When Plastic is Less Than Fantastic
Eavesdropping at autumn boat shows, always produces some curious snippets of conversation.
"I want a boat I can plow into a seawall without any worries," I heard one prospective buyer tell a friend as he surveyed the boats assembled at the Strictly Sail Miami Boat Show last winter.
As far as I could see, there was only one boat that fit the bill: a
steel-hulled Miami River tug that was pushing Haitian cargo boats just north of the show.
For any sailor who trusts his life to the thin barrier between himself and the deep blue sea, the inclination to buy an "indestructible" hull is natural. Sooner or later&emdash;such reasoning goes&emdash;either a mistake, bad weather, or some combination of the two will test the hull to its limit.
Granted, a cruising boat is exposed to natureís rawest forces, but good seamanship and passage planning can usually avoid the most worrisome. So, how strong does a hull need to be?
The question of hull strength becomes more relevant these days as an increasing number of fiber-reinforced plastic (FRP) hulls are being built using vacuum-assisted resin-transfer methods, or pre-impregnated (pre-preg) cloth. In both cases, the amount of resin can be more closely controlled than it can with conventional roller and brush lay-up. The result, if done properly&emdash;no small caveat&emdash;is higher fiber-to-resin ratios (70 percent fiber to 30 percent resin versus about 40:60 for hand lay-up) and theoretically higher strength-to-weight ratios. A core sandwich can yield even lighter, stiffer structures, but this is by no means seawall-smacking-proof. And depending on the thickness of the outer FRP skin, a reef-bound cored hull will likely come out worse for wear than a solid laminate hull.
One only need to look at the text and photos accompanying the Special Report on hull strength ("Lessons from the Boneyard," page 23) to recognize that the integrity of an FRP hull is dependent upon much more than fiber-resin ratios or how thickly the hull is laid up.
Just as critical are quality control during lamination, structural reinforcement, keel and rudder design, chainplate installation, hardware backing and reinforcement, and adequate attention to "stress risers" where extreme loads migrate and focus. The keel stub, mast partners, hull-deck joint, and rudder stocks are some of the common failure points in the boneyard boats that were inspected for the report.
As for longevity, poor attention to details like deck hardware attachment (leading to rotten deck core), poor secondary bonds, or poorly engineered hull-deck joints are what frequently doom older FRP hulls, not the strength of the laminate itself. Look at the floors and keel stub of the Pearson 32 (photo, page 9), and you get a pretty good idea why this boat is still around after 28 years.
No production FRP boats can be expected to stand a severe pounding on rock, coral, or concrete. Nevertheless, seeing what happens to some popular cruising boats when they are tested to the extreme, makes me extremely skeptical of unproven claims of engineered strength. It also makes me wonder how many off-the-line boats are actually built to their design specs.
Now matter how well built a fiberglass boat is, the material imposes strict limits. So the next time a salesman starts touting his production sailboat as "virtually indestructible," donít bother listening any further. Unless youíre in the market for a steel-hulled tugboat, the seawall will always win.