Displacement – Length Ratio: D/L

The D/L Ratio is a useful analytical tool to explore how heavy a boat is relative to its waterline length.

The SA/D ratio is another useful tool which we cover in another post.

In Imperial measurements, the Displacement – Length Ratio (D/L) is calculated by dividing a boat’s displacement in long tons (2,240 pounds) by one one-hundredth of the waterline length (in feet) cubed.

The D/L number can be used to compare the relative “heaviness” of boats no matter their size. So a yacht with a 25-foot waterline and one with a 50-foot waterline are equally heavy in relation to their waterline lengths if they have the same D/L ratio.

The lighter a boat is relative to its waterline length, the higher its potential top speed. With less water to move from the bows, drag is reduced. Some ultralight-displacement yachts, or ULDBs, are light enough to plane and a low D/L suggest the boat will more quickly and easily start planing. Hull shape is also important of course, and another factor you should consider.

D/L Ranges

Under 90 – Ultralight
90 to 180 – Light
180 to 270 – Moderate
270 to 360 – Heavy
360 and up – Ultraheavy

Types of Displacement

One of the tricky things you will come up against when comparing yachts, is the displacement figures supplied by the manufacturer. Are you looking at light displacement? Or maximum displacement?

Try to compare apples with apples. Were the sails on? The stuff that is really going to move the dial is fuel, water, sails, tender and so on.

Displacement Types

Light Displacement: The weight of the boat (displaced seawater) without any cargo, fuel, crew, or provisions.
Normal Displacement: The weight of the boat fully loaded with crew, fuel, water, cargo, and provisions.
Loaded Displacement: Similar to normal displacement, but also including the weight of additional equipment, stores, and water.
Maximum Displacement: the maximum weight that the yacht can reach and still safely navigate oceans.