Most often it is red, but actually nowadays you can get almost any color you like.

The reason for it goes back to the earliest days of sailing ships.

Back in those days, wooden sailing ships would slowly plot around the world.

A combination of their slow speed and rough hull made them an ideal breeding ground for underwater growth.

Just take a look under a pier, you will see the sort of growth these ships used to suffer, which would be barnacles, worms, seaweed, and things like that.

So what's the issue?

Well, all of these things have negative impacts on ships over time.

You get the obvious of things like damage to the hull itself due to worms and the actual growth,

then you get issues like the additional weight that they have to carry around, and reduction in maximum speed due to the extra drag.

Of course on sailing vessels, that drag-on weight would impact their ability to sail upwind, which would yet further reduce their efficiency.

What you need is a way to stop marine life from growing on the bottom of the hull, and this is where antifouling comes in.

Antifouling, fairly obviously, is just a system designed to reduce fouling by animal and plant life on the underwater sections of a boat or a ship.

Early solutions were to place copper sheets on the hulls of ships.

The Cutty Sark is a great example of this, and I'll link to the Greenwich Maritime Museum below if you want to see more about that.

The primary purpose of the copper sheets was actually to stop worms eating their way through old wooden hulls.

A secondary benefit though, is that the copper would reduce the growth of plant life.

Of course, as wooden hulls were replaced by iron, worm issues did reduce, but they've never been eliminated.

Just look at the leisure industry today, and you'll still see plenty of wooden hulls around.

And of course regardless of its construction material, we still have the same old issue of drag caused by the growth of plant life.

It's probably more important now to keep that under control, what were the cost of fuel and efficiency savings on long passage.

We still need antifouling to stop a combination of worms barnacles and weed from growing on the underside of hulls.

But instead of using the old technique of copper sheets, we now use a form of paint instead.

That antifouling paint works on the same principle and actually still uses copper as a biocide, though is mostly cuprous oxide now mixed in with the paint rather than copper sheets.

It's the natural red color of those copper oxides that's led to the traditional red color of antifouling.

Modern antifouling systems can be broken down into two broad categories: hard and soft.

Soft coatings are designed to wear off over time, continuously exposing fresh biocides as the outer layer of the paint wears off.

Hard coatings on the other hand are designed to be a lot more durable. They're meant to last a lot longer.

As the biocides are released, the durable layer of paint remains, but of course the biocides contained in the outermost layer do get used up.

Both systems work on the same principle: They gradually release biocides, commonly based on the chemical element copper.

The difference is that soft coatings slowly allow the paint to flake off as well.

As you can imagine there are environmental considerations to think of.

No matter what way you look at it, antifouling releases biocides, and possibly paint, into the environment.

That is one reason a lot of ports don't allow cleaning of hulls. They don't want the extra dose of biocides and paint released by the scrubbing process.

So what are your other options?

The cleanest one is to simply use normal hardware and paint on the other side of the hull, but that will result in a lot of aquatic growth.

That's fine on a small boat that you can pull out of the water and clean quite often, but is not so great on a container ship running around the world.

What would happen if, for example, a container ship picked up some seaweed in Asia and carried it into the Baltic Sea, where it takes hold and overtakes some of the native species?

Similar things have happened, and do actually continue to happen.

Though it's not so much from hull growth because of antifouling. It's more of an issue for a ballast water, but that's a topic for another video.

So aside from just using no antifouling, what could you do?

There is talk of systems that slowly ooze some sort of jelly from the hull.

The theory is that as the growth attaches to the hull, the oozing jelly seeps off and takes the growth away with it.

I've never seen that in use, but if anyone has, let me know in the comments below, because it'd be fascinating to look into.

Otherwise, there are some silicon based paints that make it hard for barnacles and things to stick to the hull itself.

Unfortunately, these don't actually stop the growth, but it makes it easier to clean off.

Said that, most ports don't allow cleaning anyway, not only because of the historical antifouling issues, but also they don't want to clean off species that are not native to the harbor itself.

The last thing they want is to be overcome by some sort of invasive weed from the other side of the world.

Anyway, hopefully you've enjoyed today's video, and have liked learning about the paint on the bottom of the hull.

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Until next time, thank you for watching, and good bye.