Yesterday and Today The simple fact is that the materials and methods used to build boats in 1965 remain good enough to continue to be used today. 1965 technology (if you can call it that) is perfectly suited to build good quality boats to this day. In fact, most boats are still made with the same basic materials, although the advertising has fooled you into believing otherwise. The primary difference is that they are USING LESS OF IT, and substituting cheaper materials for more expensive solid laminates. Speaking of solid laminates, that’s what I mean by 1965 technology.
When it comes to the use of core materials, they are employed not, as was originally intended, to stiffen up flat structures such as decks, but to replace more costly material with less costly material. In essence, where there used to be solid laminate, the space is now filled with air, because air is what most cores are all about. It would be one thing if these cores were serving to make structures stronger. Instead, they are being used as an excuse to eliminate frames, to make unsupported spans of flat surfaces even larger. It costs money to add frames: use a core and eliminate the frames. In most cases what we end up with is not a structure that is stronger, but weaker.
Typical examples of this are hull sides and decks. The average entry level cruiser has a cored foredeck with virtually no frames. Jump on it and it’s like a trampoline. No problem, except when you do jump on it, and that deck flexes, what is happening is that the core is separating from the outer laminates. A cored structure is designed not to bend, like a bridge. When you do bend it, bad things happen, like the bridge or deck starts to fall down. Much the same thing is going on with hull sides. Most small boats don’t use foam cores, but products like CoreMat, a material that sort of looks like that absorbent material you find at the bottom of meat packaging. It’s a fibrous material with millions of little holes or perforations through it. It would be great stuff except for a couple of things.
First, it absorbs water like a sponge. Small boats never had much of a blistering problem until products like this came along. Now they blister just like Taiwan boats that are loaded with chopped strand mat and blister like a banshee. Secondly, very thin cores like these do not create a structural truss like a real core does. Cores increase strength by separating the distance of the load between the inner and outer skins. Think of the skins as beams, and the core as columns. The effect is exactly the same as a roof truss. But not when you use a thin sheet of this stuff. All it does is replace strong material with weaker material.
Take a typical cruiser, use a core like this in the house sides and then paint it black. Set it out in the Florida or Texas or Alabama sun for a few years and watch what happens. Ooooh! Weird! It buckles and cracks. Want to know why? Or do you want to know why the builder didn’t know what was going to happen? Or did he care? Oh, no, he simply didn’t know because he doesn’t employ any composite engineers. The material salesmen designed the thing for him, so he’s happy as a clam thinking he saved some money and can now tout “high tech.” What happened, of course, is called heat distortion. Every place a laminate is held rigid, like around a window frame with screws through it, the material expands but is restrained by frame and fasteners. And so it does the only thing it can do, it buckles and cracks.
I don’t hesitate for a moment in saying no. Not with any kind of material. The risk is too high that something will go wrong, mistakes either by the builder, the owners of the boat, or someone working on it. We all know that it’s hard enough to keep the superstructure of the boat from leaking, but to keep water out of a core below the water line may nigh well be impossible. Fiberglass is known to be water absorbent enough as it is without adding more risk to the mix. To do it right requires a very high degree of care which can ultimately be compromised by something as seemingly innocent as running a screw through the laminate somewhere in the bilge. it’s just too easy to make a mistake.Should Hulls Be Cored Below Waterline?
The other problem with coring a bottom has to do with the inability to calculate and estimate stress on complex shapes. It’s easy enough to calculate stress on a flat panel, but change the contours of that panel, introduce the factors imposed by human error, and any benefit that might have been obtained by coring the bottom is long lost. The risk of error multiplies exponentially, far beyond anything that is suitable for high production building. The smaller the boat, the more true this is due to the scale and economic factors. It’s one thing to core a 110 footer that costs 8 million, something else again for a boat that sells for $100k or so. The fact that the economic viability is not there for small boats translates as the builder cutting corners to turn a profit with a process he shouldn’t be using.
All of which means that you run a big risk in buying a cored hull when you could easily avoid that risk by buying one that’s not. Could the reasoning be more simple than this?
