Boat structures 101

[Blue_Heron]

Some recent posts, and the broad range of approaches to boat restoration we see here, got me thinking it might be nice to have a “go to” resource on ClassicSeacraft.com for folks looking to fix up their boats. A lot of us, myself included, are amateurs looking to restore classic boats to their former greatness.

And make no mistake, these are great boats. Seacraft was among the early pioneers to go with structural fiberglass stringers without wood cores. They were using the highest quality cores available at the time for decks and transoms, end grain balsa, and marine plywood respectively. With the endless array of composite materials and advanced resin formulas available today, the choices can be staggering.

I hope this thread will encourage a discussion of the available options for restoring these boats without getting into a debate over what is the “best” way to do it. Ultimately, all decisions are a compromise. Cost, availability of materials, and skill level, all enter into the equation. What’s best for me may not be best for you. Let’s make this thread a discussion of the tradeoffs that come with our preferred method. We have quite a few professionals on this site, and without naming names, I hope they will contribute to this thread. Especially, I hope they will correct us amateurs if we post something that’s wrong.

Before you can make informed decisions about what materials to use, you have to have a basic understanding of boat structures, so that’s where I want to start. Fiberglass boats are made up of some pretty simple structures. There are solid laminates, like the bottom and sides of the hull. There are cored laminates like the deck, gunwale cap, and transom. And there are structural members like stringers and bulkheads which may or may not be cored.

The solid laminates used from the keel to the sheer line in our boats serve several purposes. They resist the bending forces imposed by hydrodynamic loading (a fancy term for water pressure) at planning speeds. They also protect against impact and puncture when we run over something we wish we hadn’t in the water. Assuming a reasonable spacing of stringers and bulkheads in the hull, a solid laminate (in boats the size of our Seacrafts) is going to provide the best puncture and impact resistance when it is sized correctly to resist the bending forces imposed on the hull by water pressure at planning speeds.

The deck, gunwale cap, transom, and parts of the console are made from cored laminates. Cored laminates are probably the most misunderstood structural element of our boats. Many of us think the core is what provides the strength while the fiberglass “coating” protects the core from rot. That is simply not how a cored laminate works. There are some good resources online to explain how cored laminates work. I found one here that does a very good job of explaining the function of the outer laminate and the core in terms of the loads they resist:

http://www.oneoceankayaks.com/Sandcore.htm

In a nutshell, a cored laminate works the same way as an I-beam, what the structural engineers call a wide flange section. An I-beam has three basic elements, the top flange, the bottom flange, and the web. Remember these terms for later in this post. In an I-beam supported at the ends, bending loads create compression forces in the top flange, tension forces in the bottom flange, and shear forces in the web. Why?

The best way I can think to paint a mental picture is this: Imagine if you take three 10 foot 1 x 8 boards and stack them flat on top of one another between two saw horses. Now, take your fattest friend and ask him or her to sit on the stacked boards right in the middle. Depending on how big your friend is, they will either deflect dramatically (anything more than a third of an inch in a 10 foot span would be considered too much) or they will fail completely and your friend will find him/herself sitting on a pile of splinters. Why?

Partly, because the boards are able to slide against one another. And partly because we didn’t take advantage of the shape of the boards and stand them on their edges. More on that in a moment (pun to follow).

Now, imagine taking the same three 1x 8s and glue and screw them together to make an I-beam. You can probably seat two of your fattest friends in the middle of the span without unacceptable deflection. Why?

The answer is a concept called moment of inertia. It’s a complicated concept measured in Inches to the fourth power, (at least here in the good old US of A) and well beyond the scope of this thread or even my ability to explain it. The Cliff’s notes version is this: When you have a beam or a panel that is intended to resist bending forces, the bits farthest from the bending axis (the flanges) are doing the most work. The middle bits (the web) are resisting the shear forces that want to make the flanges slide against one another like our stacked 1 x 10s. Since the shear forces exerted on the web of our beam, or the core of our panel are much lower than the compressive and tensile forces on the flanges, the most economical structure is composed of flanges with high tensile/compressive strength spaced as far apart as practical by a web or core just strong enough to resist the lower shear loads. Are you with me so far?

Translating this into practical terms, a deck made up of two layers of fiberglass laminate, separated by a core of end grain balsa (which has almost no ability to resist bending loads on its own, but has exceptional resistance to shear), creates a very strong lightweight structure with excellent impact resistance and durability in a marine environment.

Enough food for thought for now. Please feel free to chime in with questions or input of your own. I’ll post more as I think of it or in response to your feedback.

[FLexpat]

Really nice synopsis!
Here are links to a couple of articles I found to be very good:
http://www.bpspecialprojects.com/PDF...20PROBLEMS.PDF
http://www.bpspecialprojects.com/PDF...0CLOSEOUTS.PDF
I will keep looking for some of my other stuff too.

