Foam Core VS Balsa Core - no stringers

[Shife]

Re: Foam Core VS Balsa Core - no stringers

I am always amazed at what appears to be stubborn refusal by most of the DIY'ers on this site to learn about composite panel construction.

Those who refuse to learn why balsa is a great core material, or why foam from Home Depot isn't suitable for a structural core, should be opening up their checkbooks to the pros and putting down the tools before they kill themselves or someone else.

Balsa is a wonderful core for its price point. Plywood is an abysmal core at any price point. Plywood is commonly used in mass produced boats because it is cheap, not because it is a good core material. (I'm referring to the common runabout, not a home built cold molded tortured okoume ply custom) Learn to isolate fasteners from ANY core and you will have a long lasting repair.


As a good friend of mine likes to say, "You can do it right, or you can do it right now."

 

[erikgreen]

Re: Foam Core VS Balsa Core - no stringers

I am always amazed at what appears to be stubborn refusal by most of the DIY'ers on this site to learn about composite panel construction.

Those who refuse to learn why balsa is a great core material, or why foam from Home Depot isn't suitable for a structural core, should be opening up their checkbooks to the pros and putting down the tools before they kill themselves or someone else.

That's a bit more harshly than I'd put it, frankly, but I worry about people using poor materials or techniques for repair because "someone on iBoats said it would work". All you can do is speak your truth clearly and firmly, and provide references and other sources so people can corroborate what you say. Unfortunately you can't help everyone.

Balsa is a wonderful core for its price point. Plywood is an abysmal core at any price point. Plywood is commonly used in mass produced boats because it is cheap, not because it is a good core material. (I'm referring to the common runabout, not a home built cold molded tortured okoume ply custom) Learn to isolate fasteners from ANY core and you will have a long lasting repair.

As a good friend of mine likes to say, "You can do it right, or you can do it right now."

I agree on Balsa, but disagree on plywood. For its price, plywood is an excellent core material. Jacques Mertens, one of the more forward looking architects of boat plans for home builders, uses for the most part plywood cores in composite panels, including the hull, with epoxy over biax glass (no mat) for the skins. His boats are high performance, light, and strong. (check out www.bateau.com for more).

Plywood has enormously high compression strength, shear strength, stiffness, and overall strength. It's also cheap and easily available. The downsides for it are A) It can rot and B) It's heavier than some cores.

I agree on the rest of what you say though..isolate the fasteners and seal any skin penetrations, and it'll last a long long time. Do it right, or do it fast, or do it cheap. Pick any two.

Erik

 

[erikgreen]

Re: Foam Core VS Balsa Core - no stringers

With regards to the strength of the DOW foams:

Here are the specs from the DOW site for their "Tuff-R" foam board, which is foam sandwiched between two durable face skins:

Compressive Strength(1), ASTM D1621, psi, min. 20
Flexural Strength, ASTM C203, psi, min. for 1" core foam 40
Water Absorption, ASTM C209, % by volume, max. <1
(2), ASTM E96 (dessicant method), perms <0.03
Nominal Density, ASTM D1622, pcf 2
Operation Temperature Range, ?F -50 to +190

So from this we see a compressive strength of 20 psi minimum. This is typical for all the DOW foam products, but some are up to 25 PSI. For comparison purposes, we'll assume the DOW foam board has 25 psi compression strength.

There is an exception to this - dow has a foam board made for high compression strength, it's "styrofoam highload" which goes up to 100psi. Not bad, frankly. Let's assume you're not using that though, because it's much more expensive than the common insulation foams. They also have "panel core" foams made for composite core use, but again those aren't commonly available.

From the test you give, 25 psi compression strength would make sense:

i stacked 350 lbs on a 5"x8" area of a 1/2 peice of foam, after 2 days the foam is not dented or compressed.

So a 5" by 8" area would be 40 square inches. 40 * 25 psi means that area could withstand 1000 lbs of force without crushing. It seems like a lot, but frankly it's not. According to a standard ANSI test for home window glazing:

"according to ANSI Z97.1 impact standard test that a 8lb steel ball that is drop from 4 feet has apprx impact force of 400 lb"

8 lbs dropped 4 feet impacts 400 lbs.. how much force do you think is involved with a person, say 200 lbs, jumping onto the deck from 2 feet up? Hint: lots more

For comparison, standard pourable urethane foam in the 4 lb/cubic foot density has a compression strength of about 90 psi. Corecell, which is a popular foam core for composite work, has compression strengths of 80 to 350 psi depending on which type you get, and there are even stronger versions.

