Stability

The power to carry sail

A boat should survive all the waves and wind that the weather can throw at it. As far as possible, it should get from A to B in it too. Any boat must be stable through the whole range of conditions from good to frightening; that is, if any foreseeable combination of hydrodynamic, aerodynamic, or other forces disturbs its balance, the boat must have a strong enough tendency to return to an even keel. This nautical ’virtue’ is stability.

Stability is a specially significant requirement of sailing boats, as huge areas of canvas hung out on a high mast have a strong tendency to force a boat over when the wind blows.

A 27 ft catamaran sailing to windward in a breeze might easily have a total force on the sails of well over 1000 lbs (nearly a half a ton), with the centre of pressure 4 m above the waterline. Less than 1/3 of this force could be expected to be pushing the boat along, most of it would be trying to tip the boat over on its side.

The power to remain upright under strong heeling forces determines a sailing boat's ability to carry sail safely and efficiently — and consequently the boat's performance. This is called Static Stability. More static stability means bigger sail can be used, and hence faster sailing. More static stability also means safer sailing in strong winds and gusts.

Static stability is a very useful measure of how a boat will behave and perform, but, as it ignores the important effects of the boat and water moving, it only reveals part of a vessel's general seaworthiness. Getting a more complete picture of seaworthiness (which is a measure of a craft's ability to provide safety and comfort in all weather conditions) also requires understanding a boat's Dynamic Stability.

Zero power to carry anything – static instability

Imagine a log, freshly cut from a Canadian forest, and floating down to a logging collection point. The log floats very well and will not sink, but if someone without exceptional skill and balance were to stand on it, the log would probably rapidly roll them into the water before they were able to take advantage of its buoyancy. If someone mounted a mast and sail on a single log, and tried to sail the ‘craft’ it would capsize even quicker.

Although the log plus person has ample buoyancy, it is statically unstable. The Centre of Gravity is above the Centre of Buoyancy. Any small rolling of the log results in out of balance force that causes even more rolling.

Increasing static stability by adding a keel

The log in the above example has little resemblance to a sailing boat, but with a simple modification we can get much closer to a modern displacement yacht. If we suspend a heavy weight on a plate deep beneath the log, we will find that not only could we stand on the log but that it would carry sail too.

When the ‘boat’ heels due to wind pressure on the sails or due to the person moving his or her weight, there is always a force restoring the craft to an even keel. Of course the extra weight will cause the log to float lower in the water – which is a disadvantage if we want to get our log to sail fast.

Increasing static stability by increasing beam

There is another route to getting stability. This time forget the weight; we just lash two logs together. This craft has some static stability because when the vessel heels over, the centre of buoyancy moves and the centre of gravity stays put. As long as the centre of buoyancy moves outside a vertical line from the centre of gravity we will benefit from a stabilising force. Instead of lashing the two logs close together as in the above example, this time we have the brainwave of lashing them to a wide frame. This time we have taken the idea of widening the beam to a whole new level.

Static stability of displacement yachts

A modern displacement yacht uses both wide beam and heavy deep keel to gain static stability and power to carry sail. More beam and more weight and having it deeper down (lower centre of gravity) will increase the speed potential of the craft. There are practical limits though. A heavier or beamier hull will have more drag and eventually it will limit top speed and compromise seaworthiness.

Static stability of multihulls

When a ‘catamaran’ heels, the centre of buoyancy rapidly moves dramatically towards the extremity of the craft. This seemingly gives us unlimited stability (and hence sail power) as long as we are able to keep increasing the separation of the hulls. Of course, as in the case of the monohull, there are practical limits if we are to maintain good seaworthiness. However, with the multihull, we have a lot more potential to exploit stability due to buoyancy alone and still maintain excellent seaworthiness.