Eli5: How do water lines on a boat work?


I’ve been watching a lot of boat building videos on YouTube but I never got the concept of water lines.

I get that you can draw a horizontal line across the hull but how do you determine that it will be the waterline? Is it just a rough estimate based on Archimedes or is there some other witch craft going on?

In: Engineering

On the plans it is a rough estimate based on Archimedes principle. You take the weight of the boat and the displacement bellow the waterline and these should match up. But once you have the boat on the water you can adjust this by adding or removing ballast. All boats have ballast at their keel which they use to adjust the waterline to where it should be. So even if you do not build the boat exactly as planned and the waterline is a bit off you can still compensate.

Anytime I put new bottom paint on a boat thus creating a water line I’ll have the boat in the water to determine its natural line. The boat will always sit at a certain point at rest due to buoyancy. Then when I remove the boat I just trace that around the boat as even as I can and go from there. Usually if you leave a boat in for a week it will give you a good “scum” line around the boat to follow. Hope that helps.

As a former boat builder, I can tell you that the determination of the waterline before the boat is first floated is a mixture of about 20% calculation and 80% instinct. I got it right about 90% of the time

Archimedes is exactly right – you know the weight of the boat and the weight of the proposed cargo, you know the density of water, and with a little maths you can figure out where the water line will be under different conditions (different cargos, etc).

All of this should have been covered under the design process – making sure your boat is capable of carrying the intended loads, making sure it is stable and floats correctly and so on.

A lot of boats will have markers on the side displaying this – by comparing the water level against the scale you have a quick indicator of how heavily it is loaded by how high in the water it is sitting.

When it comes to paint, the big difference is that different paint types are used depending on whether an area will be submerged in saltwater or not – under the water line more expensive paint is needed to be more resistant to corrosion, and also prevent things fouling the surface of the boat like barnacles. There is no point in painting the top of the superstructure in expensive paint designed to stop sealife hutching a lift when you could save a lot of money with cheaper paint, so they paint differently above and below the water line.