The other attribute of foam to worry about is shear strength or tensile strength. Because of the way composite panels gain strength (check here for more information on how they work: http://www.fram.nl/faq/how/Corecell_in_Marine_Struct.pdf) their ability to resist pulling apart is just as important as their compression strength.

Essentially the function of a composite core material is to hold the two skins, which carry the load, parallel to each other and a fixed distance apart. So they have to avoid crushing, which brings the skins closer together, and shearing which lets them move apart.

Epoxy will glue the glass to the foam core, but it won't help you if the core is weak enough to pull apart in the middle, separate from the epoxy joint. It's kind of like using epoxy to glue cardboard together. Pull on it and you can be sure the epoxy won't have a problem, you expect the cardboard to come apart.

The shear strength of the "normal" styrofoam boards isn't listed on Dow's site, probably because they know it's not useful foam for that. They do list it for some of their foams that are specifically made for use in composite panels. Here's a link to a great doc on that from Dow: http://www.dow.com/PublishedLiterat...foam/pdfs/noreg/802-00251.pdf&fromPage=GetDoc

In the various specs you can find the shear strength for their composite panel core foam (much stronger than the home depot stuff) is about 50 psi. Compare this to end grain balsa at 262psi.

All the above isn't to say that foam cores won't work for a few boat purposes, but for all those purposes (like hard tops that won't be walked on) there are stronger, lighter, cheaper alternatives that perform better.

FYI, suggesting that those of us saying the cheap foam won't work well are just saying things and haven't "actually done it" is silly. Many of us have tried it once upon a time. There was a guy by the name of KnottyBuoyz here when I first started that did some testing with cheap foam cores, vacuum bagging, and epoxy, and he confirmed that the stuff is just too weak to work. I think you can still find the results of his experiments in a thread here.

So, there you have it. I'm saying the same thing as many of the folks here, many of the folks on boatdesign.net, several amateur experimenters from this board have proved it, and we all agree with Dow chemicals. Dow sells the cheap foam for insulation use. They sell a better quality foam for composite core use. If they thought you could use the cheap stuff for composite panels, why wouldn't they say that?

Erik

 

[Shife]

Re: Foam Core VS Balsa Core - no stringers

That's a bit more harshly than I'd put it, frankly, but I worry about people using poor materials or techniques for repair because "someone on iBoats said it would work". All you can do is speak your truth clearly and firmly, and provide references and other sources so people can corroborate what you say. Unfortunately you can't help everyone.

I agree on Balsa, but disagree on plywood. For its price, plywood is an excellent core material. Jacques Mertens, one of the more forward looking architects of boat plans for home builders, uses for the most part plywood cores in composite panels, including the hull, with epoxy over biax glass (no mat) for the skins. His boats are high performance, light, and strong. (check out www.bateau.com for more).

Plywood has enormously high compression strength, shear strength, stiffness, and overall strength. It's also cheap and easily available. The downsides for it are A) It can rot and B) It's heavier than some cores.

I agree on the rest of what you say though..isolate the fasteners and seal any skin penetrations, and it'll last a long long time. Do it right, or do it fast, or do it cheap. Pick any two.

Erik

A cold molded plywood home build and the average glass runabout are two very different animals. The average glass runabout owner doesn't understand the difference between cloth and mat, and why mat is a waste of resin. A home build owner usually does. This isn't an insult, it's just reality. There is a multitude of books/videos/web tutorials devoted to composites repair. For some reason the average joe just refuses to make use of those and would rather just botch his way through a sub standard repair. Such is life.

I replaced the deck in my fishing boat with plywood because that is what was in there to start with, and a foam core deck would have cost me more than the 70lbs in weight savings would have been worth. If money was no object, my beater tinny would be sporting a Nomex honeycomb deck with carbon skins. I would NEVER replace a balsa core with plywood unless the NA who drew the boat signed off on the change. I would replace balsa with balsa or an engineered foam core.

There is a responsibility, even at the DIY runabout repair level, to do things the right way. It's not hard and I don't buy the "good enough" shadetree excuses that permeate these threads.

You're one of the guys who dishes out solid advice on this forum, so please don't think I'm aiming at you. I am just continually amazed at the amount of just plain incorrect, even dangerous, advice that is on this site.

 

[Yacht Dr.]

Re: Foam Core VS Balsa Core - no stringers

I agree with Erik and others about balsa..

EG Balsa is light..works well with conforming to curves..bonds well.. and If properly Installed will last a very long time as long as it does not get wet.

Big boat builders spend millions for Research and development. EG balsa is still used.

