Boat structures 101

[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

[Islandtrader]

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

[Normagain]

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

[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:

[Bushwacker]

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!

[FLexpat]

Originally Posted by Terry England

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

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.

[Bushwacker]

Originally Posted by FLexpat

. . . I’m probably going to be placing an order for ½” Corecell in a couple of weeks for replacing the deck cores. . .

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! 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!

[FAS]

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...