Epoxy is a better glue in five major areas:
1. Better adhesive properties (the ability to bond to the
reinforcement or core)
Yes, I can see that. Epoxy sticks to wood far better than polyester resin. I would only tab bulkheads using epoxy (or at least vinylester) resin.
The only other place I can think of where adhesive properties is important is the laminate-to-core bond. When I've ever come across delamination in a sailboat, it's always been after core has gotten wet. Do we know if wet balsa sticks to epoxy better than it does to polyester resin? I once looked into injecting epoxy into a soft deck, but all the epoxy products I could find insisted the core had to be completely dry.
2. Superior mechanical properties (particularly strength and
Numbers I found for tensile strength of the resins are 7,000 psi for ortho polyester and 7,960 psi for epoxy (source: Marine Composites Handbook, table 2.7
That's about 14% higher, but it's actually the glass reinforcement that provides most of the strength and stiffness in a laminate, not the resin. A blend of glass and resin isn't going to be as sensitive to changes in resin strength, so the actual improvement for the total laminate will be considerably less than 14%. (The only production builder using epoxy resins uses E-glass reinforcement, which is the economy fiberglass normally used by other production builders so their reinforcement isn't any stronger.) I couldn't find what the difference in strength and stiffness would be for a completed panel using e-glass and epoxy vs. polyester.
3. Improved resistance to fatigue and micro cracking
Paul Miller of the U.S. Naval Academy studied fatigue of J/24s which are light weight, cored hull boats. His "high mileage" sample boat had an estimated 11,300 hours of use in a sailing school in the roughest part of San Fransisco Bay over 14 years. He measured the stiffness of this boat had dropped by 18%
due to fatigue and micro-cracking.
That's a fairly extreme boat usage example, yet the boat was still far from failing. Is fatigue and micro-cracking a real problem on production sailboats using polyester resin?
4. Reduced degradation from water ingress (diminution of
properties due to water penetration)
There was an old study of early USCG 40 foot patrol boats
(built in 1952) to see how fiberglass properties changed with age and exposure to water. To quote:
"In 1962, Owens-Corning Fiberglass and the U.S. Coast Guard tested panels cut from three boats that had been in service 10 years. In 1972, more extensive tests were performed on a larger population of samples taken from CG Hull 40503, which was being retired after 20 years in service. It should be noted that service included duty in an extremely polluted ship channel where contact with sulfuric acid was constant and exposure to extreme temperatures during one fire fighting episode. Total operating hours for the vessel was 11,654. Visual examination of sliced specimens indicated that water or other chemical reactants had not entered the laminate. The comparative physical test data is presented in Table 4-5."
The properties in the table didn't change much. Probably epoxy would have performed better, but there are a lot of old polyester fiberglass boats (Pearson Tritons are coming up to their 50th birthdays) that still seem to be OK.
5. Increased resistance to osmosis (surface degradation due to
This is certainly true. But how bad is blistering on newer (< 10 years old) boats? Many builders have been using iso-gelcoats and vinylester skincoats, which seem to work pretty well. I know Island Packet offers a 10 year warranty against blisters.
So, yes, I can see that epoxy resin by itself does have better properties than polyester resin, but when it's combined into the total system of a production sailboat, is the total boat measurably better? No doubt the boat will be more expensive, but do we have real benefits to offset that cost? Or, put another way, is using epoxy solving real problems rather than perceived problems?