Rhumb Lines June 2019 Issue

Fiberglass Boat Strength

fiberglass boat hull
Photo courtesy of Boat U.S.

In the battle between a fiberglass boat hull and a metal piling (or guard rail) the metal almost always wins.

Several years ago I heard a story about a boatbuilder who was demonstrating the toughness of their hull at a boat show booth by allowing passersby to wack a hammer at a sample fiberglass sandwich core panel. Each time, the hammer would impressively bounce back, leaving only a small dent. But then one dubious volunteer (an engineer, one presumes) took a turn, but this time with the hammer claw at the business end. The claw quickly pierced the thin laminate and lodged in the core, thus puncturing the myth of the “indestructible hull.”

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—no small caveat—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 in our 2007 special report on hull strength 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 (see “Lessons from the Boneyard,” PS August 2007 online).

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 that 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 robust floors and keel stub of a Pearson 32 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. Observing what happens to some popular cruising boats when they are tested to this extreme, makes us extremely skeptical of unproven claims of engineered strength. It also makes us wonder how many off-the-line boats are actually built to their design specs.

No matter how well built a fiberglass boat is, the material imposes strict limits. So the next time a salesperson starts touting a production sailboat as “virtually indestructible,” take it with a grain of salt. Unless you’re in the market for a steel-hulled tugboat, the seawall will always win.

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