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SeaJay 08-25-2007 07:39 PM

Foam Core - Deck, Cabin, Bulkheads
I recently purchased a 46' hull uncompleted hull from an out-of-business builder (Core-Cell/Kevlar/Glass), and now need to construct the interior, deck, and pilothouse structures.

I've spent several months researching materials and methods, and am strongly leaning towards foam core composite. I have opted for foam as opposed to plywood for weight and rot considerations. Similarly, balsa is out. I have Coosa, Last-A-Foam, and Divinycell samples, and will likely use one of these for my core material.

The information I’ve read regarding foam cores seems to dismiss the strength of the core other than to transfer the loading between the laminate skins. Clearly 20lb+ foam has significant strength in its own right. It seems to me the laminating schedule for a 5lb core panel could be reduced when using a denser core.

Ideally, I would like to use a foam board with a single layer FRG laminate on each side as a direct replacement for the ˝”-3/4” marine plywood/epoxy typically used in a boat of this size for bulkheads, stiffeners, decks, cabin sides etc. If I built these structures from plywood, I would need to epoxy for protection against water intrusion, and likely add a layer of glass cloth for abrasion resistance and strength. Substituting foam of appropriate density and thickness, and a single (?) layer of glass fabric on each side, would seem to offer some weight reduction and eliminate rot potential. (All tabbing and support structure would be per standard construction)

If so, perhaps someone in this forum can suggest an appropriate panel specification, and offer some insights on working with these various cores.


1. Weight - Not necessary to be ultra-light, but would strive for at least some modest weight reduction over ply/epoxy.
2. Ease of Construction - High Priority. My thinking is to use thicker, denser, stronger foams requiring less laminating.
3. Cost - willing to pay premium for materials in order to reduce labor.
4. Construction Details / Other - a) non structural screw holding ability would be nice. b) I like the idea of thicker foam for insulation, joining, details, etc. For example, 1.5” x 20lb, vs. 1” x 26lb. c) I’m not expecting to use vacuum bagging or molds. d) Thoughts on laminating/finishing the underside of deck?

All input would be greatly appreciated.

KnottyBuoyz 08-26-2007 06:24 AM

2 Attachment(s)
Hi SeaJay

Sounds like you have quite a project on your hands. I'm not a structural engineer but have been studying composite core construction for our trawler project. From what I've come to understand about the process, especially with foam core materials it's the opposing skins of composite panel that provide the structural characteristics.

I've provided some reading but I'm sure there's more out there. The fella's over at Boat Builder Central are willing to help you with the engineering and design if you buy your supplies from them. Ask for Joel or Jacques and tell them Rick sent you.

Good luck. Keep us posted.


marshmat 08-26-2007 11:17 AM

Welcome aboard, SeaJay :)

Engineering a cored panel is somewhat more difficult than it looks. There are some semi-empirical mathematical methods for calculating appropriate laminate schedules, but there's still a fair dose of experience and engineering sense involved.
A stronger core will not necessarily change the required skin properties. The reason for this is that the core and the skins are responsible for supporting different components of the load. Although the actual stress distribution is somewhat more complex, for present purposes it's safe to approximate it as follows.
Picture a sandwich panel, simply supported at each end. Apply a point load on top of it. The panel deflects slightly under the load. Now, the top skin is loaded in compression, and those forces are in the plane of that skin. Because it is thin relative to the whole panel, shear and transverse compressive/tensile forces within the skin are small. Thus it can be optimized to absorb compressive forces in its own plane. Similarly, the bottom skin is on the outside of the now-curved panel, and is almost entirely in tension within its own plane.
What's going on in the core? The core material is resisting the shear forces that would be trying to force the skins to slide past each other. It is also spreading out any forces that are normal to the panel's surface. Since the foam/honeycomb is not nearly as stiff as the fibreglass skins, it absorbs almost none of the compressive or tensile stresses which the skins are supporting.
You can see, then, what the effect of increasing the core density will be. You'll get more shear strength; thus, if the primary failure mode of the panel is core shear (you'd calculate the failure mode for your original panel before trying to optimize it), you'd get a stronger panel. You'll also have more resistance to dents or skin punctures due to point loads. But if the original panel's failure mode is skin wrinkling, or panel buckling, a heavier core will just add weight with no extra strength.
I'll try to dig up the formulae if you want them.

SeaJay 08-26-2007 12:18 PM


Yes, I do have a bit of a project in front of me. A bit frightening, but you aren’t plowing any new ground in life if your stomach doesn’t quiver from time to time. Thanks for the links…there was some good information there. I’m familiar with the Boat Builder Central site, but was unaware of possible engineering assistance. I’ll definitely get in touch with them.

Best Regards,


KnottyBuoyz 08-26-2007 12:45 PM

Posting pictures and sketches will help get reply's too Doug. I'd be interested in seeing what you've got to start with. Sounds like fun (to me anyways but I'm a sick person! *lol*). I'm about to head into the basement and infuse another foam core panel for my sump pump hole!

This was attempt #1

SeaJay 08-26-2007 02:17 PM


Thanks for the welcome!

