Epoxy vs Vinyl vs Polly

[zerogara]

A solid laminate at stress points may flex and not fail, go through elastic deformation and return to its original form. It's gel coat may crack through this transition or a repetition.
A sandwich will most likely not flex if it is built sufficiently but when it does it will most likely fail (it is less elastic) and delaminate. Usually between the FRP layer and core, or if bonded really well the core itself might fail. At that point you may be screwed and not even know it until a winch or a shroud plate comes off with part of the deck or hull. I had seen a really well made trimaran loose its whole roof of sandwich together with the 10' traveler in a strong gust.
Not a pretty picture of that "convertible".

A boat/hull that doesn't flex is over designed or under utilized. If you don't hear squicky noises inside a sailboat you are not pushing it hard enough. When you do it is flexing! Why is wiring and tubing through a boat installed properly to be loose and allow movement and prevent chaffe from that movement?

[Robjl]

RichardM,
Polyester is the oldest... cheapest...has poorest adhesion to other materials, even to itself when cured...it is not suitable to use with modern stiched fabrics that enable thinner (lighter) construction unless chopped strand matt (CSM) is laid between the layers of fabric and that defeats the purpose a bit. And most important, polyester laminates absorb water!!!
Epoxy is the most expensive...but less is used if stiched fabrics are used (no CSM needed) Excellent adhesion to itself and other materials...doesn't absorb water. Also has higher strength.
Vinyl-ester is somewhere in the middle.
So... if you are trying to compare apples to oranges it won't work.
An epoxy boat is likely more expensive but the hull should be either lighter or stronger or both. And of course osmosis is not the problem it is in poly laminates.
I only regard the osmosis as a problem if the boat is permanently in the water. If you are looking at trailer boats it's not an issue.

Cheers
Rob