: Flotation Foam, Structural or not?

Thanks for the reply Erik

 

[Spinnaker]

Re: Flotation Foam, Structural or not?

Most times the foam you find in the hulls is 2lb foam meaning it only takes 2lbs per square inch to dent it. Not very much structural property in that.

 

[Boataudio]

Re: Flotation Foam, Structural or not?

Thanks Colin, I will post some pictures of the project as it gets underway.

 

[Woodonglass]

Re: Flotation Foam, Structural or not?

You might find this interesting.

http://forums.iboats.com/showthread.php?t=367571

 

[Mark_VTfisherman]

Re: Flotation Foam, Structural or not?

One other point is that sheet foam isn't going to be structural to the boat unless it is adhered to something...

the pink foam at home centers is either 150psi or 250psi compressive strength- good for support of floors for example, or under concrete, but it won't add any torsional or tangential strength to a hull unless it is attached/adhered to the hull bottoom and/or sides.

 

[sbklf]

Re: Flotation Foam, Structural or not?

Most times the foam you find in the hulls is 2lb foam meaning it only takes 2lbs per square inch to dent it. Not very much structural property in that.

? The 2lb is 2 pounds per cubic foot. This is what it weighs not its compressive strength.

Whatever you put in your hull needs to be fuel resistant more especially if you have a underfloor fuel tank but even if not. You might try seeing if gas melts it like it does a lot of Styrofoams.

For the most part the foam itself does not offer not structural support. However, in the case of fiberglass stringers with foam on both sides of them, it helps keep the stringers upright and that is structural so in a way it does in that design.

If you want a cheaper alternative, you might consider plastic jugs for flotation. Soda bottles are a little thin and I would not rely on them but if you could get a more stout of a bottle and somehow tie them all together and maybe add an extra seal to the cap that would work.

 

[Spinnaker]

Re: Flotation Foam, Structural or not?

? The 2lb is 2 pounds per cubic foot. This is what it weighs not its compressive strength.

Thank you for the correction SBKLF. I was going by what someone had recently told me. I will be correction him as what you say makes much more sense.

 

[erikgreen]

Re: Flotation Foam, Structural or not?

Thank you for the correction SBKLF. I was going by what someone had recently told me. I will be correction him as what you say makes much more sense.

No, that's correct actually. The lbs per square foot is density, not weight. Density is directly related to compressive and shear strength.

2 lbs per cubic foot foam you can dent with a fingernail. It can still be structural depending on how it's employed, but it's no good as a composite core. Usually if structural it's used to reinforce wood supports and damp vibrations. This or 1 lb per cf is the standard floatation foam for boats.

4-8 lbs per cf foam is harder (8 is much harder) and can be used as structural cores or supports for other parts. People also use it for casting models, duck decoys, buoys, and some other things. It can survive real world use without a ton of protection.

16 lb foam is rock hard and very structural. Due to the cost it's not used often and not used extensively, but it can make a very strong, lightweight panel. People make fake rocks out of the stuff.

Erik

 

[Spinnaker]

Re: Flotation Foam, Structural or not?

No, that's correct actually. The lbs per square foot is density, not weight. Density is directly related to compressive and shear strength.

2 lbs per cubic foot foam you can dent with a fingernail. It can still be structural depending on how it's employed, but it's no good as a composite core. Usually if structural it's used to reinforce wood supports and damp vibrations. This or 1 lb per cf is the standard floatation foam for boats.

4-8 lbs per cf foam is harder (8 is much harder) and can be used as structural cores or supports for other parts. People also use it for casting models, duck decoys, buoys, and some other things. It can survive real world use without a ton of protection.

16 lb foam is rock hard and very structural. Due to the cost it's not used often and not used extensively, but it can make a very strong, lightweight panel. People make fake rocks out of the stuff.

Erik

Ok, thanks. Makes this builder feel a little stupid.

 

[erikgreen]

Re: Flotation Foam, Structural or not?

Nah, don't be. With all the folks providing information here, you can usually find two completely different opposing answers to any question.

The trick to figuring out which is right, I leave to you....

Erik

 

[sbklf]

Re: Flotation Foam, Structural or not?

http://www.shopmaninc.com/foam.html

 

[BobsGlasstream]

Re: Flotation Foam, Structural or not?

Hello all,
I normally stay out of these conversations but sometimes I can't help myself.
sbklf, is correct, The 2 pound density foam is weight per cubic foot. Density is determined by volume and weight.
As for the structural ability of the foam it depend on how it is used.
The density of the foam is also determined in the free rise state. If you restrict it's rise without venting, you can increase the density of foam in a given volume. I have double the density of foam utilizing pack density.
You can also change the hardness of foam by changing the index ratio of the mix.
Most of the foam's used for flotation today are closed cell rigid or semi rigid foams. The same foam is used in the automotive world as EA foam, (Energy Absorbing foam). This foam is currently being used in a lot of the 2010 model cars headrest to enable them to pass the newest Gov. regulations (202).
I have only worked with foam for about 30 years in the automotive industry, The plant I work in only uses several thousand pounds per day. Including the above mentioned foam.
Sorry for the long post.
Bob

 

[oops!]

