D/L (Displacement Length Ratio)

The D/L ratio is the ratio between the displacement and the length of a boat. The calculation is very popular because its simplicity and the fact that the values of the displacement and the length are easily available. The article relates the D/L Ratio to thousands of boats sailboats as well as motor boats.

1 long ton = 1 displacement ton = 35 cubic ft of sea water = 2240 lb = 1.01605 metric ton.

First we must decide what we mean with the "length" and the "displacement" of a boat. Unfortunately, neither the term length nor displacement is proper defines. We have described these issues and how we have decided to mitigate them here: Issues with 'length' and Issues with 'Displacement'.
In our calculations, we use the m_EC or equivalent mass condition as displacement, and L_H (length of hull) as the length. Both m_EC and L_H are defined in ISO 8666^[1]

Ranges for D/L have been slowly re‐defined over the years, tending toward lighter displacements within each range.

Heavier displacement requires more sail area, and is more comfortable.
Heavier displacement combined with smaller water plane area results in lower accelerations.

Lighter displacement requires less sail area, and is less comfortable.
Lighter displacement combined with larger water plane area results in higher accelerations.

This figure shows the scattering of the D/L ratio for thousands of sailboat designs.

This figure shows the mean value of the D/L ratio as function of the length of the hull. According to the general interpretation, this means that the average designs less than 7 meters and above 17 meters are "light racers", while the average designs between 7 meters and 17 meters are "Light cruisers & offshore racers".

The D/L ratio is not well defined and you will see different values for the same boat. Therefore, the effect of different D/L ratio is vague. The best interpretation is the one above "ultra light", "light", "medium weight", etc. Not as useful as many other ratios.

[Ref 1]: ISO 8666:2020