What is the L/B ratio?

What is the length/beam (L/B) ratio?
The length / beam ratio is a measure for the slimness of a boat. A low length / beam ratio indicates extra space on board, while a high value indicates a more speedy hull design. The calculation is very popular because its simplicity and the fact that the length and the width values are easily available. The article relates the L/B Ratio to 20.000+ sailboats and motorboats.

How to calculate the L/B ratio?
First we must 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. 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]
Length / Beam ratio (L/B)
L/B ratio

LohftThe length of the hull
BmaxftGreatest beam measured
The unit of the length and width can actually be feet, meter, or any other length unit as long as it is the same for both values.

L/B scattering
L/B scattering
This figure shows the scattering of the L/B ratio for more than 20.000 boat designs. The plot to the left shows the scattering for sailboats and the plot to the right shows the scattering for motorboats. The scattering seems to nearly identical for both categories. Both plots shows a clear correlation between the L/B ratio and the length of the boats. Though there are some outlayers with extreme designs.

L/B for the average boats
Length / Beam ratio per length
This figure shows the mean value of the L/B ratio as function of the length of the hull. Boats above the line are considered slim boats while boats below the line are considered spacy boats. The longer above or below the line, the more extreme boat design

One could expect that if a boat manufacturer would double the length, he would also double width. As the figure shows, this is not the case (the line would be horizontal). In fact: the longer the hull => the higher the L/B value, i.e.longer boat designs have relatively slimmer hulls.

Comparing the two graphs, one can see that the motorboats has a lower L/B values than sailboats for small boats. This means that small motorboats are generally more spacy than sailboats. For large boats, the L/B value seems to be the same for both categories, i.e. a 20+ meter sailboat is generally just as wide as a 20+ meter motorboat.

Does an L/B rate of 3 mean anything? No! Without any other information about a boat than the L/B value, you cannot conclude anything about the boat.
Let's compare three boats with the very same L/B ratio of 3.
Boat 1: L/B ratio = 3, length 6 meter (20ft)
Boat 2: L/B ratio = 3, length 9 meter (30ft)
Boat 3: L/B ratio = 3, length 21 meter (70ft)
Boat #1: Looking at the figure, the boat will be far above the line meaning it would be very slim
Boat #2: This boat is just on the line meaning that it has an average designed hull.
Boat #3: This boat will be far below the line and would therefore be extreme spacy.
So,... a boat having L/B ratio = 3 can be either slim, normal, or spacy dependent of the size of the boat.

L/B distribution
L/B distribution
This figure shows how the L/B ratio for 9 meter (30 ft) sailboat designs is distributed. The histogram shows that an L/B ratio of 3 is a fairly common boat design for a 9 meter, as most boat designs are close to this value. There are a few boat designs with L/B ratio far from 3.

Btw., if you wonder about the rightmost sailboat with the L/B ratio of 5.5+ then it is a Controversy 30. A very descriptive name for this boat.

You cannot tell anything about a boat given just the L/B ratio, you need more information. Having additional information, like access to our unique database of several thousand boats, you will be able to tell how the L/B ratio relates to the average boat design and answer questions like:
1) Has a given boat a slimmer or a more spacy design than other boats.
2) Since a slimmer hull moves less water than a spacy boat, you will also be able to evaluate if a given hull design has the potential for higher speed.
3) A more spacy hull indicate more space in the cockpit and/or inside the boat.
4) Has a given boat an extreme design.

[Ref 1]: ISO 8666:2020