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D/L (Displacement Length Ratio)


What is the displacement/length (D/L) 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 sailboats.

How to calculate the D/L ratio?
First we must decide what we mean with the "displacement" of a boat. The ISO 8666[1] has some definitions:
  • displacement: mass of water displaced by the craft including appendages
  • loaded displacement: mass of water displaced by the craft including appendages, when in fully loaded ready-for-use condition.
In the ISO standard has of course described the meaning of "fully loaded ready-for-use condition".
But the real world is different. E.g.The displacement in a brochure is often the weight estimated by the naval architect before the boat is built. Other boats are sold with and without an inboard motor and fuel tank. So,... in reality you will find different (more or less specified) displacements for a given boattype.

We must also decide what we mean with the "length" of a boat. Many formulas refers to loa (length over all), but this term is not well defined either. In the good old days, the loa was just the length of the hull (loh). The loa did not include the pulpit, bathing ladder, and the like. When you look in sales brochures nowadays, the loa includes these exterior items because the boat will appear longer. Since the loa is not well defined we cannot recommend this value. Therefore, we use the length of the hull as specified in ISO 8666[1]
Displacement / Length ratio

Nomenclature
Abrev.UnitDescription
Disp.long tonDisplacement
LengthftLength of the hull.

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

Common interpretation of the D/L ratio

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

D/L Interpretation
D/L rangecategory
100-150Ultra light racers
150-200Light racers
200-275Light cruisers & offshore racers
275-325Medium weight cruisers
325-400Heavy cruisers

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.


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

D/L for the average boats
Displacement / Length ratio per length
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".

Conclusion
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.

References
[Ref 1]: ISO 8666:2020