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 boats sailboats as well as motor boats.

How to calculate the displacement/length (D/L) ratio?

Displacement Length Ratio formula

Abrev. | Unit | Description |
---|---|---|

Disp. | long ton | Displacement |

Length | ft | Length of the hull. |

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

'Problems' with the formula

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]}

In our calculations, we use the

Common interpretation of the D/L ratio

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

D/L range | category | ||
---|---|---|---|

100 | - | 150 | Ultra light racers |

150 | - | 200 | Light racers |

200 | - | 275 | Light cruisers & offshore racers |

275 | - | 325 | Medium weight cruisers |

325 | - | 400 | Heavy 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 for the average boats

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