How to clean a boat hull in the water: method, tools and marina rules

Yes, you can clean a boat hull in the water, with no haul-out and no trip to the yard. The method works on the slippery film and short weed that build up between two bottom jobs, as long as you remove them gently and leave the antifouling paint where it is. Before you go down, check two things: the marina rules, which may ban diving at the dock, and that someone stays on board to keep an eye on you.

What comes off afloat, and what waits for the haul-out

A bottom job, strictly speaking, means stripping the hull and repainting it, and it is done out of the water. Between two bottom jobs, cleaning the hull afloat keeps it smooth while the paint is still in good shape.

The International Maritime Organization (IMO) has published guidance for recreational craft on hull fouling. It considers in-water cleaning suitable for light fouling, such as the slime layer of microalgae, using gentle techniques. The caution has two reasons behind it: limiting the biocide the paint releases, and keeping the paint from wearing out before its time.

The same document advises against scrubbing off hard or bulky growth attached to antifouling while the boat is afloat: barnacles, tubeworms, long fronds of weed. Tearing them off scatters debris and a pulse of biocide into the water, and the depleted paint then fouls again faster. The IMO also recommends hauling the boat out at least once a year to clean it on land, where the waste is captured.

Why bother when the hull has only turned a little green? Because a thin film already slows you down. For commercial ships, a study published by the IMO estimates that a slime layer 0.5 mm thick (about 0.02 in) covering up to half the hull can raise greenhouse gas emissions by 20 to 25%, depending on the ship and its speed. On a pleasure boat the logic holds: a clean bottom moves through the water more easily.

Before you dive, read the marina rules

In France, every port applies its own set of port regulations, which you can consult at the harbormaster's office. Read them before you pull on your wetsuit, because they may rule out cleaning at the dock. The rules adopted in 2023 for Villefranche-Darse and Villefranche-Santé, two département-run harbors in Villefranche-sur-Mer, are a clear example. There, in-water maintenance of the underwater hull is reserved for a specialist contractor, with the port authority's approval. And diving, whether freediving or on scuba, is banned within the port limits.

Other ports are more flexible, but the substance stays the same. The good-practice guide to hull maintenance from the Iroise Marine Natural Park, in Brittany, points out that France's Environmental Code penalizes letting substances harmful to wildlife and plants run into the water. It specifies that this provision targets hull work done outside the designated areas and slipways, and that some towns, such as Douarnenez, ban unauthorized hull cleaning by municipal order. A brush that lifts antifouling and spreads it around the basin produces the same kind of discharge. The IMO guidance points the same way: check with the local authorities before any in-water cleaning.

Some regions even give guidance on frequency. In California, the boater's guide to using hull paint from the state's Division of Boating and Waterways recommends waiting at least 90 days after applying a copper paint before cleaning it. After that, clean it only when needed, no more than once every 30 days. For a biocide-free paint, the document suggests a cleaning every two to four weeks, adding that the right frequency depends on the product and local conditions.

Four questions to ask at the harbormaster's office:

• Can I dive under my boat in its slip?
• Does in-water maintenance have to be done by an approved contractor?
• Where is the hull maintenance area, and how do I get to use it?
• At anchor, outside the port limits, which rules apply?

Pick the right day and gather your gear

Aim for a calm day: little wind, little current, and water clear enough to see the hull at arm's length. If the rules allow it, a sheltered anchorage sometimes has clearer water than the marina basin.

Take the age of the paint into account as well. Freshly applied antifouling should not be brushed, and each manufacturer explains on the product data sheet how to maintain it. The IMO asks boaters to follow those instructions.

• A soft-bristle brush or a non-abrasive sponge. No wire brush, no scouring pad on the paint.
• A scraper for the propeller and shaft, when they are not painted.
• Something to hold on to. A suction-cup handle, or a line pulled tight under the hull from one side to the other. Without it, every stroke of the brush pushes you away from the bottom.
• Gloves and a wetsuit. Barnacle edges cut.
• A little weight, on a quick-release belt. Near the surface you float, and you end up finning more than brushing.