The One Main problem with Any core or anything on a boat is going to be water..

If water can be introduced to any core..it will fail.

I would say 90% of all boats have water intrustion of some kind.

Damage is done when water freezes..or stagnates.

Were not even talking about Micro-organisms. Or how they affect things.

YD.

 

[mrdude]

Re: Foam Core VS Balsa Core - no stringers

well you guys seem to have all the answers to shoot down the way i am using the styro core but i doubt you all have read exactly the way i am doing this and what it is going to be anchored to... dont know why all these old guys dont want to go beyond the "industry standard" and try some stuff!!!! it works for my application guys!!! if you all wanna go buy balsa or say divinycell for a core thats fine!! some people dont want to spend or have the money to spend $150 for a 4x8 sheet of balsa or $200 for divinycell.. then spend more on surfacing these materials... its up to the individual to figure out whether something will work for their application.. and my application works well for me... and can work well for another as well.

the strength is not in the core ok guys!!! the strength is in the 1/4" x 1/8" epoxy filled ribs going parallel to the anchor points and fiberglassed faces that are bonded to the ribs... if you really read what i wrote and think about it, it will make sense. all i see here is "industry standard" well the culture is changing !! new materials and uses... look at coosa board... only difference from mine is styro vs polyurethane core

 

[Shife]

Re: Foam Core VS Balsa Core - no stringers

well you guys seem to have all the answers to shoot down the way i am using the styro core but i doubt you all have read exactly the way i am doing this and what it is going to be anchored to... dont know why all these old guys dont want to go beyond the "industry standard" and try some stuff!!!! it works for my application guys!!! if you all wanna go buy balsa or say divinycell for a core thats fine!! some people dont want to spend or have the money to spend $150 for a 4x8 sheet of balsa or $200 for divinycell.. then spend more on surfacing these materials... its up to the individual to figure out whether something will work for their application.. and my application works well for me... and can work well for another as well.

the strength is not in the core ok guys!!! the strength is in the 1/4" x 1/8" epoxy filled ribs going parallel to the anchor points and fiberglassed faces that are bonded to the ribs... if you really read what i wrote and think about it, it will make sense. all i see here is "industry standard" well the culture is changing !! new materials and uses... look at coosa board... only difference from mine is styro vs polyurethane core

Yep, Go styrofoam. Foam is foam, right? You better pray that deck you're building doesn't fail while underway and kill someone. I don't know how to tell you this in a nice way, but from what you have described you don't know what you're doing. There is no strength in a 1/8 x 1/4 inch channel of cured plastic. Strength in FRP construction comes from engineered orientation of the reinforcing fibers. Unless you're varnishing wood, pure resin is a waste of resin.

*I have used that kind of foam in various projects. In order to gain enough strength from that core you will have to overbuild the skins to the point where you might as well not even had the foam in there to begin with. Yes, you can use cheap foam to build a stringer hat form, IF you know what you're doing. This is VERY different from building a load bearing cored FRP panel.

If the type of foam you are referring to was suitable for structural composites applications DIY'ers like me would be using it for all sorts of fun projects. We're not. Would I build interior furniture panels with it? Sure. Would I use it in structural application? Heck no. I value safety and reliability higher than a sheet of foam from a box store.

This is what will happen with that foam: It will seem solid at first. Quickly, after even light use, the foam will dent and begin to leave voids in the laminate. The panels will become spongy and cracks will appear. This will continue until the laminate fails. This will not take long. I know this because I've done it. If box store foam worked we'd be using it. It doesn't so we don't.

Can I ask what engineering you did to verify your assumptions?

 

[itchin4fun]

Re: Foam Core VS Balsa Core - no stringers

wow lots of disscussion here. I have a repair shop and have replaced lots of floors. Use stringers fiberglassed,use marine grade coast guard rated foam,1\2" treated ply then fiberglass the entire floor keep it water tight. The floatation foam has great bouyancy and is strong enough to walk on and expands 25x its volume ,i always cut a one inch circle on the ply and pour foam in that way if u pour too much it will expand out that hole and just cut it off, lot of work but worth it, if u are needing foam i have a couple gallons left. I am from wichita originally by the way

 

[sasto]

Re: Foam Core VS Balsa Core - no stringers

That "go by industry standards and try some stuff" has already been done. Styrofoam has been around for years and long before these new composite panels were. I've been building boats (Buddy Davis, Donzi Yachts, Garlington Landeweer, American Custom Yachts, Palm Beach...) for along time. Styrofoam and blue board have been tried, with no success. There are a few guys here that know what they are talking about, and they know who they are.

