Here are some thoughts:
(no core). No core
problems! But you want it thick enough that it hasn't fatigued due to flexing. Very early fiberglass
boats built in the '60 and '70s were overbuilt because people didn't understand the material very well. These 40 year old hulls are as strong as new. Since the gas crisis in the early '70s it's become much more expensive to build solid boats so many "price" builders moved to cores - at least in the deck
. Also, boats that need to be light (catamarans) need to use cored construction for performance.
Blistering bothers both solid and cored boats. This happens when water
get into the fiberglass skin and reacts with the resin. Most common in boats from the '80s. This could be fixed but it was expensive. Boats built in the last 15 years or so have epoxy
barrier coats and use more water
resistant resins so the problem has pretty much gone away. The good news is that if the boat is more than 10 years old and hasn't blistered (or blistered but been well repaired) it's not likely to start.
Very early or very cheaplly boats sometimes had cores of plywood
or mystery wood. You're unlikely to find a boat with this core
that hasn't either been repaired or been cut up with a chain saw.
Fiberglass with end grain balsa is usually fine if the builder
was a quality builder
who made sure that they got good adhesion between the fiberglass and the balsa. During the last 15 years most cored boats were built with a construction method that used a vacuum to push the fiberglass hard onto the core. This has helped a lot but the old hand method was fine if carefully done.
The biggest problem with cored construction is where a hole is drilled to mount something and the core wasn't removed near the hole. Unfortunately this is a very common problem with lower quality builders or when later owners weren't always watchful when someone approached their deck
with a drill. Even a hole for a bimini
snap can cause problems.
Having said that, the great, great majority of end grain balsa cored boats from reputable builders don't have a substantial problem. Surveyors are pretty good at finding core problems. This is probably the biggest "walk away from the deal" survey
problem. You do not want to take on a major core repair!
Foam cores were viewed by many to fix balsa's problems but has problems of it's own. Again, a good builder will turn out a trouble free boat. Foam is less bothered than balsa by the "drilled hole" problem.
Here's a petty scary site with lots of pictures of badly built boats. Don't let it discourage you. Almost all of the examples are from lower quality builders and most are fast motor
boats (where slamming puts even more strain on a core). Rule
#1 - buy the builder not the boat.
Yacht Survey Online: David Pascoe, Marine Surveyor