Depending on the seriousness of the infection, getting rid of them by pressure wash or scraping will increase speed and gas mileage. If you are a competent diver, get a wide putty knife and scrape away. (Watch out for your capac meter though)
I generally find that so much pressure is required to remove barnacles --and I think zebras stick just as tightly if not tighter -- that it will loosen or blast away paint.
If you are using Interlux, it needs to be their Trilux 33. ANything else in the Interux line and the problem is magnified.
I am glad that those critters have not yet hit the Chesapeake Bay! At least not in my part where the water is brackish.
Joel Albert, Potomac MD
"Charlie B" - 1988 32' FBS
Twin 318's/FWC/16x15 nibral props
docked Deale, MD