Resources · The problem
Yachts roll because waves push sideways against the hull, and every yacht has a natural roll period at which that motion builds easily. Roll damping is the reduction of this rolling motion, either passively through hull features or actively through stabilizers that push back against the roll.
Roll is not a flaw in your yacht. It is physics doing exactly what physics does, and once you see what sets it off and what makes it build, everything about it (including why it's worst at anchor, and why some days are fine and others miserable) makes sense.
Waves are moving walls of pressure. A wave meeting the bow gets split; a wave striking the yacht from the side, in a beam sea, generates a rolling moment that rotates the yacht about its longitudinal axis. That rotation is roll. The bigger the beam sea relative to the hull, the bigger the push.
Every yacht has its own natural roll period: the rate at which it naturally rolls from side to side when disturbed. This is determined by factors such as beam, displacement and weight distribution. On its own, this natural motion is usually harmless.
Problems arise when the rhythm of the waves starts to match the natural roll period of the hull.
When waves arrive at intervals close to the yacht's natural roll period, each wave reinforces the rolling motion. The effect is similar to pushing a swing at just the right moment: the motion grows with each cycle. As a result, relatively small waves can produce significant roll angles when their period matches the hull's natural rhythm, while larger waves may have less effect when they arrive at different intervals.
This is why seemingly moderate sea conditions can sometimes produce uncomfortable motions. The effect is often most noticeable at anchor, where the yacht cannot alter course or speed to change its response to the swell.
Sailors switching to motor yachts notice this immediately, and Ton Schoolderman, fifty years of ocean sailing behind him, said it precisely:
"The sail support and the balance of keel and mast simply gave much more stability, less rolling, than the motor yacht."
Ton Schoolderman, MY Beau, Pilot 44
A sailing yacht carries built-in roll damping: a deep ballasted keel below and pressed sails above, both resisting the rhythm. A motor yacht has neither. What it can have instead is deliberate, engineered damping.
Roll damping is anything that takes energy out of the rolling motion. Passive damping (bilge keels, a hull's own friction) resists a little, always, indiscriminately. Active damping measures the roll as it begins and pushes back in time with it, breaking the resonance instead of merely muffling it.
How the pushing works, type by type, is its own guide: every stabilizer type, explained. The approach built around low-speed and at-rest strength is the solution built on spin, and for the anchor problem specifically there is damping without forward speed.
Roll damping is the reduction of a vessel's rolling motion. Passive damping uses fixed hull features like bilge keels; active damping uses stabilizers that sense the roll and generate a counterforce in real time.
At anchor a yacht cannot choose its heading relative to the swell and has no speed to steady it, so the hull takes the waves' rhythm broadside. If that rhythm is near the yacht's natural roll period, the motion builds through resonance.
Generally yes. A sailing yacht's ballasted keel and pressed sails both damp roll continuously. A motor yacht lacks both.
Physics explains the roll. It doesn't oblige you to live with it. Tell us your hull, displacement and cruising grounds, and a RotorSwing engineer will advise the stabilization solution best suited to your yacht.