Those aren't barnacles, they are zebra mussels and require paint formulated for freshwater. They are a major problem as invasive species go. They clog municipal water filtration systems all the time.
If a boat sits still zebra mussels will cover the bottom. An ablative paint needs motion to expose a new layer of biocide and drop the critters off.
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1978 30' Hughes Northstar 1000
1964 20' Bertram Moppie