[flyingfrizzle]

The Elements of Boat Strength for Builders, Designers, and Owners By Dave Gerr


This is one of the best reads that I have found for boat building or restoring. It was recommended by a member up here and I went out and ordered it. The book has plenty of how to instruction, scantling rules/figures, diagrams, and descriptions/definitions. It is a must have for anyone doing a restore, building a boat or doing repairs.



http://www.amazon.com/The-Elements-B.../dp/0070231591

[Blue_Heron]

Originally Posted by flyingfrizzle

The Elements of Boat Strength for Builders, Designers, and Owners By Dave Gerr


This is one of the best reads that I have found for boat building or restoring...

X2.

[bibijawa]

Thanks for the recommendation, I just ordered a copy.

[Bigshrimpin]

Originally Posted by flyingfrizzle

The Elements of Boat Strength for Builders, Designers, and Owners By Dave Gerr


This is one of the best reads that I have found for boat building or restoring. It was recommended by a member up here and I went out and ordered it. The book has plenty of how to instruction, scantling rules/figures, diagrams, and descriptions/definitions. It is a must have for anyone doing a restore, building a boat or doing repairs.


http://www.amazon.com/The-Elements-B.../dp/0070231591

x3 - that's a great book!!

I also liked "The Fiberglass Boat Repair Manual" by Allan Vaitses

It's much less technical, but it's a good place to start.

[Blue_Heron]

So, I’ve explained what cored laminates are. They are a successful construction method for conventional fiberglass boats. Maybe I should also talk about what they are not. There are construction methods that aren’t true cored laminates, but kinda look like them. Some are quite successful, others not so much.

A good example of a successful alternative to the cored laminate is the type of plywood/epoxy construction used in stitch and glue boat building. Stitch and glue is a process of wood boat building that uses reinforced epoxy joinery to build hulls and other boat components of plywood. They are then covered with an epoxy/glass laminate inside and out to protect them from moisture, abrasion, and puncture. Typically, the laminates in this type of construction are not as thick as in a pure cored laminate. The plywood core contributes more stiffness to the assembly than in the cored laminates I described in my previous post. This method can be used in lieu of pure cored laminates to successfully repair a fiberglass boat. But if you use an epoxy plywood construction method, you want to do it right, or it may not hold up.

Probably the most important thing to remember about plywood/epoxy construction is EPOXY. You can’t use polyester boatyard resin in a thin laminate over plywood and expect the same results. Cured epoxy resin has far superior physical properties for this application. It is more flexible than polyester, it bonds better, and it is much more impervious to moisture.

You can build a deck by coating plywood on both sides with a couple layers of mat and polyester resin, but it won’t be anywhere near as durable as a plywood/epoxy deck, particularly if you live in an area with high humidity and lots of rainfall like I do. It will appear quite strong when first installed, but over time it may absorb moisture and/or delaminate. This was the method used on the previous “restoration” of my Hewes Bonefisher project boat , and it lasted probably less than10 years. By the time I got the boat, the deck had been mostly removed, but what was still there was a delaminated waterlogged mess.

For more info on stitch and glue, or plywood epoxy construction methods, I would refer you to Joel Shine, our resident expert on the subject. Or you can learn quite a bit from his web site:

http://www.boatbuildercentral.com/

Dave

[FLexpat]

Gerr's book is good however the e-book version of it was a bad choice - an hour later I ordered the hard copy. I love to pay twice for something.

[Terry England]

Originally Posted by FLexpat

Gerr's book is good however the e-book version of it was a bad choice - an hour later I ordered the hard copy. I love to pay twice for something.

Some days we still pay TUITION even when we've been outa' collage for decades!

[flyingfrizzle]

Some info on the strength of foam, honey-comb, marine ply, and balsa - shear & compressive





But don't forget about good ol Marine grade ply!

Marine Plywood: Compressive strength is 5000 psi- more than 5 times better than the best foam (at 10# density).

Marine Plywood: Shear (transverse) strength is 4500 psi- more than 5 times better than the best foam.

End grain Balsa (10# density): Compressive strength is 4000 psi- 4 times better than the best foam (at 10# density).

End grain Balsa (10# density): Shear strength is 433 psi- here is the weakness of Balsa, this is at the low-mid range of foams, many foams will outperform balsa in transverse shear. This is significant if a cored bottom skin is supported by a bulkhead or stringer, the pressure load will put the core material into shear at the bulkhead connection. Easily managed by tabbing, etc, if the builder knows about this characteristic.

Just remember this is comparing just the core alone, the strength on the foam comes from the glass laminated to it. But wood gains from the lambent too once glassed just the same but just is heaver and can rot if not sealed 100%.

This below is another good diagram backing up what was being said about the core thickness and strength by bushwacker on my 25' seafari thread, the thicker the core the stiffner it will yield with the same inner and outer lambent: