Welcome to the site. You don't mention what type of boat you have, and replacing the core can be a little bit different depending on the boat, but this thread and several others, walk you through how to replace the core. It is a bit of a long thread, but covers demo, materials and re-build. If you have any questions, feel free to ask. There are plenty of guys to steer you in the right direction here.

http://www.hydrostreamforums.com/viewtopic.php?t=373

Bill

My floor was rotten ! I cut it all out and found the Bulsa core was bad to. I have all of the material to put it back in just looking for the best way to go about it.

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