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
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.
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