Yes, I know exactly how draft reduction works.
It is hard-coded.
The help gives you an overview, but if you want the nitty gritty, I’ll give you the moderately technical explanation:
Your vehicle has a boat hull percentage. This is ratio of vehicle squares with boat hulls to total vehicle squares. It’s roughly a measure of how much of a boat your vehicle is: at 0%, your vehicle is a car, and at 100%, it’s very much a boat.
Your vehicle’s watertight hull height is 0.3m + 1/2 the boat hull percentage in meters - so 0.3m for a car, 0.8 meters for a perfect boat. If your draft is more than your watertight hull height, entering deep water will cause water to flow into your vehicle, sinking it.
Your vehicle effective boat hull area in m^2, which is roughly the half the width in tiles * length in tiles * boat hull percentage (minimum of 0.1). Basically, the more of a boat your vehicle is, the more of the hull that counts for displacing water.
Your vehicle is assumed to have a roughly cubic back half and a tetrahedral front half - this is an approximation of a boat hull form that have the enormous benefit that is it is easy to calculate. Draft in meters is 12 / 7 * mass in metric tons / hull area, which is really just calculating the draft necessary get the volume to displace as much water as the vehicle weighs. Because that’s how Archimedes’ principle works.
So, for a typical converted security van:
5 tiles wide, 8 tiles long gives an area of roughly 20 m^2.
Each boat hull increases the boat hull percentage by 2.5%.
Weight is roughly 5 metric tons.
The first 4 boat hulls you add increase your boat hull percentage to 10% and your watertight hull height to 0.35m. Boat hull area is 2 m^2, and draft is around 4 meters, so it very much sinks.
The next 4 boat hulls you add increase your boat hull percentage to 20% and your watertight hull height to 0.4m. Boat hull area is 4 m^2, and draft is around 2 meters, so it still sinks.
The next 4 boat hulls (total of 12) increase the boat hull percentage to 30%, watertight hull height to 0.45m, boat hull area to 6 m^2, and draft to 1.5m. Still sinking, but you’re already seeing a reduction in the efficiency of each additional hull in the second 4 reduced draft by 2 meters, but the third 4 only reduces it by 0.5 m.
Going to 16 boat hulls puts boat hull percentage at 40%, watertight hull height to 0.5m, boat hull area to 8 m^2, and draft to a little over 1m.
Cutting to the chase, putting 32 boats hulls raises the hull percentage to 80%, the watertight hull height to 0.7m, boat hull area to 16 m^2, and draft to 0.5 m, so the security van will now float.
Hope that helps!