Re: Flotation Foam, Structural or not?

Nah, don't be. With all the folks providing information here, you can usually find two completely different opposing answers to any question.

The trick to figuring out which is right, I leave to you....

Erik

speaking of definitios and opinions....a wonderful discussion about the use of silicone has begun on my thread.


as far as structural foam.....some manufacturers tried to cut costs on resin and glass.....so they actually used the flotation foam as a reinforcement to the fiberglass on the hull.......while the foam alone is not strong enough to withstand any force....in conjuction with a light hull....it is very stable and will withstand hydrodynamic forces.
i.e.....take the foam out of your capri hull, hit some wakes......and start swimming !

your larson is NOT in this class.

the main example of structural foam is the boston whaler

 

[erikgreen]

Re: Flotation Foam, Structural or not?

Hello all,
I normally stay out of these conversations but sometimes I can't help myself.
sbklf, is correct, The 2 pound density foam is weight per cubic foot. Density is determined by volume and weight.

Um.. I think maybe you're confusing a couple people. sbklf said:

? The 2lb is 2 pounds per cubic foot. This is what it weighs not its compressive strength.

I think we're all agreed at this point that the weight/cf numbers give density, which is closely related to strength, which is what we care about if a foam is "structural" or not.

As for the structural ability of the foam it depend on how it is used.
The density of the foam is also determined in the free rise state. If you restrict it's rise without venting, you can increase the density of foam in a given volume. I have double the density of foam utilizing pack density.
You can also change the hardness of foam by changing the index ratio of the mix.

Actually, since most of the polyurethane foams supplied by boat repair shops etc. are in a two part pourable form with a fixed ratio, you can't alter the mix at all. You can restrict expansion by confinement or temperature, though.

Most of the foam's used for flotation today are closed cell rigid or semi rigid foams. The same foam is used in the automotive world as EA foam, (Energy Absorbing foam). This foam is currently being used in a lot of the 2010 model cars headrest to enable them to pass the newest Gov. regulations (202).

Yea, the rigid PU buoyancy foam sold by USComposites and others can "catch" bullets in the right shape and size. It absorbs the impacts pretty well.

I have only worked with foam for about 30 years in the automotive industry, The plant I work in only uses several thousand pounds per day. Including the above mentioned foam.
Sorry for the long post.
Bob

Just to let you know, we do appreciate your contributions here and look forward to more.....which is why I'll tell you that it takes away from your credibility to post your job experience.

It's the same sort of thing we see from people who "worked all their life repairing boats" etc... it's the sort of background that would make a witness in court credible to a jury, but logically speaking it's just fluff.

It's possible to work for 30 or 40 years in a boat repair shop and not learn a damned thing about boats or fiberglass, no matter how many boats get fixed or how much glass is used.

I'm sure that's not the case with you and that you know something about foam, but it's a pet peeve of mine that people try to substitute an impressive background for logic or supporting information in a discussion here

Erik

 

[Mark_VTfisherman]

Re: Flotation Foam, Structural or not?

...it's a pet peeve of mine that people try to substitute an impressive background for logic or supporting information in a discussion here...

But, but, I stayed in a Holiday Inn Express last night??

 

[Lou C]

Re: Flotation Foam, Structural or not?

I would contact the boat manufacturer and ask them if it was structural....I did this in my 88 FW rebuild and FW told me that the foam was for both structural and flotation reasons...so I felt that I had to replace it....and if you look around at various re-builds here and on other sites...boats that had foam for structural reasons usually do not have as many transverse bullkheads as boats that did not have foam...in my 88 FW there are just 2 transverse bulkheads...one at the front of the gas tank area and one at the rear....I recall a rebuild of a 22' Wellcraft (built with no foam) that had a lot more bulkheads. So what I'm saying is if the boat was built with foam in its design and you don't replace it you may have to add structure to add back the stiffness that the foam provided...I'd hate to see someone do all the work to replace a deck and wind up with a flexy...banging...junky feeling boat...
If you glue the deck to the stringers in addition to using SS screws to hold it down...seal all the holes....AND GET RID OF CARPET ON THE DECK....STORE IT RIGHT WITH AN ADEQUATELY SUPPORTED COVER...you'll have something that will most likely last way longer than the factory build...

We all have to keep in mind if you are not an engineer then don't pretend you are one...ask the people who designed and built your boat before leaving out foam...

After my rebuild I was really pleased to see how solid this old boat felt out there on the waves of Huntington Bay and LI Sound....

 

[BobsGlasstream]

Re: Flotation Foam, Structural or not?

erikgreen,
I can assure you I'm not trying to impress anyone. I also will not get into a battle of wit's with anyone either.
My intension is to is to give correct and accurate information for people to use and not just opinions.
I agree with the fact that people often work in or around a lot of processes and think they are experts in that field.
That is one reason I stay out of the foam conversation most of the time. I feel we should state the facts and let them make their own decisions.
If I have offended anyone that was not my intension.
Have a great day.
Bob