I know this because I power washed and scraped 8 million of them from the bottom of Ya Sista's A$$ on Saturday. There was barnacles growing on barnacles. The trim tabs must have been sprayed with barnacle attractant. I had to power wash the street when I was done; it smelled like Wagner's at low tide. I prolly shed 300lbs of 'em....
....and there's still a few more to be had in some hard-to-reach areas I will target tonight.
The crusty demons were helped by a 1/8" thick coating of green slime which deflated any ablation.
Overall she's looking good now; the funky weather last week as well as family obligations, pushed back the project some. I hope to finish prep and paint this week.