Base of compression post.
is where you find it in some boats. It is under the cabin sole in most cases at the base of the compression post. This is a post that extends from the I-beam which is located over the keel to a point under the mast. It counters the loading of the mast upon the top of the coach roof. Afterall, something must hold it all up or it would collapse. Thus, a corroded I-beam or rotten compression post can be a real problem. This is probably more explanation than you needed.
