Know your boat by numbers

Øyvind Bordal explains how form stability and polar diagrams can help you choose the right boat for your kind of sailing

We’re all familiar with the questions that arise when looking for a new boat. Is it the right size and does it need work? How does it sail and is it within budget? Next you might look at the specifications – LOA, waterline, beam and draught – and maybe drill down a little deeper on the engine, for example.

The specifications give you a good framework, but more interesting is the information that’s hidden. Or, at least, appears hidden to the everyday boat owner. Aside from a test sail, much of a boat’s performance can be ascertained by looking at some calculations.

Designers use a variety of tools when creating a new boat. Using calculations they can predict things such as stability, seaworthiness and speed potential under a range of conditions. Keel percentage, form stability, VPP and STIX–- these terms may sound off-putting, but get to know them and you’ll better understand your boat

Kasper Wedersøe is a partner and designer at CDE Danish Marine Design. He uses calculations frequently to predict how a boat will behave in the water.

“There are quite a few models that are useful, each in different ways,” he says. “But it’s important to remember that these dimensions are capable of predicting potential – not necessarily the actual performance.”

In other words, the numbers we get from dealers can be a bit optimistic. They’re based on ideal conditions, so not always representative, but can still be useful for comparing boats. Here, Kasper explain some of the more noteworthy terms.

Waterline length