Wooden/Fiberglass Scantlings

[JEM]

I'm currently reading Elements of Boat Strength by Gerr. Addresses a lot about foam composite. I love the formulas.

But there is little to no information regarding scantlings for using plywood core in a composite sandwich instead of foam.

Anyone know of a good resource for such information that’s presented in a similar fashion as Gerr’s?

Thanks,

Matt

[gonzo]

Use the scantlings for wood core with fiberglass sheathing.

[Not A Guest]

foam, wood, and plywood are different materials and have much different properties.

In the order I have written them they are in order of increasing strength and density.

I would expect that plywood could be thinner than wood or foam.

[JEM]

I bounced my ideas of a couple of designers and also ordered Designing Power & Sail. I prefer to use plywood/fiberglass as a composite so hopefully I can find or even develop some charts to use.

Does anyone think Gerr's ratios apply to plywood as well as foam? In other words, once you establish your basic hull lamination, do you think Gerr's ratios for hull bottom, speeds over 10 knots, etc, would be accurate for plywood/fiberglass construction?

[SeaDrive]

The only designer that I know of who has made any sort of engineering study of a glass/ply composite is Jacques Mertens (http://www.boatplans-online.com/index.php). I believe he developed his scantlings by building and testing panels, and he considers the results to be proprietary, i.e. a trade secret. I mention this not because it helps you much, but because it suggests that there is something to know that is not in the public domain. Most designers use scantlings for ply and a single layer of light fiberglass just to make the surface finish as durable as possible.

[Bavarian]

A plywood core boat misses the point of coring - light and stiff by using strong skins and light core. If you are building a "plywood core" boat, you really want to build a plywood boat and sheath it in a single layer of boat cloth for abrasion resistance and moisture protection. Scantling rules for plywood sailing boats are in ABS Rules for Building and Classing Offshore Yachts.

[JEM]

With the proper lamination schedule, one can utilize thinner and lighter sheets of plywood. Example, instead of 1/2" with a layer of 6 ounce standard weave, use 3/8" with biax.

That's just a rough example.

[duluthboats]

JEM
There are many ways to measure a laminate. A thin plywood/glass laminate might be equal to a thick foam/glass laminate in one characteristic, but very different in another.
Gary

[JEM]

Very true. Ok I'll rephrase and say plywood "core" in a "fiberglass composite" layup is my prefered method for now.

Hopefully I'm not mincing words or cross-pollenating terms. ;)

[SailDesign]

If you use 3/8" ply instead of 1/2", you finish up with only 43% of the stiffness. This can lead to all sorts of unforseen problems with structure. Plywood was never intended to be used as a spacer material, with reason. It is also denser than solid wood, and less stiff except in a cross-grain application.
Steve

[Not A Guest]

Jem ---

If you read Gerr's book a little closer he discusses plywood as cores and plywood as planking.

I have not examined plywood issues enough to endorse his statements.

[Robert Gainer]

I don't think plywood is a suitable core; it doesn’t transfer the sheer from the outer web to the inner web of the structure. It is worthwhile as a core where you need the compression strength in the core material for bolts, as in an engine bed for instance. The peel strength is nowhere near high enough to use as the core in a boat. As others have said the weight is also a problem. The bottom line is why would you want to use it.

[JEM]

Simplicity of build with my current skill set.

[Robert Gainer]

Use the rule for a balsa core and you will get reasonable numbers. Make sure the bond is good or you end up with a real problem. The weight will be much greater then balsa but you already know that.

[SeaDrive]

The bottom line is why would you want to use it.

Ply is about the only material that can be the mold as well as the core, which has appeal to amateur builders.

[Robert Gainer]

Sea Drive:
I admit it; now I am confused. If the mold is built with Airex foam over battens doesn’t the Airex become the core. How is that different then plywood over stringers as a mold? Its may be easier for the backyard builder to slap together the foam rather then build with finely fit joints the wood hull needs. Of course if you don’t have the skill to build the wood mold the rest of the project may perhaps suffer anyway. Six of one and a half dozen of the other.
All the best:
Robert Gainer

[SeaDrive]

Robert:

Confused? Seriously? Somehow I doubt it. Anyway, we are probably in the realm of perception here rather than objective reality.

The fact is that an amateur builder can see his way clear to stitch & glue a ply hull over bulkheads and cover it inside and out with 1 or more layers of fiberglass. There may not be any longitudinals at all, or perhaps just at the keel, chine and rail. Just about all the material that he has to buy ends up in the finished boat. The ply is "self-fairing" for the most part. He can join panels with butts, or with taped joints. He has been told that, in the age of epoxy, a close fitting joint is actually undesireable.

The typical amateur builder does not know anything about foam core since none of his buddies have used it, and none was used in building his house. He doesn't want to buy material for, or spend the time to build bulkheads (mold frames) or stringers that don't end up in the finished boat. He distrusts any method that requires stringers close together because that emphasizes that the burden of getting a fair hull is on him. He doesn't know about getting bends in the foam, or how to achieve a fair curve at butts in the foam planks.

Of course, you understand all that. Your point is that with the expenditure of a little time for education, he could build a better boat for similar money using foam core. You might well be right, but are not many amateur builders using foam, and I'll bet the resale value of an amateur foam hull would not be any higher than for a ply core hull.

The final caveat is that there are amateur builders who are as skillful as any professional.