I have never seen barnacles
cause significant damage to the bottom of a fiberglass yacht.
I once had growth of six to eight inches of tube worm/barnacles on the bottom of Exit Only when we were anchored in the Hatea River in Whangarei, New Zealand
. I had been away from the boat for four months because of a car accident
on the North Island. I was in the hospital for two months and living next to the hospital for another two months getting physical therapy in New Plymouth until I could return to Whangarei four months later.
We hauled Exit Only out of the water
and cleaned the bottom of the boat with flat blade shovels scraping off the 6 to 8 inch growth. In all we filled up five 55 gallon drums with bottom growth. I had never seen anything like that before. We cleaned the bottom, and in spite of all of the growth, it looked perfect after the job was done.