Use Aurora [url=https://www.auroramarine.com/store/algae-strip.html]Algae Strip[/url]. Put it on with a paint roller. Let it stand for about 20 minutes and rinse off. If the rings are will developed, you may have to scrape the softened bases with a wooden scraper (so you don't scratch the hull) and repeat the process.
When the hull is clean, we recommend scrubbing the bottom with [url=https://www.auroramarine.com/store/boat-scrub.html]Boat Scrub[/url] to remove all traces of the cleaner and other dirt, grime, stains and oxidization, then protecting your hull against further barnacle attachment with [url=https://www.auroramarine.com/store/vs721-bottom-coat.html]VS721 Bottom Coat[/url].
Thanks for your question,