How to Clean the Underside of a Boat

small boat lying upside down

A boat spends most of its time at rest in the water so it is inevitable that algae, barnacles and other marine creatures will settle on it. This extra growth is not necessarily harmful but it does lower the performance of the boat.

If the Boat Stays in the Water

If the boat has to be cleaned while in the water, you will have to use scuba gear or a hookah to be able to clean the bottom. For most boats, the bottom can be cleaned quite easily with a scraper or scrubber. It is possible to use a pressure washer underwater to clean wood, fiberglass or copper clad bottoms.

Taking the Boat out of the Water

Cleaning the bottom is easier if you can take the boat out of the water or land it on a sand bar as the tide goes out.

happy man pressure washing the bottom of a large sailboat

Copper Bottomed or Steel Clad Boats

You can safely clean copper bottomed boats with a pressure washer. It is always best to scrape as much of the growth off as possible and then finish with the pressure washer. As long as you don’t use chemicals, you will be able to allow the washer water to flow back into the river or the sea. In the case of really heavy growth, especially if the boat has been out of the water for a while so that the growth has dried out, you may need to resort to a sand blaster or chemicals.

Wooden Bottomed Boats

Boat cleaning for wooden bottomed boats tends to be more challenging and more physical, hands-on work. The type hull will dictate whether a pressure washer can be used. Obviously it is not a good idea to use lots of pressure if the hull is heavily caulked. Generally a wooden bottom should be scraped clean.

Fiberglass Bottomed Boats

Fiberglass bottomed boats should be scraped clean as soon as they come out of the water. A pressure washer can be used on fiber glass, but you need to be very careful and spray at oblique angles so that the spray glances across the surface of the fiberglass. The top coat of a fiberglass bottom is called the gel coat. This coat can be damaged by high pressure sprays.

Stubborn Encrustations

If the growth on the bottom is proving really difficult, you will have to try a commercial cleaner. These can be sprayed or painted onto the growth and then pressure washed off after a specified time. When using these cleaners, you must ensure that the water from the washing is not allowed back into the river or sea.

Trisodium phosphate mixed in warm water at I cup per gallon of water will get rid of algae and most algae stains.

Another friendlier cleaner can be made with six parts of hydrogen peroxide and four parts Lysol. This needs to be painted onto the algae several times over a period of two or three hours and then washed off with a pressure spray.