If you plan on leaving your boat in the water for an extended period of time applying antifouling bottom paint to you boat required (usually). Most locations will result in bottom growth after a few weeks, though there are exceptions.
The demo Sage 17, AIR BORN, spends most of the Colorado sailing season on Lake Dillon, a reservoir lake, about 55 miles west of Denver/Golden. Dillon is a cold water lake … I mean COLD water. Most of the year the lake is frozen over. When I put AIR BORN on the lake, usually, the last week of May the temperature is just above freezing. In August the lake may be 50 degrees and by the end of September the water is again getting close to freezing. AIR BORN does not have antifouling paint because Lake Dillon is so cold only a bit of slime grows on the boat over a six or seven week period (the longest AIR BORN is on the lake between summer travels to regattas and messabouts). I know folks that keep their boat on the lake all season (last week of May through the end of September) and don’t anitifoul their boats. Such water conditions are very very rare.
AIR BORN has spent 15 days in the San Juan & Gulf Islands of Washington State and British Columbia, Canada. In those 15 days she had a slime coat greater than she experiences on Lake Dillon and had a couple of very small barnacles attached to her hull (these came right off with a scrape from my fingernail).
So … 99% of the time … if you are going to keep your boat in the water more than a couple of weeks you need protect your boat with bottom paint.
There are many types of bottom paints. For a trailer sailor, like the Sage 17 and the new Sage 15, the best paint option are usually what are branded as ‘hard ablative’ and ‘copolymer ablative’. Ablative means the paint, over time, sheds its outer layer or ‘used’ chemicals exposing ‘new’ paint that will discourage creatures from attaching to the hull. Some paint manufacturers call this ‘self polishing’. Hard, in the situation of a trailerable boat, means the paint will stand up well to the rubbing involved in launching, retrieving and when the boat is bouncing down the road. (NOTE: paints sold with just the ‘hard’ notation, in almost all cases, will not work on a trailerable boat … see below on why.) Copolymer ablative paints have a different chemistry than, and ‘shed’ their outer layer similarly to, hard ablative paints.
Another paint on the market is called ‘thin film’. This paint is applied in very thin layers with a very thin paint. In general these paints work best in ‘low fouling areas’. These paints do well on trailerable boats as they dry hard and resist being rubbed off the boat going on and off the trailer.
But it isn’t just that simple. Many-to-most bottom paints are formulated to be wet all the time; this is especially true of ‘hard’ paints. If the paint, after initial application, is kept out of the water for more than a few months it looses the ‘antifouling’ ability. Such paints will not work for a trailerable boat that will likely spend more time out of the water than in. The requirement for a trailer sailor is a paint that is formulated to keep anitfouling abilities with an, “indefinite maximum time to launch”.
Obviously there is a lot of chemistry going on with bottom paint. This means that some bottom paints cannot be used over any, or different chemically, old antifouling. You also need to confirm that the boat’s building material (ie, fiberglass, aluminum, steel, wood …) is compatible with the paint you wish to apply. To add more complication some paints require a primer paint and and special cleaning agents.
Not so fast … there is another consideration. Each body of water, sometimes different locations on a body of water, and even differences within slip locations in a marina, result in one antifouling paint works better than others. What you need to do is ask around the location you will be keeping your boat to see which paints boaters, preferably folks with trailerable boats, are using.
Yep, there is more research required: is the paint you want to use allowed in your home waters, or waters you will visit? Some locations don’t allow specific antifouling paints, or chemistries, to be used. You can contact the paint manufacturer and ask if there are any use restrictions in your area. Be sure to check as you don’t want to be violating any laws by applying an ‘illegal’ paint.
Now that you have made the decision your boat needs bottom paint, narrowed your paint selections to those that will work best where the boat will be moored, you can now pick a brand and color.
Next installment in this antifouling bottom paint series will be prepping the bottom of your boat for painting.
PS: Linked below are more reading material on bottom paint selection –
This is a good summary from the folks at Practical Sailor –