Tuesday, November 2, 2010

Cats are designed to flex: The Oak Tree and the Willow

by Lenny Rudow

Are cat boats really designed to flex like an airplane wing? Sure - in fact, to one degree or another, most modern high-quality boats are. I'm sure you remember the fable about the oak tree and the willow. As the wind blew stronger and stronger, the willow bent over farther and farther. But the oak tree stood firm, and refused to bend. Soon the oak's roots were ripped out of the ground, and it came crashing down. The willow, meanwhile, survived the wind without sustaining any permanent damage. When it calmed down, the willow simply sprang back upright again. Believe it or not, boats are exactly the same way-whether they're built of fiberglass, steel, wood, or any combination of these materials.
A boat that can't or won't flex is one that's full of cracks, breaks, and eventually, will probably suffer catastrophic failure of one sort or another. Stringers break free of the hull, bulkheads get jarred out of position, and cracks radiate from stress points. Why? For the same reason that the willow survived where the oak tree couldn't; a little give and take goes a long way. When a boat crashes into a wave, the impact (lessened, of course, by a cushion of compressed air in the case of most modern cats), sends shock waves from the hull up through the deck, console, cabin, and other parts of the boat. If these parts can't flex enough to absorb that shock, there's only one alternative: something's going to break.
On the other hand of course, too much flex is also a bad thing. It can also lead to massive failure, and commonly creates stress cracking (crazing) in the gel coat, leaky windows and door frames, and stringer or bulkhead separation. A well designed, well built boat-be it a cat or a monohull-will flex just enough but not too much. Luckily, fiberglass as a construction material is quite well suited to flexing. Unlike aluminum and other metals, which tend to fatigue over time, it can flex over and over again and remain resilient. That's why most airplanes, which you can watch flexing in the wings as they taxi down the runway, are “retired” after a specific fatigue-limited life expectancy.
Oil-canning hull or cabin sides, springy decks, and hullsides that visibly flex when a boat moves through the water are all examples of excessive flexing, and if you see them in any boat, you should question its longevity. But on most modern powercats, you're unlikely to see such clear signs of future failure. In fact, you're unlikely to see them on any modern production powerboat; the “new” economy has done an amazingly efficient job of weeding out shoddy boat builders.
Just what amount of flexing is desirable? According to 30-year veteran marine surveyor David H. Pasco, in Hull Design Failure, “there are limits to just how far a designer can go with flexibility. In terms of rigidity, we're talking about the difference of the bottom flexing a quarter inch to half an inch or not at all.” That might not sound like much, but in reality, half an inch of flexing is quite a bit. Remember: if you can visibly see flexing in the boat as you run it, it's probably flexing too much. So the next time you run a powercat, watch the hullsides closely and see if you can notice any flexing. Chances are you won't. But trust us, there is some give and take in that fiberglass. Just enough-but not too much.

A well built cat will flex just the right amount. Note how these high-stress areas on this Glacier Bay haven't cracked at all, despite years of hard use.

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