There was a time when you could read the insurance study online but I have searched and not found it lately. The jist of the study was that marine insurance companies began noticing a trend that more extensive hull damage was occuring in accidents on older boats relative to newer boats than should have been predictable from the accident descriptions. The study began by looking at actual damaged boats of all periods and trying to quantify the extent of damage that occurred in comparatively routine types of collisions. There was a conclusion that in general the damage to older boats was more extensive than would be expected. (I considered that part of the study to be a little suspect since its hard to quantify the amount of impact that the vessel incurred.)
The study went on to do destruction testing of panels taken from actual boats built over a long period of time. What they found was that the older panels had subtantially lower impact and bending strength than could be expected by calculations based on the typical calculated strengths.
The actual bending and impact strengths were very low (I don't recall the actual numbers but I seem to recall that they were something on the order of 50%-60% of what would be expected). The report then looked at why these numbers were coming out low. The report attributed the problem to a variety of reasons. As I remember these in no particular order, the report mentioned the following factors:
-How the glass fabrics were handled; the report explained that early builders routinely cut the glass fabriics and then stored them folded prior to layup. since some point in the 1970's this practice was changes such that laminates are no longer folded. The fold
lines weakened the glass fibers.
-Early boat builders were less precise in their mixing procedures and used larger amounts of accellerators. This resulted in a resin that started out more brittle than later resins and became increasingly more brittle over time.
-Early boat builders used excessively resin rich laminates which contributed to the higher fatigue rates noted on older laminates, as well as to the internal sheer failures parrallel to the reinforcing that were observed in destructive testing.
-The method used to produce fiberglass fibers in the early days produced shorter, more brittle fibers that as a result required more careful handling prior to layup (which they did not receive) and which were more prone to fatigue than new fabrics.
-Early laminates used a proportionaly larger percentage of non-directional materials (mostly mat but in some cases chopped glass) to bulk up the laminate. Impact and fatigue related failures tended to occur more frequently in these non-directional laminates.
-The panel sizes employed on earlier boats were several times the span of those on later boats with interior framing. This resulted in larger amounts of flexure, a higher number of bending cycles, higher concentrated stresses at hardspots and higher tensile stresses within the laminate, all of which resulted in significant and measurably higher amounts of fatigue.
-Safety factors were actually lower on earlier designs because to achieve the later post Fastnet safety factors without using framing the boats would have been prohibitively heavy.
While I know that this seems counter-intuitive, if you spend some time walking around boat yards looking at these older 1960's and early 1970's era boats and compare them to just slighly later boats, it is pretty easy to find fairly large areas with signs flexural cracking as compared to even slightly later designs with internal framing systems and better laminate materials and methods.
Seabreeze, its amusing and easy to make statements like your Bristol 32 would sink my Farr but with all due respect I'm not sure that kind of rhethoric really adds much to this discussion. In that vein, my only anecdotal evidence to add to this discussion (and I realize this is a very small not very scientific sample using the Laser 28 which were unusally well constructed using vinylester resin, kevlar laminate, a closely spaced framing system, vaccum bagging and a high density foam coring) is one that I cited the last time someone made a similar rhetorical statement.
Back in the mid-1990's a thunder storm came through just before the start of one of the Annapolis Wed. night races and in the blinding rainstorm a Laser 28 and a Alberg 30 (at literally twice the weight of the Laser 28 and with a slightly lower ballast to weight ratio) came together. They saw each other at the last moment and spun head to wind, colliding topside to topside.
After the race, there was no sign of the impact on the Laser 28 while the Alberg had a large spider cracked area roughly 2'-3' feet in length at the point of impact and torn tabbing on the bunk flat below the point of impact. The Laser 28 was sold roughly 5 years later and the surveyor looked for damage in that area and found none. I raced on that Laser 28 roughly 6-7 years later and the boat still showed no signs of damage at the impact area.
Respectfully,
Jeff
Linear Mode
