different core types (foam pvc, divinicell vs balsa)

[schoonerdog]

over they years I've talked to various people (insurance agents, brokers, lots of builders) about the types of cores used in multihulls and was wondering the groups opinion.

Basically it came down to this, balsa is easier to work with and cheaper, so a lot of boat builders choose it to keep down costs. But on the negative, it's significantly heavier and can have much greater water penetration through the core and more difficult to repair. A boat insurance rep mentioned that insurers look at very negatively at balsa cored boats because of their tendency to have water penetration rot the core for a far greater area than one would assume given the relatively small void that might have been the cause.

The synthetics are more water resistant and lighter, but more expensive, so typically boat builders which are interested in keeping weight down and also making the boat have a more serviceable life. I've used it as a bench mark (one of many) on the quality of a catamarans construction.

Typically the cheaper boats use balsa and the more expensive use synthetic, though there are exceptions.

Any opinions? Any reason to think that Balsa would have merits above the synthetic foams? I'll put in one, from underwood (general manager of broad blue and prout), he believed that synthetics were more prone for offgasing and therefore hull delamination. I found it difficult to believe though since PDQ had been using sythetics since the late 80s with none of theirs have offgasing problems, and I'd personally seen a broadblue 380 being injected with resin throughout the hulls to counter delamination.

[44'cruisingcat]

Balsa is about twice as dense as foam, but it has about 3 times it's compression and sheer strength. So, if it's used correctly a balsa cored boat can be stronger and lighter than a foam cored one. You do need to take care that hull penetrations are properly sealed though. Have a look here :
http://www.schionningdesigns.com.au/www/welcome.cfm

Then go to material supply, then material choice.

[cat man do]

Hi guys,
Schionning use Duflex panels or strip in their boats and this is fine. Reason being is that it made using epoxy resins and does not use contour balsa. Like contour foam ,this style of balsa is in 1 inch approx squares with a 2 to 3 mm space to alow easy laying into a mould. If the voids are'nt filled, adding weight and expense, they both suck up water if the hull is damaged. Also , I would have to say all production builders would use poly or vinylester purely as a cost issue certainly not because its stronger, and as most of us should be aware, epoxy is the only adhesive that should be used with a timber core.

Unfortunately the Balsa rots really well, the foam does not. But this does not mean I'm a "Foam Freak". Far from it.

I have been involved with a bit of composite construction over the years and was able to witness diffeernt core samples going through a drop test [a large rounded weight dropped onto various layups from different heights to measure impact and delamination on other side] for QLD Transport survey requirements in the late 80s, early 90s. I don't think duflex was around then, duracore was the equivelent. It was interesting to note how well heavy polyester layups on foam stood up to heavy poly layups on end grain balsa compared to light layups with stitched fabrics with epoxy on western red cedar and Duracore.

The heavy poly layups performed quite poorly compared to the light epoxy layups, and in fact end grain balsa usually had a neat hole punched through it if taken to destruction. Foam absorbed the shock somewhat, and strip plank with its longitudinal strength,seemed to stack up best of all as a weight for $$$ comparison. Unfortunately from a production point of view it is faily labour intensive.

My current boat is being built using KIRI as a core,lighter than western red cedar ,slightly heavier than balsa, far cheaper than foam[in my opinion if I was using foam in a multi, it would have to be foam/kevlar/epoxy]and does not rot. In deflection tests it was far superior than western red cedar.

Again my opinions, but interestingly, a lot of KIRI knockers are now using and selling it as an alternative.

Dave

[GordMay]

Back in 2003, Jeff H wrote an excellent primer on FRG construction, which led to an interesting & informative discussion. I highly recommend reading, at least, Jeff’s original post :
“ A Primer on Fiberglass Construction” ~ by JeffH
A Primer on Fiberglass Construction

A little additional reading:

“Foam Core Materials in the Marine Industry” ~ by Trevor Gundberg (Diab)
“For over 60 years foam cores have been utilized in marine applications to lighten, stiffen, and strengthen everything form hull bottoms to fl y bridges. Butwhat exactly is foam core? What type do I use, and where can I use it? Should I use it instead of balsa or plywood? With the seemingly endless variety of foam core materials on the market today, it can be frustrating for the average boating enthusiast to find the right foam core for a particular application. This article will attempt to inform boat builders about the properties and correct manufacturing procedures involved in constructing foam core sandwich laminates...”
Goto: http://www.boatdesign.net/articles/foam-core/

“Core Materials in Polymeric Composites” ~ SP Systems (Gurit)
http://www.azom.com/details.asp?ArticleID=1092

”Cored Sandwich Construction” ~ Diab Group
http://www.diabgroup.com/americas/u_...andwich_hb.pdf

See Also:
Baltek Core Materials: http://www.alcanbaltek.com/
Then Products - Balsa - Processing Guidlines - etc

[Canibul]

Playing the puzzled newbie ( all I gotta do is, act naturally) but one with years of woodworking experience, I cannot easily understand why any end-grained wood with its inherent tendency to split along growth rings could be anywhere near as strong as a correctly applied interwoven mat. Is the balsa cut from between growth rings? Just my ignorance, sorry.

