Take a grinder and grind off all the exposed metal. Put a wet towel
over the portion on the 'other side of the hull' that your are grinding
to keep the metal relatively cool .... so as not to overheat the
fiberglass. Stop grinding when all of the flange is 'gone' and so
you dont grind fiberglass. Then simply whack out the stub thats left.


In article , engsol
wrote:

My boat has a really old engine sea water intake. On the water side
it looks like shower drain...you know the kind...round with lots of tiny
holes. Inside is a threaded steel pipe (yep, old boat) with a hose barb.

Nigel Calder has told me all about installing a new one, but not
a word about removing the old one. So, what do I do? A slege
hammer from the inside to drive out the old thru-hull comes to mind,
but I bet that's not the correct way.

Regards...NormB