 

[drewpster]

Re: Foam Core VS Balsa Core - no stringers

Hey sasto, Did John Graviscus show you how to build while you were at American Custom? I'm a kiddder yanno.

I am very reluctant to do any fiberglass structural work without a great deal of research. Most, if not all, of the hair brained ideas I can come up with have already been tried. I prefer to use other folks misfortune to my benefit. So I generally go by the book...literally. I suggest you do the same. Better safe than sorry.

 

[erikgreen]

Re: Foam Core VS Balsa Core - no stringers

dont know why all these old guys dont want to go beyond the "industry standard" and try some stuff!!!! it works for my application guys!!!

Years ago, that's almost exactly what I thought when I started participating here. It's pretty easy to get the idea that the guys here are older, set in their ways types who either don't know anything about modern materials or don't want to try anything new. Some few are that way, but you'll also find some of the most cutting edge types here, along with a good mix of in-between folks.

Although it's an attractive idea to think you know something the folks here don't, or haven't tried, that's all it is. The only things that haven't been discussed here are the very expensive or very new materials.

if you all wanna go buy balsa or say divinycell for a core thats fine!! some people dont want to spend or have the money to spend $150 for a 4x8 sheet of balsa or $200 for divinycell.. then spend more on surfacing these materials...

That's the thing... many of us are in exactly the same position and we don't want to or can't afford to spend $$$ for high tech core materials. That's why we recommend plywood.

We're not telling you to use other materials because the cost is irrelevant to us, we're doing it because we've already made those mistakes.

its up to the individual to figure out whether something will work for their application.. and my application works well for me... and can work well for another as well.

This is the main reason we're bothering to have this discussion at all. It's one thing to say "even though you all think it won't work, I'm gonna try it" and another entirely to try to present your work as a safe standard for others to use.

One thing many of us here have seen is a lot of "experimental" types with no engineering experience trying something then trying to evangelize it to get other people to agree with them. It reinforces your self worth if other people use "your" techniques and materials, doesn't it?

The problem is that usually the person trying something new has no idea how well something will work long term, and has no idea how well it works in situations different from their own boat.

If you want to claim "other people can do this and it works fine" make sure you test it for a few years in different conditions, on the ocean, rivers, and lakes, and above all make it fail and find out how it happened. Trying something and not having it fall apart won't teach you anything, it'll just tell you it worked that one time.

the strength is not in the core ok guys!!! the strength is in the 1/4" x 1/8" epoxy filled ribs going parallel to the anchor points and fiberglassed faces that are bonded to the ribs... if you really read what i wrote and think about it, it will make sense. all i see here is "industry standard" well the culture is changing !! new materials and uses... look at coosa board... only difference from mine is styro vs polyurethane core

Again, the idea that we're a bunch of stodgy old guys who refuse to change their ways is flat out wrong. We like industry standard because how it holds up long term is well known. We also recommend experimental ideas from time to time cautiously, but the main reason we stick to the basics is that they're cheap. Most folks here are repairing old boats that will never be worth using fancy materials, and they don't want to even use epoxy (which is really necessary for composite panels) if they don't have to.

FYI, Coosa board is absolutely nothing like what you're using. Differences include:

• Strength - coosa is suitable for transom use, it's stronger than most core materials, compression and shear are both above 200psi
• Composition - coosa is PU foam surrounding a glass fiber 3d structure, IE it's a composite material in its own right
• Coosa can hold screws put into it. Try that with your foam
• Price - Coosa is about $260 for a 1/2" 4x8 board. If it's just the same as what you're using, why is it so much more expensive?

Last note now...

the strength is not in the core ok guys!!!

In any composite panel or stressed skin structure, the core is a major component of the strength and stiffness. That's just how the structures work.

Putting it another way: If the strength of the foam doesn't matter, then why put it in at all? Why aren't you just leaving an air gap?

Here's a suggestion - do things your way, post pics as you do it, then test it out... step on it, abuse it, and run it in your boat a few weeks in a variety of conditions. If you do all that and it holds up maybe you'll change some people's minds?

Erik

 

[sasto]

Re: Foam Core VS Balsa Core - no stringers

Hey sasto, Did John Graviscus show you how to build while you were at American Custom? I'm a kiddder yanno.

QUOTE]

I met John when he first started the show at another marina, some 15 years ago. He has come a long way since then. An all around good guy and he found a good home at American.

 

[Yacht Dr.]

Re: Foam Core VS Balsa Core - no stringers

I dunno..is it just me..or has the OP last posted some how can you help with his blueboard HD stuff BACK start of June.

