Boat structures 101
 

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.

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.

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://i1291.photobucket.com/albums/...ps985c0571.jpg

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

X2.

Thanks for the recommendation, I just ordered a copy.

Heavy stuff man. What is a scantling? Some kind of European currency?

Don, it's how women are dressed not wearing very much clothing. I think. :)

:D

Exactly. Scantling clad. Like the ones in Ole's threads. Nice!

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.

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

I started out on these 2 books, both very good!

Attachment 9200

I started out with this book. Maybe not the best but some good information there, entertaining and enough to get me started/hooked.

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.:eek:

Tuition
 

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

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

Attachment 9203 Attachment 9204

Attachment 9205 Attachment 9209

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:

Attachment 9207 Attachment 9208

Great thread, Dave! I agree it definitely deserves to be a "sticky" because of the valuable info it contains!

I also bought Dave Gerr's book based on your recommendations, and I agree it's a very comprehensive book. I'm still wading through it, but I've run a few calculations on his recommended Scantling Numbers and compared them to what SeaCraft used in the early 4-stringer 20' hulls. Although I believe Moesly arrived at his structure by trial and error, including some very creative use of thin laminates and brittle resin in prototypes to find the high stress locations, he ended up with laminate thicknesses that are very close to Gerr's recommendations! The tall stringers are probably even more massive than needed, but he certainly did plenty of proof testing in the extremely rough offshore powerboat races, and none of his boats ever broke up, so I'd be very careful about changing anything he designed!

Friz, thanks for adding the material property info on the various core materials! Many people are quick to condemn balsa, but it actually has better shear strength than most of the foams, and better water resistance than plywood, so I consider it one of the best core materials in terms of strength/weight ratio! You just have to know how to use it and properly seal and backfill any holes you put in it! It also tends to absorb resin better than most foams, so getting a good bond between it and the outer skins should be a bit easier than with most other core materials.

One good reference I'd like to add to the discussion of cores are the extensive articles on "Cores and Structural Issues" by Dave Pascoe. As an experienced surveyor, he's seen first hand what works and what doesn't, and his articles provide some dramatic examples of problems with poor laminate bonds and water intrusion that can occur when cores are not properly installed!

I forgot to read the fine print on the tuition statements a long time ago – apparently there was something along the lines of “… and it never stops being due until you quit making bad decisions or die, whichever comes first.” Hoping for the former, for a long time.

As a self-imposed penalty for not reading the fine print, I did some homework on cores… I don’t think there is a ‘perfect’ core material; all have advantages and disadvantages. For each application (transom, decks, backing plates, etc.) and each individual using it, some work well and some don’t work as well. There are lots of them out there; plywood, balsa, and a whole raft of completely synthetic ones.

Like Frizz says, plywood is pretty awesome and has some great qualities – starting with being one of the least expensive of the core materials I know of. It has been used in boats for a long time and a lot of work went into developing really good products for marine use. I’m pretty sure the PT boats in WWII were made of plywood – not as a core but as the structure because it was light, strong, cheap, and didn’t set off magnetic mines.

Plywood weight is ~35-40 lbs/ft3 (dry) but some types are more and some are less – it depends on the density of the wood used, the glue used, and how may plies there are. The Okoume and Meranti marine plywoods are arguably a couple of the better ones and are also included in the more expensive varieties. Okoume is pretty light; 27-35 lbs/ft3 and is registered with Lloyds for marine applications. There are lower cost ones too with different qualities. I don’t know what was originally used in my ’76 23 Seacraft but my transom was so wet/rotten that strength was near zero and weight was probably getting close to that of water (62.4 lbs/ft3). I do know that it was 2 layers of 5/8” plywood stapled together.

Plywood fails far more gracefully than other core materials; the wood layers and individual wood fibers do not usually all rupture (fail) at once and it seems to retain a good bit of strength as it fails (most other cores don’t). Since the grain of the layers are usually at 90 degree angles to each other, it is pretty close to having equal strength in length and width directions. It also has great compression strength. Plywood can flex a lot without failing too. Plywood holds screws really well. And it rots in the right (wrong) conditions. It also can delaminate (usually a glue failure or the wrong plywood type being used). With the development of core materials that are lighter and don’t rot, plywood is being replaced for some applications.

