Cored composite with wood inner sheathing

I am exploring different methods of constructing the hull for a large (60-70 foot) one-off blue water cruiser

One method of building one-off boats makes use of a male mold sheathed with plywood, fared and prepped for use in composite construction. After the hull is finished, the plug is removed, producing a hull with inner skin-core-and outer skin. As an alternative, would it be practical instead of using plywood, substitute a lightweight, but aesthetically pleasing wood, maybe spruce, or cedar (3/8” or so) for the mold surface? After the hull is finished, the mold frames would be torn out, with a laminate cross section consisting of outer skin-core material-inner skin-wood interior. Would this not be lighter, stronger, and faster to build than traditional strip plank; with the advantage of a wood interior?

 

Yes. Look at ProBoat #69 2001 "Designing and Building in Wood/Composite Construction" by John Fox. A good primer for your line of thinking.

 

Thanks Doug
Ah yes! I found it; good article with much data. But this is not exactly what I had in mind, as this technique is not truly cored construction (page 160). Because of the size of the hull I want to rely on wood as little as possible for strength and stiffness (wood is heavy); thus incorporating true cored construction, with an inner skin of cedar for the look of wood I can see that I am going to have to build some test panels, and compare it to some of the data given in the article.

 

Nothing new about strip plank hulls, been doing them snce 1984. Have a look at KIRI as a core. lighter than cedar, stronger than foam. The timber core is antegrall part of the strength calculation. It allows you to use less glass and epoxy.

Don't be too concerned about weight, sounds like you'll be strapping a huge lump of lead on the bottom anyway, unless of course it's going to be a multi-hull.

Dave

 

Yes again, but do think of wood as a lamina within the laminate, not nesc. as the core. You can build skins on either side of a low density core using wood layers as you wood cloth layers, just consider the specific proporties of the material to be laid down. and ONLY USE EPOXY.

 

Kiri is interesting. I couldn’t find specs on it though, and it isn’t listed in my giant Encyclopedia of Wood reference. Also I couldn’t find a supplier hear in the states. I did find some info here http://www.paulowniawood.com/characteristics.htm I am not convinced it would be as good as balsa for a core material though, because it is slightly heavier. (25% or so) even if it could be cut thinner for the same strength to weight ratio, that is not necessarily advantages for a core material, as the laminate gains stiffness with increased thickness of the core; at least to a point. I guess I need to determine the panel strength I need first, and then work up a laminate schedule. I think it best to use a program like Composite Pro as indicated in the article mentioned by Doug. In any case it would probably be best to use a core material that can’t rot, and is impermeable to water. As you suggest Doug, I will consider the inner wood ply as a skin within the laminate schedule.

 

Veneer-foam-veneer sandwich laminates have been used to good effect, (see Rage by Schooner Creek BW) as have veneer-foam-carbon, but those guys just couldn't figure out how to attach the outside veneers.

The first boat I ever worked on was a veneer-endgrain balsa-veneer sandwich. (the Accent prototype designed by Peter Norlin, winter of 1973-4)

Yoke.

 

Man over board, Kiri is better than end grain balsa in many ways .
It is highly resistant to rot, unlike balsa.
It does not drink resin like balsa.
IT is stronger than balsa
It is not sold as end grain, so has longtitudenal strength, unlike balsa

Sure it's a little bit heavier than balsa, but your resin saving's make up the difference.

The fact that Aussie and N/Z boatbuilders are starting to swing across and have been using it for the past 5 years should say something.

My 50 foot cat is biult from it. I did several different tests on Kiri VS Western Red Cedar VS Balsa, and I found Balsa being the lowest performer, but cheapest, which is why you'll find it used in production boatbuilding so much.

This is where we got our's from http://www.highpointtimber.com.au/

You'll find on this site info about rot resistence and uses etc

Also search Tennant design and Ron Given , Kiwi designers who have used ply,foam,ply for large panels in their cats.

Have fun

Dave

 

Dave, I finally got a chance to study the website that you gave me. Indeed it is a light wood I calculated the specific gravity to be about .25 based on the info in one of the articles on that site. (Balsa is .17) There isn’t a species of wood that grows in the United States that is as light. Cedar comes the closest. Kiri shrinks very little (.37% volumetric; I find that kind of hard to believe) It does seam like it would be a good core material, but there might not be a supplier here in the states. One other thing, it is a very good insulator. That’s probably not a big deal down where you’re from, but up here in Lake Superior, the water temp rarely gets above 47 Degrees. ( 8.3 C )

 

Proboat #69

If anyone has a copy of the Proboat #69 article that Doug refers to, I'd dearly like to have a read....

 

One advantage of using AIREX as a core is the sound and heat insulating value of the material. The AIREX is tied to a wooden matrix , outside laminated and then rolled over and matrix removed.Then the inner skin is laid on.

The hull is VERY stiff , and bulkheads frequently need not be structural .

Not the lowest cost option , but Epoxy is required for encapusulating wood , std polyester or Vynilester is fine with AIREX.

So the lower cost of 20 drums of resin might pay for the more costly core.

FF