Definition

Bottom loading is the displacement mass divided by the area of the planing surface (usually somewhat less than the total wetted surface).

Since the area of the planing surface constantly varies, it's common to just use B_WL * L_WL as the reference area.

Units: Usually kilograms per square metre, or pounds per square foot. One might think of bottom loading in terms of pressure, but that can lead to confusion- so I prefer to avoid using units of pressure (Pa, or N/m²) to describe bottom loading. Just think of it as another arbitrary ratio that can be used to compare boats.

What it's used for

Planing performance

Bottom loading is a very useful figure when trying to determine how well a hull will plane. Higher bottom loading indicates that more lift must be produced by a given size of planing surface, so the boat will have to run faster or at a steeper trim angle to get on plane. We must confirm, of course, that the hull shape is amenable to planing- straight buttocks, roughly prismatic afterbody, etc.- before this ratio will be of much use.

In general, lower is better, within reason. A boat whose bottom loading is much less than her competitors' will be quicker to pop onto plane and better at low to moderate speeds, but may have a bouncy ride in rough weather.

Less than 100 kg/m² (20 lb/ft²) - This implies a very light hull that should pop up on plane quickly at low speeds.

About 200 kg/m² (40 lb/ft²)- For boats in the 4 to 7 metre range, this is the point where getting up on plane needs a bit of effort, and there may be a range of "no-go" speeds where the boat plows around with its bow in the air. Once up, planing can be easily sustained.

About 300 kg/m² (60 lb/ft²)- For a boat in the 4 to 7 metre range, overloading is indicated, and the boat probably won't plane very well. Given enough power, it may run OK at high speed, but will be a dog from 8 to 20 knots or so. In boats from 10 to 15 m L_OA, bottom loadings in this range are typical and indicate reasonable performance.

About 400 kg/m² (80 lb/ft²)- Bottom loadings in this range on a 5 to 10 m boat indicate almost no ability to plane. On a 10 to 15 m boat, this bottom loading would indicate slight overloading and poor low-to-moderate speed performance, and on a 20 m boat would imply typical, but unremarkable, planing performance.