The Ballast to Displacement Ratio Explained (with Formula)

Written by Shawn Buckles in Sailboat Keels Explained

Ballast to Displacement Ratio Explained

The ballast to displacement ratio is one of the ratios sailboat enthusiasts use to quickly compare sailboats. It is simply the percentage of ballast against the displacement of a boat.

What is a good ballast ratio?

Sailboats with a ballast ratio of over .40 are generally stiffer, more stable, and have better handling in rougher conditions. Sailboats with a ratio below .40 will typically be less stable, have a higher heel angle, and be more prone to rolling. However, the ballast ratio doesn't account for other important factors, such as righting moment.

Ballast to displacement ratio formula

The formula for this ratio is:

ballast displacement ratio = ballast / displacement

In other words, it tells you what percentage of the sailboat's weight is in the ballast.

The average value is between 0.35 and 0.45 for cruising boats. This means that on average, 35-45% of the weight of a cruising boat will be carried in the keel ballast.

It is a simple indicator that can give you a rough idea of how stiff and stable the boat will be. However, there are limitations to the use of this number, since the ballast weight doesn't tell the whole story. For example, it doesn't tell you how much righting moment the keel design provides, or how much wetted surface there is - while both of these factors greatly affect handling and stability.

To get a more complete picture of the boat's handling, one should also look at the capsize screening ratio (as explained by Ted Brewer) and the comfort ratio.

How Do Sailboat Keels Work?

In short, a sailboat keel's weight is used to increase displacement and lower the center of gravity, while its surface is used to increase wetted surface. This helps to stabilize the boat, but also provides directional stability. For a more detailed explanation on how sailboat keels work, I recommend reading this article.

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