Join Date: Feb 2000
Location: Annapolis, Md
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We are just now starting to understand the problems with non-oriented materials. In actual testing performed by the US Naval Academy (from a paper presented at the 2002 SNAME Chesapeake Bay Sailing Yacht Symposium), non-oriented fiber reinforcing fabrics were found to be the primary mode of failure in point impact situations. This paper outlined that Naval Academy cutters, which are used in training exercises, are subjected to frequent collisions, but the Academy cannot afford to take them out of usage for long repair periods. As a result, impact resistance was very critical. In order to test the impact resistance a large pendulum with a massive weight was constructed. On the leading edge of the pendulum was a steel replica of the bow and stem fitting of a Naval Academy cutter. Test panels were constructed that matched both known (prior cutter lay-up schedule and J-24 topsides) and conjectural hull panels. The panels were aged and then tested warm (some resins lose strength when warm). The tests consisted of retracting the pendulum with a forklift and then releasing the restraint cable. The results were very dramatic.
To begin with. Solid hulls did far worse than cored hulls. In examining the panels after the collisions, the failures almost always occurred in the non-direction material being used and not in the core materials. The test sample that faired best used an oriented glass laminate, NO non-oriented materials, vinylester resin, and a high-density foam core. The pendulum never entered the outer laminate and microscopic analysis further destructive testing showed that core was still fully adhered to the skin and that the deformation was within the elastic (memory) properties of the core.
This is bad news for those with older heavier hulls. Through actual testing it has been known that these heavy solid hulls did not have the strength of newer lighter hulls but the failure mode was not completely understood. As mentioned above, it was generally believed that the issues were inferior resins and fibers, poorer handling of the materials, poor resin ratios, and the extensive use of accelerators and fillers. What is implied in the NA testing is that the problem may also lie in the extensive use of non-oriented fiber type laminates. These old heavier so-called solid glass hulls actually used an enormous proportion of non-oriented materials greatly reducing their impact resistance, stiffness, and tendency to resist fatigue.
Everything else being equal, twice the laminates take twice the time to abrade, but heavier cloths are not more abrasion resistant than lighter ones. Kevlar is enormously more abrasion resistant than any other laminate. The other factor is the force of the impact. A lighter boat hits with less force than a heavier boat so the rate of abrasion is greater on a heavier boat. On the other hand there is typically more material to resist this greater impact and abrasion. As far as I know resin has little bearing here.
If one had to design a boat solely to abrade for a day or two against rock it might be thick steel. If that was not your only criteria for designing a boat (in other words you were concerned about sailing ability and motion comfort), then it makes sense to build in FRP with outer layers of kevlar over a medium density foam core over more layers of S-glass and Kevlar.
Here more laminates is not necessarily better. Fiber type and fabric type is most crucial. Proper load distribution is crucial. This means reasonably small panel sizes, good fiber orientation and a bit of luck. Kevlar helps. Resins again have can have a major impact on performance. In the US Naval Academy testing mentioned above Vinylester Resin of a type used to build military and motorcycle crash helmets performed much better than less ductile resins. The high tech fibers, Carbon and Kevlar, need resins that can withstand higher tensile loads without developing small stress cracks. Epoxy and Vinylester can deflect more without getting the microscopic fractures that are the beginning of the end for FRP.
Polyester is the cheapest and most common resin and as laid up is not impermeable to water. Polyesters vary widely in quality and performance. They are more prone to fatigue problems than other resins. One source of water penetration is the microscopic passages created as polyester fatigues. Early polyesters were particularly brittle and fatigue prone. This problem was further aggravated by the tendency by early boat builders to use accelerators and retardants depending on temperature and the nature of the operation. Another issue is with accuracy of the metering. Early boat builders used pretty imprecise methods to proportion resin. Today metering pumps make precision metering a piece of cake, but back then mixing was more hit or miss. For example I installed an instrument through hull in a Triton and found a pocket of uncured un-reinforced resin probably a decade after the boat was built.
Vinylester resin does better than polyester so many better boat builders are now using it in the outer laminates and with high tech fibers. Epoxy seems reserved to custom builders and secondary bonds, because it is expense rather than some other flaw.
Kevlar is harder to laminate than the other fibers. It is hard to cut and floats to the surface. It dulls cutting tools and is hard to tool. The key is to use sharp tools to cut the laminate vacuum bag the lamination and use glass mat buffer laminates. Both carbon fiber and Kevlar require Vinylester or epoxy resins to get any real advantage out of them.
Hopefully you will find this helpful. I appologize for its length.