How long you can stay under the hull

On a breath-hold, a good share of each dive goes into getting down and back up. With a mini scuba tank, you stay in one spot long enough to finish a whole section. At OXSEA, a 0.5-liter tank filled to 200 bar (about 2,900 psi) gives up to 8 minutes of air, and a 1-liter tank about 15 minutes. Those times assume calm breathing. They drop with effort and depth, and scrubbing a hull is real effort.

How many tanks for the whole bottom? Don't start from a number you read on a forum. Split the hull into sections: port waterline, starboard waterline, bottom, keel, rudder, propeller and shaft. Do one section per dive and time the first one. You will know your real pace, on your own boat, with the growth it has right now.

Refilling then sets the rhythm. The 12 V / 220 V mini compressor runs off the boat's battery or shore power. With two tanks, one fills while you use the other, and the break gives you time to warm up. The guide on how to refill a mini scuba tank compares the hand pump, the compressor and the transfer station.

Brushing takes two hands: one to hold on, the other for the brush. A tank strapped to your back suits the job better than a mouthpiece clenched between your teeth. The OX500Plus can be worn that way thanks to its harness and external regulator, and so can the OX1000, with twice the air. The OX1000 and mini compressor pack puts the tank and the means to fill it on board in one box.

The method, dive by dive

1. On deck. Shut off the engine and take the key out of the ignition. Hand it to the person staying on board. Hoist the Alpha flag. Agree on simple signals with them: all good, coming up, I need help.
2. Look before you touch. Swim the length of the bottom without brushing. Spot the heavily fouled areas, worn antifouling, blisters, the anode. According to the IMO, the propeller, the shaft, the rudder and its fittings, and the water intakes and outlets foul faster than the rest.
3. Start at the waterline. The sunlit strip just below the waterline is often the first to turn green. Brush in long, light strokes.
4. Watch the color. If a cloud the color of your antifouling comes off the brush, you are taking off paint. Ease up or switch to the sponge.
5. Finish with the propeller and shaft. Engine off, key out of the ignition, no exceptions. On bare metal, the scraper pops the barnacles off. A painted propeller gets the same treatment as the hull.
6. Surface with air to spare. Check the pressure gauge each time you move to a new section. Decide to go up while there is still air left, and breathe out gently all the way up.
7. After the session. Rinse the tank, regulator and brushes in fresh water. Write the date, the sections cleaned and the condition of the paint in the logbook. The IMO suggests keeping this information in one place, along with the name of the antifouling product.

Safety: four rules, no exceptions

• Engine off, key out. Neutral is no protection: one throttle lever bumped by mistake is enough to put the propeller in gear.
• Never alone. In diving, the basic unit is the pair, as the safety sheet from France's Ministry for the Sea points out. Under a hull, someone stays at the surface, keeps you in sight and knows how to raise the alarm.
• Alpha flag up. According to the same sheet, the support boat must fly this blue-and-white flag as long as a diver is in the water. It tells passing boats you are there.
• Keep breathing all the way to the surface. The air in the tank is compressed. DAN's Alert Diver magazine reports that rising about 1.5 meters (5 feet) while holding your breath can be enough to cause a gas embolism.

The ministry's sheet also advises seeing a doctor to rule out any reason you should not dive. The mini scuba tank safety guide goes through each point, and the guide to mini scuba tanks for boats helps you pick a model based on how your boat is equipped.

Mistakes that cost you a paint job or a day on the water

• Scrubbing hard to get it over with. Antifouling is designed to wear away slowly. Every heavy pass takes off a little more.
• Putting metal on the paint. Wire brushes and steel scrapers scratch the gelcoat and tear off the antifouling.
• Counting on the brush to skip a haul-out. The IMO advises against in-water cleaning as a way to stretch paint beyond its service life. Cleaning maintains the paint; it does not replace it.
• Staying in the current. If it picks up, stop and work your way back to the boat along your line.