Converting plywood scantlings to Coosa/Nidacore?

Hi,
I am having some discussions with a traditional builder who is using a solid glass hull, and marine ply glassed in on both sides for everything else. He builds semicustom boats, so negotiating a change is always a possibility.

I am thinking it would be nice to substitute Coosa board for the structural plywood in the stringers, bulkheads and transom, and probably to use NidaCore for the decks and wheelhouse. The wheelhouse is good sized, so the weight savings there should make the boat more stable, and of course a wood-less boat would give more peace of mind regarding rot.

Even to open the discussion though, I'd need to know how to substitute Coosa and Nidacore for glassed in plywood. I realize that the properties are not identical between these materials, so *truely* equivalent is not possible. The way that I'd put it though is that I would like to maintain the strength of the plywood cored boat, while dropping the weight. Nidacore has some pretty good info, but Coosa doesn't publish anything, and of course, who knows tensile, compressive, shear strengths of glassed in plywood?

If you have a member that is glassed in 3/4" plywood, can you substitute, for example, 3/4" Coosa Bluewater 20? Coosa Bluewater 26?

I do see that the Nidacore panels need to be finished, and the factory laminate is to use 18oz woven roving. What about the Coosa board? Do you just coat it with resin?

Thanks for your help,
Barry

 

You probably know, but there is more too it than just the thickness. The foam/honey comb you propose is a core - the stiffness will mostly come from the skins of fiberglass. Coosa is very heavy and I would not use it except for high compression areas like the transom, or inserts where harware will be added. Why use 20 lb. foam when 5 lb will suffice.

I have built a boat from foam core and nidacore (it was origanally designed for plywood). You can save weight, but you need someone to look at the original scantlings and then find a suitable "match" in composites. One thing to consider is that the materials will cost between 2 and 3 times that of plywood, which is due to all the additional fiberglass needed. The labor might be double also.

 

Hi JR - thanks for your reply. I do know that there is more to the term 'strength' with these materials than meets the eye. I have surveyed all the books that I could find on Amazon re fiberglass boatbuilding, but most of them are of course older, and there was no coverage of Coosa or Nidacore. Balsa core is pretty well covered in that literature and foam core less so.

I can see just by looking at it that Nidacore should have great strength orthogonal to the plane, but be much weaker in the plane, due to it's anisotropic (directional) core structure. I can see it being great for things like decks and roofs where forces will be orthogonal to the plane.

Foam cores would seem to lack orientation to their many small cells and I'd expect them to be fairly isotropic in properties. I am aware of the I-beam concept you allude to for skinned foam cores, but don't feel that I understand 100% how such a composite would respond to forces coming from any direction. Any discussion I've ever seen just says something like 'Yeah, it's like an I-beam, the skins take most of the compression and tension loads at the inside and outside faces, respectively', and then they just move on, but to me that analysis may be correct as far as it goes, but it doesn't go very far. For example, if you take a skinned foam core and turn it on it's edge, how strong would it be to compression forces directed along that edge? (ie, if it were used in bulkheads supporting decking or holding up an inboard engine).

If I can find results of comprehensive strength tests for these materials, all done in the same manner so they may be compared, and I can find one that meets or exceeds plywood in each of the tests, then I believe it should be alright to substitute it for plywood 1:1 in dimensions. That's about all I can hope for, because I doubt that any modeling software would actually have accurate internal models for materials like Nidacore or Coosa - they are not bulk materials and have pretty complex internal structures. The other information that would be very valuable of course would just be lots of experience - whether or not analysis shows it, extensive real world testing can of course also reveal how to substitute materials - that's why I posted on a designers forum!

 

In a boat with ply stringers, the plywood does the bulk of the work. In a foam/honeycomb cored stringer, the glass does the work. In fact, the strength of the core is not considered at all. The only consideration for the core is whether it can stand up to the bending/compression loads exerted on it by the skins. As for the compressive loads, I use the same type calculations I use for wood or metal, only substituting the strength properties of the glass.

I agree with JR - Coosa is overkill unless it is in an engine room. Even then I would recommend straight urethane foam as opposed to Coosa. I don't believe the glass in the Coosa does much of anything and their numbers when compared to more traditional foams tend to bear that out. I am not putting down their product as I have customers who use it and are happy with it. I just don't believe the glass imparts much additional strength, if any.

I do think you will find the cored stringers will be far more expensive than the plywood. That core material is very expensive. Have you considered treated plywood? Some of it comes with a 50 year warranty if I remember correctly.

 

Interesting discussion.

All I can add is:

1. Coosa takes screws, try doing that on a foam core glass skinned panel.

2. The air in the honeycomb cells of Nidacore may expand with heat (or sun) giving you a puffy look which can't be that good for strength, can it?