Displacement theory

What does your boat's measurements say about the way it handles? It's not as straightforward as you'd think, as Ali Wood discovers

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When I returned to marine journalism after a spell working on other magazines, I was surprised to hear it suggested that heavy displacement yachts were a thing of the past.

“But what if you hit heavy weather?” I asked.

“You don't, if you've got the right routeing apps.”

And so began my sobering education on all things that had, or were soon to become, redundant, from SSB radio to paper charts and radar. I'm glad to say that a decade later, all of these things are still very much in use, and I did feel at least a little vindicated when ARC skipper David Diaz told me unequivocally, that no boat can outrun a squall, not even his lightweight performance catamaran.

However, talking to David after his 3,000-mile voyage reinforced my understanding that seaworthiness and safety rely not just on boat design and/or the right tech but also the skill and preparedness of the crew. Knowing how your boat reacts in different sea and wind conditions is part of this, and your boat's displacement-when combined with other factors-can tell you a lot before you've even left the dock.

It's true that modern production boats are built with much lighter materials, and generally have a lower displacement for their length than in the past, but of course, there are still medium and heavy displacement boats on the market, both new and secondhand, which might be perfect for your type of sailing.

What's meant by displacement?

A displacement hull (as opposed to a planing hull, which skims the surface) pushes water aside as it moves: the heavier the displacement, the more water is pushed aside. Displacement boats are generally slower, sturdier and more fuel-efficient (up to a point).

Do you choose a heavy boat that holds its line through big seas and strong winds, or do you opt for a light, quick one that skips and bounces across the waves?