“Engineering a cored panel is somewhat more difficult than it looks. There are some semi-empirical mathematical methods for calculating appropriate laminate schedules, but there's still a fair dose of experience and engineering sense involved.”

I agree. I’ve a bit of an engineering background, and can follow the principals and understand the complexities (read expense) involved in a properly engineered core/laminate schedule. Because of this complexity I’m looking for empirical guidelines from folks that have maybe glued some of this stuff together and then proceeded to let their overweight friends wander over their foredecks.

Actually, after thinking about this a bit, what I am after is more akin to Coosa Bluewater 26. Their literature seems to imply that it can be used as a direct size-for-size replacement for plywood although I have not confirmed that in my own mind. Even if a thicker Coosa board is required, we are probably at about the same overall weight as plywood construction. At that point, I envision the “skins” as contributing some additional strength but primarily bonding the panels into a monocoque structure…the same as plywood construction.

My only issues with Coosa Bluewater 26 are that it is unpleasant to handle and that you pay for the fabric that is imbedded in the panel, but would still need to apply fabric to the surfaces to lock everything together.

Thanks for the input, and please send the formula if you can find it.

Best regards,


Bentwood 08-26-2007 06:43 PM

SeaJay: I am using coosa bluewater-26 on the bulkheads, sole, and deck of a 32' sailboat project. My reasoning was very similar to yours, as a direct replacement for a similar thickness marine ply. I am convinced that the final layup with coosa is lighter than the comparible plywood, after manhandling it around the shop. I do apply a layer of fabric to the surfaces to get good bonding to the subsequent tabbling.
Let us know how your project progresses.

kengrome 08-26-2007 08:28 PM

You could always just use plywood that's been pre-treated with copper naphthenate, then it will never rot. This is not new technology either, CN has been used for decades and is extremely well documented and proven to permanently prevent rot.

Personally I cannot understand why all boat builders that use wood in their boats do not treat it to prevent rot. I suppose it all comes down to money -- it is "cheaper" to ignore the problem than to prevent it.

SeaJay 08-26-2007 10:43 PM


That's the kind of information I'm looking for. What weight/weave fabric did you use? Do you have any suggestions for handling the Coosa board? After handling my samples for only a couple of minutes my hands were full of "splinters".


SeaJay 08-26-2007 10:54 PM


I'm with you in terms of being willing to pay what seems like a small premium upfront in order to eliminate a potentially serious and expensive problem down the road. While I believe that plywood can be successfully encapsulated in epoxy, a quick search in these forums on 'replacing stringers' gives countless examples where good intentions have come up short. Although $250 for a 4'x8'x1/2"sheet of Coosa does give one pause, it the grand scheme of things it is a small percentage of the total cost of the boat.


Bentwood 08-26-2007 11:07 PM

SeaJay: I prepped the panels with 12 oz biaxial fabric on each face sheet. I found that it was necessary to pre-coat the panel with epoxy thickened with colloidal silica, then lay the fabric on the wet panel, then finish saturating the fabric with epoxy. Coosa will absorb unthickened epoxy out of the fabric leaving a dry bond if the panel is not primed. The panels cut easily with bimetal jig saw blades and carbide circular saw blades and router bits. It is also easy to grind bevels into them with 40 grit; they cut faster than plywood.
Many have complained about the dust. I use vacuum tools and the best dust mask I can find. I also run a central vacuum hose as close to the cut as I can get it. I wear woven cotton gloves. In general, it is no worse than working with fiberglass.
Good luck,

SeaJay 08-26-2007 11:18 PM


Great info, thanks! Did you leave an "un-epoxied" edge for tabbing to the hull or did you run the fabric all the way to the edge of the panel? What was your thinking on whatever method you chose?


SeaJay 08-26-2007 11:22 PM


What is the purpose of the bevels?


Bentwood 08-27-2007 01:32 PM

SeaJay: I fiberglassed the entire panel so that the tabbing would have a good full fiberglass skin to bond into. I did not want any weak dry-bonding problems at the hull-bulkhead joint. The edge of the panel needs to be beveled to correspond to the curvature of the hull over the thickness of the panel. The strength of the bulkhead joint comes from the fillet/tabbing on the side of joint, not the direct bond between the edge of the panel and the hull (see "The Gougeon Book on Boat Construction")

marshmat 08-27-2007 06:35 PM

If anyone's worried about the secondary bond at the hull-deck joint being a potential trouble spot because of the difficulty of laminating to the smooth surface of the cured epoxy, may I suggest trying a peel-ply fabric (you can get pretty good ones from Airtech for about $60 for a 32-square-metre roll). Press a piece of it into the surface of the resin once you're done the layup, then once the epoxy's cured, peel it off- and all the oils, waxes and contaminants come with it, leaving a surface that bonds very well without any extra sanding or scrubbing. The stuff saves so much time I wish I had known about it when I built my boat.

I've never used Coosa and so can't add any insight on its structural properties, sorry. If you need information about honeycombs or closed-cell foams I might be able to help.

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