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

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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.”