Choosing a deck core

Well, I'm taking donations if you want me to build in 100% core cell. I'd love to!

Maybe my new source will be cheaper. As it stands, I can't build the boat if I use 100% Corecell.

I have no idea how balsa people test sheer strength numbers, or how foam guys do either.

 

What I am saying is beware of the numbers,One cannot test brittle materials and springy in the same method.

 

You people are talking of putting cores in the bottom of these boats? Not on my life!
Hey, different kind of boat here but this is the deck I did for my boat twenty some years ago. I was able to lift the the 12' x 13' nom. thing myself ( I had to cut the foam one I removed into four pieces to get it off the boat). Not to scale.


This is what I mean by bring the sandwich down to solid laminate. I built a shelf for the deck to rest on but it was overkill. I thought it would make less work than a lot of glassing under after it was layed on but I was probably mistaken. I would support the deck in place (not quite touching the hull) and glass under, come topside, pucky and glass the the top now. I had a crane operator drop a (500lb.?) four wheeler from about the height of that drop-tested skiff. I did not hear a dreaded "crack" - just a thud and simply repaired the gelcote. Pretty amazing stuff, that balsa. There are no penetrations but were there, I'd bring it down to solid laminate rather than fill core on my boat.

I realize that the boats you are talking about are likely not set up this way but something to think about for rails to last a lifetime;


No screw heads to rust on top, very clean, easily removable if you crash one. The best part is that it is easier to make a stiff hull-side than a stiff rail. Cored hull changes the rules. Use 5200, as well, for a thru-deck penetration.

All this said, you are talking a kinda boat out of my realm. I'll just leave the thread to you guys pushing the technology envelope. Let's talk again in thirty years about what has survived.

 

Mark, the issue is we're talking about catamarans. You can't build a well enough performing catamaran without a cored hull (unless she's ply/epoxy or cedar strip planking or some other wood base).

A solid glass catamaran will be a dog. I would have done it if I could have, but you can't if you want it to sail well.

 

Ok, point taken. I'm just trying to figure out how to do the substitution here... I know balsa is different than core cell and that core cell absorbs impact and compresses, then springs back, etc...

I've had balsa decks in 3 previous boats and didn't seem to have any problem, so I figure I can shave some $$$ off the price here without compromising quality to a huge extent.

So here is my question, put in a more simple way:

If you have a certain glass layup schedule over a 3/4", 5lb Core Cell core, then you remove that Core Cell core and replace with balsa, what do you have to do in order to have the same strength?

Is it the 3/4" that matters most? Is it the 5lb density? Is it both?

Balsa weighs more than Core Cell per cubic ft. Can I use a thinner piece of Balsa core in the unchanged laminate, so the total weight of core is the same, or do I have to use a 3/4" piece of balsa that weighs more in order to preserve the "infinite I-beam" of the laminate?

 

Strength ??? Depends on location and shape for one,you reduce Cross section you lose stiffness and some benefits gained from the cored panel and may lead to a self destructive situation if you go too far,personally if going with the cheap core why not use 1".

 

Core-Cell A-Foam ........ 400 _ 500 _ 600 _ 800 _ 1200 .... BALSA
density ....................... 70 __ 90 __ 116 _ 150 _ 210 ..... (150)
compressive strength ... 0.6 _ 0.9 __ 1.4 _ 2.1 __ 3.9 ..... (13)
compressive modulus ... 41 __ 64 __ 83 __ 117 _ 217 ..... (4,080)
shear strength ............. 0.7 _ 1 ___ 1.2 __ 1.6 _ 2.6 ...... (3)
shear modulus ............. 22 __ 26 __ 34 __ 47 __ 76 ...... (162)
shear % ..................... 65 _____________ 50 __ 46

War Whoop, I don't doubt that foam is better but from the numbers balsa seems to be? Maybe % shear???

Is failure usually at the bond line and not of the core in almost all cases?

 

understand ductile cores a little ,well lets see if the core was loaded and it deflected slightly but it did not fail that would be marked as failure in most testing methods,Bond line problems are shop related no matter what is in the panel. a real test would be Hydromat.

 

Another way would simply be to mill a pattern into foam - mostly anything suitable will do - and vacuum form the pattern. That way the glass will be the stength and you can bolt it if you want to.

If you are going to use foam as a core the foam should be dense enough not to dent under the intended weight. Balsa would add strength where no foam I know of would be strong or stiff enough to make structure.

 

rethink that statement ,We build Foam cored transoms everyday.

 

So what would the problem be for a deck ?

 

No problem just build it.

 

Catbuilder,while obviously cored hulls are the way to go for stiffness you are mistaken when you make a blanket statement that a solid glass hulled catamaran will be a dog, i owned a Macgregor 36 cat for years that was no dog, solid glass hulls, klegecell cored decks.A sistership finished the ensenada race 2nd,7minutes behind Double Bullet,a 66ft full on racing cat in 1983 in 10hrs 39min.While a handfull of multihulls have done it faster in the ensuing 27yrs only one 80ft canting keel multi million dollar racing monohull has managed to do the race a few minutes quicker just a couple of years ago, so,a pretty fast dog.
Ok, back to your deck, have you priced out other foams,while corecell is probably the best choice i would explore the pricing of klegecell,divinycell etc before comitting to balsa, they may be closer in price.To your question regarding substituting balsa for foam with the same glass layup,you need to get past this Strength bs, its a meaningless term, what you are after is stiffness and this comes from shape and core thickness,in your example of a 3/4"core and the same laminate schedule and you want to use balsa the stiffness will be about the same,it will be a little heavier,if you downsize the core it will not be as stiff,core thickness is the most effective way to gain stiffness. Forget about shear strength,it doesnt matter if balsa or corecell or whatever has the most shear strength, klegecell or divinycell have plenty. We have a 40ft,10000lb sailboat in the shop right now which has a deck comprised of 4mm occume,3/4" 5lb klegecell,with 2 layers of 10oz woven glass cloth on the top, 30yrs old now and holding up fine so far.It did get some repairs 2yrs ago where water got in from a poorly installed prism over a quarter berth but it did much less damage than the typical balsa deck we see.
Steve.

 

Core Sizing

Ever wonder how a core is chosen? Here it is, arranged in an excel format (in metric of course). Your fingers do the walking, the computer does the thinking.

The Excel program will compute the thickness and the required shear strength of the core. Most likely you will find in a core layout, pressure varies on different regions. If you are aiming for single thickness with different core densities, the program comes in handy evaluating different core densities.

Core database range includes cardboard, honeycomb, end grain balsa, to various types of foam. A whole range of core can be analyzed. The database is mostly generic. You may replace the data with the material property of your preferred core.

Using the Shear Specific Method and coupling it with the cost per unit weight this index enables the user/designer to choose the most bang for the least buck. For example, if a new core is ¼ pound heavier per cubic foot but 50% cheaper, it will not add much weight to the boat, only in the pocket by the money saved.

Of course as the saying goes “Core it properly or do not core at all”. If the choice was done properly but the build is not, the responsibility goes to the boatbuilder/project supervisor. The excel program is only a tool and many cored boat failed because the builder did not do it properly.

The instruction is on the front sheet.

 

CatBuilder , I priced corecell and could'nt get anyone to deal. I went with H80 and bought case price from fiberglass supply, I ended up buying glass and silver tip epoxy from them as well. I did have to wait awhile for my foam orders but for the dollar saveings on all my materials it has been well worth it.