A thru hull is a hole in a boat which allows water (and other things) to go in or out of the boat, you are able to control the water (or whatever) by a valve called a sea cock. If a thru hull or a sea cock fail it is not a good thing, which is why a mariner should always carry emergency plugs on their boat..I have 3 different types of emergency plugs on Si Bon...they are one of the many things you have on board that you hope never to have to use.
I had several thru hulls and sea cocks on Si Bon which looked pretty questionable due to corrosion, and not wanting to test my many different types and sizes of emergency plugs decided to bite the bullet and have Si Bon hauled today and have about 6-8 of her thru hulls/sea cocks replaced. I already sleep very well on Si Bon but this won't hurt my nights rest.