Bottom Paint

Everything below the waterline of your boat is part of the marine ecosystem. Left without any protection, your boat will start to attract multiple organisms, including algae, slime, seaweed, barnacles and mussels. 

barnacles, hull paint

How do you prevent growth on your hull?

The most common method to prevent this growth (also known as biofouling) is the application of a bottom antifouling paint. There are three broad groups:

  • Ablative
  • Hard
  • Hybrid 

Ablative paints gradually wear away, continually revealing fresh biocide (chemical to kill microorganisms) as your boat moves through the water. With hard bottom paints, the biocide slowly dissolves (rather than the paint itself), allowing water to penetrate deeper into the paint until all the biocide is depleted. Hybrid paints incorporate the benefits of both ablative and hard paints into a single product.

Today, the most commonly used biocide in antifouling paint is copper oxide. Copper is a naturally occurring element, but at high enough concentrations, it interferes with cell metabolism making it challenging for life to grow on your boat’s hull. The problem with using copper in antifouling paint is that it also leaches into the marine environment and can accumulate in filter feeders such as clams and mussels, and damage the larval stages of aquatic invertebrates and some fish species. Marinas and protected anchorages with little current or tidal movement are particularly vulnerable, as they allow the buildup of copper to toxic levels.

What are the eco-friendly options?

The Environmental Protection Agency (EPA) provided funding for a three-year project undertaken by the Port of San Diego called “Safer Alternatives to Copper Antifouling Paints for Marine Vessels”. The project evaluated three factors (application, performance and cost) to determine whether alternative paints were comparable to copper hull paints in both warm- and cold-water regions. The results determined that some alternative antifouling paints are less toxic environmentally, and can save money because they last longer than copper paint. An easy-to-use table was produced that enables boaters to identify the most appropriate coating for their specific marine area and requirements.

hull paint, antifouling paint

Based on the study, the top performing alternative bottom paints were: 

1. Hempasil X3 (87500) - Hempel USA

2. Intersleek Pro (replaced Intersleek 900) - Interlux

These two paints are nonbiocides, which are commonly formulated with silicon compounds creating a slippery surface. Hulls coated in nonbiocide paints can be cleaned relatively easy and some have much longer lifespans, ranging from 5 to more than 10 years. 

What type of bottom antifouling paint is best for your boat?

Selecting the best bottom antifouling paint for your boat is far from a simple decision. It’s important to understand how alternative hull paints work, and the short- and long-term costs.

Factors to be considered:

  • What type of boat you own, the frequency of use, and the average speed
  • Cost and desired paint lifespan
  • Existing hull coatings
  • Realistic maintenance schedule

Tips for removing your old bottom paint:

  • Scrub your hull on land during a haul-out period.
  • Place a tarp or filter cloth under the boat to catch paint and scraping chips.
  • It is dangerous to remove any large areas of antifouling by dry sanding, both to the operator and environment. If possible, place a tent over your boat to contain airborne particles or choose a still, windless day to sand. Try to wet sand or use a vacuum sander to remove old paint.
  • When you have finished, collect any waste for safe disposal in a hazardous waste receptacle.

Tips for applying new bottom paint:

  • Make sure that the hull is thoroughly cleaned and rinsed, and follow the manufacturer’s instructions for paint application.
  • Place a tarp or filter cloth under the hull to catch drips or spills.

Did you know?

In San Diego Bay, 72% of the copper entering the water is due to discharges from antifouling paint and in-water hull cleaning (Office of Naval Research).

Green Boating Guide: