Threegills-
Most issues with a fiberglass boats arise from improper bonding regarding core materials or water intrusion into a core material. There are of cours other issues such as: osmotic blistering but I will not cover that here.
Back in the late 50's and early 60's the first fiberglass boats came out to market from companies like: Hatteras, Pearson, Cape Dory, and many more. These boats had solid laminates, multiple layers of plyester resin and fiberglass cloth.
Solid hulls are very heavy, many tmes too heavy to be very efficient. Now, not only were the hulls of these boats solid glass, they were also reinforced with fiberglass coated plywood for bulkheads, stringers, transoms and the like.
It would be very difficult to make an efficient outboard powered boat with this type of building technique. Although... there are manufacturers that have,
Ocean Master is one such builder.
In order to make a more efficient boat the use of core materials came into play. The core material takes the place of multiple layers of glass and strenthens the hull while not bing as heavy as a solid multi layer lay up.
Where the core material meets the glass is where issues tend to take place. Sometimes it is just unproper adhesion and the layers of glass will seperate from the core material rendering the boat severly damaged. Other times, most often, water gets into the core, from improperly sealed screws, thru-hulls, etc. The water actually causes and chemical reaction with the resins and the glass seperates from the Core. Most often the water gets trapped between the core and the glass. If the core material is wood it will rot, if it is a composite it may absorb the water and/or continue the delamination issue.
How does one avoid issues with a cored fiberglass boat? You have the right train of thought to avoid hulls that are cored below the waterline. I am not saying all boats that are cored below the waterline should be avoided, they should definitly be scrutinized by a surveyor. If a thruhull weeps water into the core material it could be a very costly repair.
To avoid problems with most of todays boats with a cored transom, deck, hull sides, etc you have to make sure you seal all holes. The best way to do this is with thinned out epoxy. First drill your hole, clean it out, then apply a thinned out solution of epoxy. This epoxy will seep into the core material and will ward of water in the future. You are not done here, do not forget to use an adequate marine sealant when inserting the fasteners. The sealent is the first line of defence where as the epoxy is the last ditch effort protecting the core material.
If drilling into something covered with gelcoat, do not forget to countersink the hole a tad, this will prevent the unsughtly cracking often seen around stantion bases and screw holes.
Also, one can not simply screw into core material like wood. You will need a to thru-bolt whatever you are mounting or add a backerplate. Problem often arise when people mount things improperly to a cored surface.
Now as for different type of core materials: We could go on for many pages. People like to avoid plywood, I disagree, wood is excellent if you need a strong solid mountable surface. The disadvantages are: if improperly sealled it will rot and it is still very heavy.
The composites:
Nida-Core ,
Divinycell ,
Kledgecell Balsa, Airex , Balsa (a wood but it is supposed to suck up the resins when used to prevent rot) each have their own characteristics but you can not screw into them like marine plywoods, you need to through bolt.
In my 20CC restoration I am utilizing 3 different types of Core materials. Marine Ply-wood, Nida-Core, and ATC Core-Cell. I feel as though each core matrial has its own place with in the boat.
Marine ply, for the transom and stringers. Transom for strength as long as the holes are properly sealed and the stringers for strength and weight. I wanted any weight I had, down low. The Nida-Core is for the deck, it is very strong and has fantastic sound dampening charachteristics. The Core-Cell is for the hatches and various custom componants, engine shroud etc.
Now there are many opinions on all of what I wrote above so please do not take above as the way it is, it is just the way I feel and how I like to do things. I did not even go into the different types of resins, Epoxy, Polyester, Vinyester, and so on.