A Practical Guide to Boat Stability

The stability of yachts in large breaking waves

The first question that we have to ask ourselves is WHAT CAUSES A YACHT TO CAPSIZE?

Experience shows us that, in flat water, gusts alone cannot capsize a yacht.

Even when encountering high and steep waves, the story remains the same.

The action of wave slope in heeling a dinghy or day sailer may assist the wind in producing a capsize, but a conventional yacht’s stability is such that it cannot be capsized by even the combined action of wind and waves, no matter how high or steep

It is breaking waves that cause capsize;

A) Beam-on to a large wave.

B) Crest begins to break.

C) 90° heel angle (transom visible).

D) Upside down (keel and rudder pointing to the sky).

E) Nearly upright again.

F) Returned to normal!

if the yacht is caught beam-on to breaking waves of sufficient size, then the exaggerated steepness of the breaking wave front, coupled with the impact of the jet-like torrent of the breaking crest, will knock the yacht down to a point where the mast is well immersed.

At this point the yacht’s fate is decided by its stability characteristics; it will either return to an upright position or carry on to an inverted position, where the boat may remain for some time until another wave disturbs it sufficiently to flip itself upright. If the wave is high enough or the encounter with it is timed appropriately, then a full 360° roll will be executed.

So the Next Question How big do breaking waves need to be to cause this type of behavior ?

Unfortunately, the answer is ‘not very big’.

When the breaking wave is 30 per cent of the hull length high, from trough to crest, it could capsize some of the yachts,

while waves to a height of 60 per cent of the hull length would comfortably overwhelm all of the boats.

In real terms this means that for a 10m (33ft) boat, caught in the wrong place, when the breaking wave is 3m (10ft) high, this presents a capsize risk;

and when the breaking wave is 6m (20ft) high, this appears to be a capsize certainty in any shape of boat.

The word breaking is in italic to stress that it is breaking waves that present the danger, while big waves in themselves are not a problem.

So the next question is obvious How can capsize be avoided?

The simple answer to avoid capsizing is to avoid breaking waves. This does not necessarily mean staying tied to a mooring, but rather in avoiding certain sea areas in wind or tide conditions where breaking seas may be thrown up.

Once the boat is to one side of the breaking part of the crest the danger is over, and even delaying the moment of impact until the breaking wave has dissipated some of its energy will reduce the capsize risk.

The wave is at its most destructive at the point of breaking and immediately afterwards. Active sailing also keeps the boat from being caught beam-on to the seas, which is its most vulnerable position.

The risk is that a mistake in steering might cause a broach which results in the boat being left beam-on to the waves.

So far we have identified the general terms about stability, but we cannot go much further without explaining in more detail the physical mechanisms that keep a sailing yacht the right way up, and how things behave once the mast is below the horizontal

The figure shows a typical righting moment curve; this describes the variation of righting moment as heel angle increases. All of the yacht’s weight can be assumed to act vertically down through the center of gravity, while the buoyancy force opposes this through the center of buoyancy.The righting moment is the torque generated by the increasing misalignment of the yacht’s center of gravity, which remains fixed on the hull centerline – unless the crew sit out – and the center of buoyancy which shifts to leeward as the yacht heels. Intuitively one can see that in the normal sailing range of up to an angle of, say, 45°, an adequate righting moment to resist the heeling moment of the sails can be achieved either by a wide beam, so that the center of buoyancy shifts outboard more, or by a low center of gravity, so that it is further away from the center of buoyancy. Some boats are stiff, ie the righting moment rises quickly with increasing heel angle, while others are more tender, ie with a slower rise of righting moment. In the latter case, such boats very often require the crew to sit out to produce an adequate righting moment, by shifting the center of gravity outboard.

At the point of maximum righting moment, the center of buoyancy is as far to leeward as it is going to get. It then starts to move back to the center as heel angle increases further.

This means that at 90° the righting moment can be quite low, hence the ease with which the broached yacht can be held with its mast close to the water by the flogging spinnaker. A few more degrees past this point and the center of gravity and center of buoyancy are in line again, but the wrong way up.

This is the angle of vanishing stability (AVS). At this point the boat is balanced in unstable equilibrium like a pencil on its end and can fall either way, ie back to upright or to end up floating upside down. At 180° heel – ie upside down – the two centers are in line, and this is another stable position unless the boat is fully self-righting. Before the hydrostatic forces can act to turn the boat the right way up some external force must push the boat back past its angle of vanishing stability.

We see in the picture that there is a upper curve and a lower curve the lower curve indicates that the boat is upside down and the size if wave that will be needed to make here turn upside again.

To calculate the righting moment curve for a yacht, the position of the center of buoyancy is calculated from the hull drawings, using a computer. The position of the center of gravity is determined by physically inclining the yacht a few degrees and measuring the heeling moment required. The ability of a yacht to recover from a breaking wave encounter depends on the hull and coachroof shape. This is not only for its influence on how the breaking wave affects it, but also for its influence on the shape of the stability curve.

The continuation of this post is on the Full boat stability article