The synthetic materials are usually foams like PVC, urethane, or SAN and are sold with trade names like Corecell, Divinicell, and a couple of others. They are available indifferent densities from about 5 to 10 lb/ft3. The honeycomb plastics are really good in some structures but I don't know as much about them. Like Friz says the synthetics don’t have the same compressive strength as plywood or balsa but they don’t rot either. They also don’t hold screws well but they are somewhat flexible. Coosa is a hybrid – it is a rigid foam with fiberglass embedded in the outer skins. It comes in 15, 20, 24, and 26 lb/ft3 densities and is pretty strong. It also holds screws ‘ok’ and has good compressive properties. A disadvantage of the Coosa is that it is friable; it breaks up instead of flexing if overstressed. A properties table for it is below. All the synthetic stuff is pretty expensive.


Denny is right about balsa- it is hard to beat in something like a deck if it doesn’t get water in it.

In a perfect world my transom and decks wouldn’t have rotted and I wouldn’t be replacing them. The reality is that they are toast and I never want to have to re-do them again. I picked 1-1/2” Coosa 26 for my new transom core. I was originally going to use ¾” Coosa 26 for my decks but started thinking about both weight and flexibility; I’m probably going to be placing an order for ½” Corecell in a couple of weeks for replacing the deck cores. Some of the other foam products would probably be equally good. I am still using plywood for backing where I can get at it to replace it.

Another tidbit; the area of the transom on a 23 is about 24 ft2 and the volume of a 1-1/2" transom core is about 3 ft3.

One thing to note about cores – you also need to consider the whole laminate; the core, glass and resin system as well as how you are going to put it together when you are making a decision. For me Nidacore (a plastic honeycomb material) was off the table since I don’t have the proper tools to really handle the assembly correctly and I don’t know how to repair it.


Now I’m looking forward to reading the discussion soon to come about polyester vs vinyl ester vs epoxy.

Attachment 9212

Good info Mitch! Pascoe seems to agree that Core-Cell is one of the best foams. This article is now about 10 years old, so I would assume we would have heard something by now if there were any problems with it.

I have a neighbor, who along with a partner, claims to have invented Core-Cell; I believe he said they began production of it in Switzerland. He claims to be an expert in composite construction, and with a partner, founded Affinity Yachts. They built the 90' sportfish/yacht using Core-Cell throughout, even in the bottom! He claims the 90' yacht is the Ferrari of it's class and about the same weight as a 70' Viking, with similar performance, and a good example of what can be done with extensive use of composites. However the quoted 2500 mile range is at about 10 kts; at a 30 kt cruise, range on 5000 gallons of fuel is about 900 miles! :eek: They mention that carbon fiber is used in it's construction, so maybe they used that to for impact resistance; I'd worry about hitting something like a shipping container at 30 kts in a boat that big with a cored hull! "El Lobo" was moored behind his house for a few weeks and a few of us got a tour of it . . . definitely a fancy yacht with all the bells & whistles, complete with automatic sliding doors opening to the cockpit!

Good stuff Flex, there are lots of great new composites coming out time after time,mostly for the better of the environment,not necessarily for strength for the consumer..similer to paints and adhesives of nowadays.structurely, you cant beat the stiffness, and screw holding of modern marine ply wood coated both sides with glass such as 1708, that's strength !at least 6 glue up layers with 1/2 ply ,more on 3/4,and ,nothing like AC house glue up.Foam, divinicell,corecell, coosa,or balsa are also great products to use in a certain parts of the boat.you are also counting on the glass over, on this stuff,and not much else,besides foam or balsa for strength .maybe a 1/8 "glasscenter on the good stuff.Transom use ,maybe, various bulkheads,ok.Floors,OK,as long as you don't have to put a screw into it more then twice or more....If you want a screw to hold,&be confident it will,more than once,put it into glassed over plywood,its even better than solid glass, and easier to fix after the screw strips in the other stuff. did Seacraft , in the past use marine wood,?doubt it,not that checkerboard chit I seen and delt with .However, on a NEW boat, it has to be wood free,totally. sorry ,just sayin...

I'll have to agree with Fas. When I bought the 23 last year I replaced the deck with 3/4 ply and epoxy. Then I replaced the fuel tank platform with 1/2 coosa. I made my deck in 1 piece then dropped it in. It was HEAVY. After putting the coosa in for the new tank it flexed. So I asked a few questions on here and my schedule wasn't correct. Cut it out, more 1708 and tabbed it back in. Super strong and light weight was the end result. Now I'm on my transom replacement. Coosa is what I'm going with because of my prior experience with the fuel tank platform. I'm upset with the deck in the boat and I may next winter hack it out and redo with either coosa or core cell. I don't know anything about the latter but I'll keep watching this post. Very informative!

