When the bottom of your boat starts growing a sea food salad, the options are simple, pay for an expensive haul-out and use a power washer to clean it off, or take the plunge and clean it in the water – which is what I usually do.

The type of growth you get on the bottom of your boat depends on the water you’re in; its temperature, and the nutrients and the type of sea life it supports.

After three months in a bay where boats weren’t using holding tanks and a nearby distillery was discharging molasses into sea, my boat hull had a beard longer than Neptune’s.

A round file, a wire brush and an old bicycle spoke work wonders on a saildrive when in-water hull cleaning. Credit: Andy Pag

I then moved the boat to the crystal-clear waters of a neighbouring island, and within 24 hours was amazed to find the fish had picked the hull completely clean.

It’s not just a question of moving from dirty to clean water, but any dramatic change in the water environment will challenge the growth that’s built up.

Hull cleaning: Scrub or scrape?

Despite imagining that the Coppercoat antifouling I applied would mean my hull would effortlessly glisten for years, I have to get in the water every four to eight weeks to give it a scrub or a wipe.

The frequency depends on the water.

I rarely get hard growth, it’s usually soft algae and it comes away easily when I scrape it.

After experimenting with an expired credit card and washing up sponge, I found the best way of removing hairy growth is with a plastic scraper.

I use a couple of 6in plastic putty scrapers.

Compared to metal ones, they do wear away quickly, though, which is why I use two for each clean.

Plastic scrapers and a scouring pad are your first weapons of choice when in-water hull cleaning. Credit: Andy Pag

I then have to trim them straight ready for next time or the middle of the scraper no longer makes contact with the hull.

I’m too nervous to use metal scrapers on my hard antifoul as I worry it will wear the surface away.

I like to be as light as I can be, because every scrub releases toxic copper into the water.

That’s bad for the sea life, and bad for the longevity of my boat’s protection.

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If using metal scrapers, round off the corners with a file or grinder as a sharp edge can scratch the hull.

It takes about 90 minutes to remove the beardy growth on my 40ft catamaran, and after that I take a break for a cup of tea and biscuits before round two.

This gives me a bit more energy, and gives the krill a chance to settle back on the hull if they’re going to.

Then I dive again and give it all firm wipe with a scouring pad.

While the scraper gets rid of the ‘forest’, the scouring pad removes the ‘topsoil’, a very thin layer of slippery green algae which encourages the next round of growth.

It comes away in a cloud of dust and with it I wipe away any tiny shrimp that have swum back to the hull.

This second round is extra work, but it’s worth the effort as it helps to significantly delay the next time I have to give the hulls a clean.

It really helps to clean the boat in a slight current, even though it’s physically harder work, because the stuff you’ve scraped off doesn’t get the chance to drift back onto the hull.

Start at the bow and work downstream.

Fish also help by eating the algae as it comes off.

When in-water hull cleaning, make sure you dress for protection. Credit: Andy Pag

I use a scouring pad on the saildrives and props which have traditional antifoul on them.

The anodes get a firm brushing with a stiff steel (not copper) wire brush. I sometimes get hard growth on the anodes.

I use the back of the brush like a hammer to smash the shells up and then they’ll brush away easily.

I have an old bicycle spoke and a round rats-tail file that I use to clean out the water channels in the saildrives, and I use the file inside the thru-hulls too.

Accessories to make staying underwater easier. Credit: Andy Pag

I always wear a wetsuit and swim cap. I used to use a rash vest and shorts, but the little beasties living in the growth loved setting up a second home in my hairy legs or belly button as soon as they’d been displaced.

I also wear work gloves as any hard growth can deliver a pretty nasty infection if you get a cut.

The whole job is a real workout but I’ve found a few ways to make it a bit easier.

I have a bathroom handrail with suckers that I use to steady myself against the hull. I found a sink plunger also works.

This saves energy positioning my body, although when I release the catches it can take a bit of effort to detach it.

Fins also make the work easier, especially when applying force against the hull while scrubbing.

A couple of kilogram weights on a weight belt reduce the effort needed to stay in position as I’m not constantly fighting the body’s natural buoyancy to float up.

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