When I worked at HydroStream, I was taught that the purpose of the core was to separate the layers of glass. That way it gave strength and rigidity similar to an I-Beam.

Balsa vs. foam is sometimes a heated topic. I like both and depending on how forces are applied, each has it's strong points and weaknesses.
Poor lay-up practices leading to water intrusion gave balsa a bad name. It wasn't the balsa's fault. Foam is more tolerant of getting wet and not turning to mush. In a "nut shell", the core material is used to separate two skins of fiberglass to create a rigid panel . Use what makes you feel good. I use Balsa.

Here's a Viper rebuild thread I wrote for Mark Casper. It explains how to and why with lots of pictures. It's a good read and will get you familiar with the process. You may not need to split the hull and deck. I'm sure you will not have any more damage than I had.

http://hydrostream.org/ArticleArchives/AlaskaViperResto.htm

Hi, your questions are more complicated than they appear. As I understand foam and balsa are essentially installed the same. But why foam? Because It doesn't rot...as fast. :cry: If not sealed properly it'll hold water (weight) and ultimately rot, just like wood. ...just sayin. ;)

As far as only replacing part of the core, I can only think if two possible issues. If you can slide a blade between the wood and the glass, it's not bonded anymore, so continue removing. Secondly, I would be concerned about using a butt joint where the old meets new, as it might be a little weak.

Here's a link to a foam recore by ccRick... [url]http://www.hydrostreamforums.com/viewtopic.php?f=13&t=85[/url].

How difficult is it to replace the balsa core with closed cell foam? Can some-one give me a step by step? The balsa on my boat is good from the dash forward. I plan to replace up to deck. If I go with foam should I remove the complete core? Is the foam core as strong as the balsa?