Manage hull fouling

Hull fouling can have a big impact on your boating if you have a moored boat. Fouling is also one of the main ways marine pests can get spread into New Zealand waters and around the coast.

What is hull fouling?

Fouling or "scum" is growth on the hull that happens when a boat is left in water for periods of time.

Many marine species can grow on a boat hull. First you get algae that form a pale green slime layer and then larger algae that attach with a holdfast. At the same time creatures that attach like barnacles, shellfish, sea squirts and sponges start appearing. As the fouling grows, other creatures like fish and crabs can come and live among the attached species.

What harm does hull fouling do?

Hull fouling reduces your boat’s performance, increases fuel consumption and if it’s left, becomes a growing maintenance issue.

Many of the fouling species are native and many will not cause harm beyond the impact on your boat, but some can be extremely harmful and invasive.

The main issue is that if you move the boat with hull fouling, you can accidentally move harmful invasive species around and risk damaging our marine paradise.

The basics to manage fouling

The longer you leave the hull without cleaning, the worse the fouling gets, becoming a bigger job.

Owners of boats kept in-water should:

  • clean regularly to keep the hull free of fouling
  • apply anti-foul protection
  • tie in checking for fouling with your regular underwater checking of the anodes that are protecting your boat’s metal parts, like the propeller, drive shaft and trim tabs.

Best practice is to keep no more than a thin slime layer on your hull. Slime is easiest and cheapest to remove and there’s less disturbance of the anti-fouling coating.

Options for managing fouling

  • Anti-foulings and slippery coatings can assist with limiting fouling growth and build-up.
  • Dry storage reduces the chance of fouling.
  • Berthing in a marina with high freshwater content can slow the speed of fouling growth.
  • Haul-out at least once a year.
  • Talk with a professional for advice – which may be a specialist contractor or paint shop, or even your boatyard operator.
  • And remember: using a boat frequently is one of the best ways to keep fouling growth down and reduce maintenance costs.

About anti-foul protection

Anti-foul is usually a special kind of paint that repels organisms. The type of antifoul you choose will depend on your boat, its location and how you use it.

Once you have selected a product, ensure it is applied precisely to the manufacturer’s instructions. Paints generally last one to two years.

Know the rules

Many regions and marinas have rules about fouling.

manage biofouling 2

Haul-out for cleaning

Haul out the boat any time there is more than a light slime layer – and do it at least once a year. If you plan to do a summer trip every year, it makes sense to do this in spring; or if you’ll travel in winter, get the boat hauled out in autumn.

Use an approved haulout facility to ensure that all by-products from the cleaning process are properly captured and treated.

Plan ahead for your haulout and book ahead if necessary. In some centres, there is pressure on availability. Some operations offer great deals to encourage you to get your work done through winter.

A good haulout operator will be able to connect you with everything and everyone you need to keep your boat maintained and free of fouling.

Ask your haulout operator to ensure that they clean beneath support blocks and under keel bulbs because these areas can sometimes be missed.

Find your closest haulout operator

Do-it-yourself cleaning out of water

The cleaning steps:

  • Remove any large fouling matter by hand and dispose of it in a bin going to landfill (not back in the sea).
  • Water-blast, hose-and-brush or scrape to remove the rest of the fouling.
  • Pay special attention to niche areas which are parts of the boat that protrude or are difficult to access or that hold water. These include:
    • keel underside and rubbing strips
    • water intakes and outlets
    • propellers, thrusters, shafts, sail drives and trim tabs
    • anode mounts and recesses
    • rudders, rudder stocks and casings.

Cleaning at tidal grids is strongly discouraged, because in reality all the material taken off the boat will go into the sea as soon as the tide comes back up. Regional councils could have rules on this too.

In-water cleaning

If you clean in the water, first check the regional (and if relevant the marina) rules.

Best practice is to use only a soft cloth to remove a slime layer, to avoid damaging the anti-foul coating.

Make sure to clean before you leave your mooring or berth so marine species don’t hitchhike out with you.


Top of the page photo credit: Jamie Troughton/Dscribe Media Services.