[44'cruisingcat]

Originally Posted by Canibul

Playing the puzzled newbie ( all I gotta do is, act naturally) but one with years of woodworking experience, I cannot easily understand why any end-grained wood with its inherent tendency to split along growth rings could be anywhere near as strong as a correctly applied interwoven mat. Is the balsa cut from between growth rings? Just my ignorance, sorry.

The core material doesn't need to be strong in bending, which is how you would split any end grain timber - along its grain. What the core is there for is to keep the laminate skins apart - so when the entire panel is bent, the core is under compression, as for the panel to bend, the two skins have to come closer together. Also the core has to resist sheer between the two skins. This is where end grain balsa is very good as a core.

A lot of the rot reputation comes from when balsa was used with polyester resins, which arent completely waterproof.

[hellosailor]

I've never noticed growth rings in balsa, perhaps because it is a "tropical" wood and growth rings in them should not be as distinct as in woods grown in seasonal growth areas, i.e. up north.

Nor have I ever noticed balsa splitting or sheering along any particular lines, besides the grain in general. You could consider balsa to be "nanotechnology" foam, it is after all *built* the old fashioned way, with a cellular structure.

And why judge foam, in the abstract, when any foam really has to be judged as part of the entire construction system? Build it right, seal it right, and core getting wet should just not happen. IIRC there aren't any synthetics that match the stiffness and overall performance of balsa core.

[Canibul]

Is the balsa being totally saturated with resin? If it is, it would seem that the lightness of it is being neutralized. If it is not being saturated, then it has air bubbles in it. Air bubbles in the hull material is not good, right? The air bubbles are supposed to be in the foam floatation!! Or have I still got it all confused???

[hellosailor]

"Is the balsa being totally saturated with resin?"
If it was, there would be no point in using a *cellular* foam. That would be blocks of resin, or solid FRP. Bubbles are not just there for floatation, in any foam they aid in insulation and weight reduction. A solid/filled object sometimes has structural properties very similar to a much lighter (and less expensive) hollow or cellular one. That's why we make masts out of hollow tubes--instead of solid metal bars.

Balsa companies and epoxy/repair companies alike have descriptions and pictures on the web of balsa, showing ery nicely that it is incredibly resist to saturation *except* down the grain. Seal the two ends of the grain, and it just doesn't saturate from the other directions. Like a test tube--you've got to pour into the top to fill the cells.

[Canibul]

Thanks, Rick and Hellosailor. I got it now. I had been thinking that wooden hulls including balsa cores, were basically an older technology that was being largely supplanted by fiberglass. I can see how a sealed end grain "plug" or "tile" would have longitudinal compression strength. like a 6x6 post on end, with out the grain or splitting tendencies...

[Moby Dick]

Most builders that use balsa core have solid glass below the water, so water penetration in the hull is not likley anyway, I feel.

[schoonerdog]

Gord,

Thank you from the links. It's not as cut and dry as I thought. It appears that both have different best applications in different parts of the boats and some builders who make the luxurious catamarans where cost isn't an issue use different types of foam for 95% of the construction to save in weight and balsa in areas where high compressive strength would be needed such as in the fore and aft sections of the bridgedeck. I also get the impression from the luxury builders that in areas with complex or greater curves that balsa is easier to work with. Again, excellent links!

[Pblais]

A synthetic material will not rot should the structure fail but it does not make up for a failure in the structure itself. Coring is not meant to carry a load. Even in a solid deck of glass the outer layers carry most all of the load. When you walk on a deck the bottom layer is in compression and the top layer is in tension. The center is uneffected.

If you consider a wooden truss used to hold up your roof the center of the truss is nothing but sticks to hold the outer memebrs of the truss in place so they can carry the load. The sticks just keep the main members in proper geometric alignment to carry the load. A cored surface is just a truss. When it fails the skin buckles and becomes cracked because it was too weak to begin with and cracks that develop that allows moisture to enter. This is the root casue rotted cores. A different core won't rot but it won't make it any stronger.

To the extent that different core matrials will bond to the outer layers easier or better then it becomes a better core material. Doing a poor job in construction is the problem. If a synthentic material is easier to work with you can use less skilled labor to make hulls and the deck structures could be made stronger if well designed. It is no wonder rotted cores are found in the same boats all too often. They were never designed or constructed properly in the first place. Many times both poor craftsmanship combined with a weak design.

The fatigue of comrpression and tension cycles eventaully breaks the outer layers of glass by crushing or stretching of the outer layers first because the outer layers carry the most load. This exposes the core to moisture but the outer layers have already died before any rot develops in the core. You see the rot well after the fact. The rotted core did not cause the probelm it is only the evidence.

[schoonerdog]

Paul, which catamarans have you seen rot due to material failure of the foam or balsa core sandwich? I'm not a surveyor, so not familiar with specifics.
I have seen water intrusion on the boats I've had due to improperly sealed fittings (around hatches, stanchions, etc.)

[Pblais]

Water intrusion from a poorly cut hole or mounted fittings is not the same thing, but in most respects a violation of the core. I'm not clear if you can make pentrations in a synthetic core and get off with no end problems even if the material in the core won't rot.

The base of my argument above is the physics of compression / tension structures. Widespread failure of a deck or a hull isn't about a poorly mounted fitting. Many cored hulls do leave areas where fitting are to be installed void of coring since the forces in these places are not the same.

My main point is that it isn't the core that fails it's what you see at the end.