Please explain. I would have thought a fast boat would loose the
ablative faster...

This is what you said:
This all raises a serious question in my
mind as to whether ablative antifouls are that useful -maybe they are
best suited to smaller boats that never really get going fats enough to
displace the slime? Any thoughts?

You didn't say anything about faster boats removing paint. In my opinion
faster boats would do a better job of removing the slime but they would have to
be faster than any sailboat..
S/V Express 30 "Ringmaster"
Trains are a winter sport