Then 1 solo post from Danny brings it up again in October...

Then a slew of argumentives happining here ? and its still on the top of the help guys out list ?? I dont get it.

IMHO VH opinion let this thread go..

There are others that can use the help on better things that how to stinger or deck with HD blueboard ..

Dont gemme wrong here.. Im not trying to bash ANYONE.. I just think theres a better use for Information for those that Really Need it right now.

Its winter commin.....

YD.

 

[mrdude]

Re: Foam Core VS Balsa Core - no stringers

this reply is for mr eric green. the dude that likes to quote and shoot! eric, you havent done a thing with these materials in any way like i have! i have already done it dude!!! after i use it a couple years i'll post again and maybe i'll explain the process.. the board is extremely stiff and i have solved any delamination problems that some others have experienced. until u actually do something yourself maybe you should be neutral in the future!! you have no idea of my experience and nor do i of yours but i do know from your comments your are not speaking from 1st hand experience. and i dont need to tell every1 about my engineering experience either ok..

 

[Shife]

Re: Foam Core VS Balsa Core - no stringers

this reply is for mr eric green. the dude that likes to quote and shoot! eric, you havent done a thing with these materials in any way like i have! i have already done it dude!!! after i use it a couple years i'll post again and maybe i'll explain the process.. the board is extremely stiff and i have solved any delamination problems that some others have experienced. until u actually do something yourself maybe you should be neutral in the future!! you have no idea of my experience and nor do i of yours but i do know from your comments your are not speaking from 1st hand experience. and i dont need to tell every1 about my engineering experience either ok..

How did you fix the delam issue? Do you know WHY this foam has delam issues? I do, and the only way to fix is to use the correct friggin foam. You can't "fix" the fact that the foam you are using has properties that make it unsafe for the application you are using it for.

The foam you are using has low compressive strength and does not rebound when impacted. It stays compressed leaving a void after the skin detaches. That is why your panels will delam and fail. Even foams engineered for use as composite cores suffer from this malady to one degree or another. By using this foam as a core in a structural panel you might as well load a revolver with a couple of bullets and randomly aim it at your passengers and yourself. That is the kind of game you are playing with their lives. You've already over powered your pontoon. Might as well make it real fun and install a disappearing floor deck.

 

[toey77]

Re: Foam Core VS Balsa Core - no stringers

I don't know a lot about balsa core but I found out a while ago and have researched other composite materials (poly core for one) but the bottom line is if it isn't recommended for marine use DON'T use it. I work on cars (smash repairs) and understand industry standard's they are in place for a reason and that is to help save lives.... my boat has had a chunk out of the balsa core for about 10 months now I will be adding stringers and a floor which it never had but it is still like this because I don't wont my boat sinking and someone drowning from a botched up job I done. and the main reason it is like this is MONEY I bout this boat knowing very little but new to look for soft floors then when I seen this one didn't have a floor just a hull I thought beuty a blank canvas, no rot here to worry about. well that bit me in the arse next time I am going the soft floor I hate getting out my core as it is very solid so it seems good but it is wet and thanks to advice on here I have been told to remove all wet core, its been a sucky job but one I want to get done so my kids are safe out on the water

 

[joe_nj]

Re: Foam Core VS Balsa Core - no stringers

Erik,
The plywood on part of the port side of my deck is rotted between the skins which opened up along the edge by the gas tank - the starboard side seems fine. I don't know how I would ever stop water from seaping in through the screw holes where the seats are mounted and I'd rather spend extra once and not have to worry about rot anymore (although this first deck has lasted 23 years and I probably won't be around for the second replacement 20 years from now.) It sounds like I can replace the plywood with Nidacore and have at least comparable strength. I see one source here: http://boatbuildercentral.com/proddetail.php?prod=NIDA_H8PP_finished

I was going to cut out the whole floor between the engine compartment and console but the Fiberglass repair book (David Aiken) says that I may be able to just cut out a portion of the core that rotted and replace that - a less daunting project.

So, if I remove the top skin, scrape out the rotted plywood, install the nidacore, and re-install or replace the top skin. Does that sound as though it will work?

 

[es844]

So how did u end up replacing the balsa core? I'm replacing mine now and was wondering what your results were???

 

[GA_Boater]

So how did u end up replacing the balsa core? I'm replacing mine now and was wondering what your results were???

es844 - This thread is over 4 years old and the thread starter is long gone. Please start a new thread with your questions. Thanks.

 

[es844]

Thanks man i thought he'd still b on here and could give a result. I started thread titled replacing core between 2 skins yesterday