Quote:
Originally Posted by Terry England
Some days we still pay TUITION even when we've been outa' collage for decades!
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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.
