How to Clean and Sanitize Your Boat's Freshwater Tank and Lines

Switch the water heater off at the breaker (and at the shore power panel). Practical Sailor says to remove carbon canisters and micron filter cartridges before sanitizing: strong chlorine uses up a carbon filter and can damage it. Take out faucet aerator screens too. Leave the wire-mesh pump strainer in place.

Run the pump to empty the tank through the faucets, or open the tank drain if your boat has one. With the tank nearly empty, open the inspection port (the round screw-off or bolted lid on top of the tank), if there is one. Shine a light inside. Look for:

• Slime or film on the walls (feel it with a gloved hand; slick means bacteria)
• Sediment, sand or rust flakes on the bottom
• Black or green growth

VETUS recommends having enough inspection lids to check, rinse and clean a tank properly as part of annual maintenance. If your tank has none, sanitizing alone (steps 5 to 9) still helps a lot.

Practical Sailor's advice is simple: nothing beats a good hand scrub and a strong rinse. Use a long-handled soft brush and a little dishwasher detergent in water, then rinse thoroughly with a hose. Pump or sponge out the dirty water. Avoid metal scrapers, which scratch plastic and give bacteria a place to hide.

Every tank has a vent so air can get in as water goes out. Find the vent fitting on the hull or deck, clean its screen, and flush the vent hose. A clogged vent can make the pump struggle and can let air or insects in if the screen is missing. Practical Sailor added a small inline strainer to a water tank vent to keep bugs out. Check the inspection port gasket and replace it if it's cracked, so the lid seals when you close it.

Use the method Practical Sailor gives, which is based on the standard sanitizing procedure for RVs (ANSI A119.2):

Tank gallons × 0.13 = ounces of plain bleach (that's about 1/8 cup per 10 gallons of tank capacity)

This formula assumes regular bleach of 4 to 6% sodium hypochlorite. The EPA notes that many bleaches now say 6% or 8.25% on the label. With 8.25% bleach you can use about a quarter less. Use regular, unscented bleach whose label says it's for disinfecting. Skip scented, color-safe and splash-less bleach. Fresh bleach works best; it weakens with age.

Never pour straight bleach into the tank. Mix the measured bleach into a gallon of clean water first. Practical Sailor says this mixes better and reduces spot corrosion in aluminum tanks. Pour the mix into the deck fill (or the inspection port), then fill the tank the rest of the way with drinking water. If the vent is where you can see it, let a little solution come out of it so the vent line gets sanitized too, but don't let it spill into the water.

Turn the pump on. Open each faucet, cold and then hot, one at a time, and run it until you smell chlorine. Do the same for the shower, transom shower, cockpit sprayer, any freshwater-flush head, and the ice maker or drinking-water dispenser if the boat has one (BoatUS points out that an ice maker's supply line often sits in a warm spot behind the unit where water goes stale; check the ice maker's manual before running sanitizer through it, and dump the first batches of ice afterward). The hot side runs through the water heater, so it takes longer to fill with solution. Then close everything and leave the pump on so the system stays pressurized.

Practical Sailor says the standard solution needs four hours of contact time to disinfect completely. Doubling the strength cuts that to one hour, but stronger mixes are harder on seals and metal tanks. A second Practical Sailor article uses a stronger mix and says to leave it at least three hours but no more than 24. Pick one method and stick to its time. Don't leave strong chlorine in the system for days: Practical Sailor notes that the rubber and neoprene parts inside your water pump are also damaged by heavy chlorine exposure.

Drain the tank completely through the faucets (and tank drain if you have one). Before you drain, think about where the water goes: chlorinated water dumped in a marina is hard on sea life. Drain ashore where you can, or check your marina's rules. Refill the tank with drinking water and run every faucet again until the system is flushed. Repeat until you can't smell or taste bleach. It often takes two or three fills.

If a chlorine smell won't go away, Practical Sailor suggests adding food-grade hydrogen peroxide to the tank, which turns the leftover chlorine into harmless salt and oxygen. They say not to use vinegar for this, because it can ferment.

Install new filter cartridges (not the ones you took out), put the aerators back, close the inspection port tightly and turn the water heater back on only once it's full of water. Check the pump strainer and clean it if the sanitizing loosened debris into it.

• Fill with a drinking-water-safe hose that you use for nothing else. Keep the ends capped.
• Practical Sailor recommends filtering dockside water as you fill. A simple inline fill filter catches sediment.
• Use the water. A tank that sits half full in summer heat for weeks will go stale. Top it up with fresh water and drain stale water.
• If you winterize the freshwater system, use only non-toxic potable-water antifreeze and flush it out in spring. Xylem (maker of Jabsco pumps) says to use only pink propylene glycol antifreeze on board and never automotive ethylene glycol, which is toxic. Pump until pink shows at every faucet, then flush and sanitize before drinking the water in spring.
• Label the water fill plate clearly, especially if it sits near the fuel or waste fill.