There is no guarantee that a boat may not have core issues. I know of two almost new (one was on delivery and the other been used about 6 months) very high end sport fishers, which had foam core delimitation--and the end of the boat. Both losses in the millions, and litigation.
The risk of core problems increase when the boat has been exposed to multiple freeze thaw cycles. I encourage to have all core penetrations, sealed with epoxy, with the undercutting technique.
Generally there have not been many hull core issues. But they do exist, and I am probably aware of more of these than most. It never has dissuaded me from buying anotherwise good boat. Foam cores do not become rotten, but if there is laminate penetration, not properly sealed, there will be water intrusion, and delamination of the foam core from the glass.
Balsa is an excellent core material--not many other materials have the shear and compressive strength. I have sailed 10'x of thousands of miles in balsa cored boats--way offshore, and never worried about it.
Do check for regular water intrusion. If it appears in places it does not belong, then be suspicious. There is probably something wrong. Do moisture tests on a dry floor (easy to do on the hull inside the cabin in both the 22's and 25's. Cockpit in pre 2006/7 22'. Look for penetrations into the core--especially the earlier 22's with screws into core, rather than glassed on strips in later models.
If you can get safely under a dry boat, without bottom paint, and do moisture readings and percussion this will give you a good idea.
The Tom Cats are a different story.. It is fairly hard to access the bottom of the hulls, and much of the bottoms are under tankage fuel and water. The C Dory 25 iinner hull under the cockpit has a fuel tank over it. If the tank is intact, and properly secured there should not be any issues. But there can be damage in a boat which as been abused.
Also the boats in areas where they are left out in the "cold"--subjected to multiple freeze thaw cycles. are at greater risk.
Let me address what my friend Barry mentioned. There is no commercially available ultrasonic technique which will give all of the answers for delaminate and wet cores. The ultrasonic waves only are conducted thru solid media. When there is an air gap, then that is the end of the ultrasonic diagnostic test. You can infer a wet core--if there the bottom measures more than the outer or inner glass layer
s. The sort version; one of my close friends owned NTD of Hunting Beach CA--and designed most of the ultrasonic instrumentation still used by NASA, the Military and civil aviation. He and I had 25 boats to do proof testing on of a technique called pitch and catch. He designed a basic instrument which would retail in the $3000 range and we proofed it on many hulls. The normal model of this type of instrument is in the $16,000 range on up. We took the concept to the various Marine surveyors society and individuals, plus boat builders. Some high end boat builders were interested and would buy. Marine surveyors said "No". They would not pay $3000 to come into the 22 century or take the time to learn how to use the instrument which in the long run would save alot of time. So marine survey remains in the dark ages. Infrared photography can be of some help in looking for moist core, but the definitive test is sample boring--and the seller is not going to allow that!
Xray is not really available or practical. With digital X ray now being the norm, it could become of some value--but I don't know of anyone doing it.