Why my big deck hatches are bowed and proper edging of cored structures
 

Reading Jeff’s post started me thinking about something I ran into when I started on my 23 redo... The original hatches for the fuel tank and the forward fishbox in my 23 Sceptre are bowed. The cores don’t appear to be wet and they are not super flexible so it took me a bit to figure it out; the edges of the cores in the hatches had not been properly finished.

Like Dave mentioned, cored construction creates something kinda like an I beam. When it is supported on the ends and a weight is in the middle (like someone standing on a hatch), the top of the laminate is in compression and the bottom of the laminate is in tension. That is great if the lower laminate is rigidly tied to the upper one at the edges – near zero flex or bending. My factory hatches have a balsa core that just fits inside the shoebox-lid shaped hatch and a thin glass lower laminate that only touches the upper laminate along a 3/16” edge – or it did when it was made. As a result of the lower laminate being in tension whenever someone walked on it, it broke loose along the edges. The result is a pair of hatches where the upper and lower laminates are not tied together anymore – and took a set with a shallow dish shape over the last 40 years.

So what does this say about cores? The edges of the core and the structure have to be properly finished with the upper and lower laminates being tied together. It was done properly on the hatches for my stern wells; the core stops an inch or two from the inner edge. It is tapered to the underside of the top laminate (~30-45 deg angle) and the lower laminate covers the core AND is wrapped fully into the bottom of the upper laminate. That means the tension from the lower laminate gets spread across a 1-2” wide joint to the upper laminate which is in shear.

This cartoon shows the way the hatches are and the way they should be.
Attachment 9217

So that is what I need to do to my larger hatches. I also need to properly finish the edges of the core (Core-cell) when I replace the decks. If I just have a 90 degree angle on the edge of the core in the deck and don’t tie the upper and lower laminates together properly I will cause the Corecell to fail in shear on the edges. Then the deck starts falling apart and delaminating from the edges. And I will have to redo it.

There is a good bit of discussion with diagrams about this in Gerrs book and some in Bruce Pfunds article on core failures.

Thanks to everyone who has contributed so far. This is exactly the type of conversation I was hoping to stimulate. There's a lot of talent and skill on this board and it's good to see it come together.

I want to go back to a couple things Bushwhacker said and explore them in a little more depth. In regards to Gerr's scantling rules, Denny said:

Denny is right. In the early days of fiberglass boats, not much was known about how to design them structurally. There was a lot of trial and error. Carl Moesly knew from his background as a pilot that weight and balance matter. So he started out with lightweight structures, but he made a point to test his boats to find the weak spots and potential failure points before he put them into production. Carl was a pioneer on many fronts.

That brings me to another point Denny made:

My signature line is "Reinventing the wheel, one spoke at a time". This is an inside joke I have with myself to remind me that when I have a great new idea, there's about a 95% chance that either it's not a great idea, or it's not a new idea. But I can't help over-thinking stuff. I suspect there are others like me in this group.

So, any time you find yourself thinking you've got a better way to rebuild your boat than the way it was originally constructed, ask yourself why it wasn't done that way to begin with.

Moesly and Potter, just like all the other builders of production fiberglass boats, then and now, had an economic mandate to make them as economically as possible. They had some options (hull, stringer, and liner molds, for example) that aren't practical for us. But just because you can't pop a new deck and liner from a mold and install it in your boat doesn't mean you can't copy the same laminate and core thicknesses for the transom, stringers, deck, gunwale cap, etc. and rebuild your boat as good or better than original. Why substitute a different deck structure when the original lasted 40 years?

That being said... As Flexpat's hatch example demonstrates, there are a few things that could have been done better in these boats. Mainly, you'll find them in the details, not in the basic structures. So think about what you're doing, but try not to over-think it.

Dave

It is 10F up here this morning. At least it is 10 ABOVE today.

I compiled a spreadsheet of commonly available resins and properties. It isn't as complete as I'd like, but it compares polyester, epoxy vinylester, and normal marine epoxies. All made for lamination or infusion. No oddball chemistries like BPA fumarate, or methacrylate modified systems.