Boat stability

A boat's stability exerts a powerful influence on its sail carrying ability and seaworthiness. A stiff ship or tender? It helps to understand the forces at work.

The stability of a boat is its ability to return to an upright position after being subjected to external forces. For sailboats, the principle heeling force is obviously the wind in the sails but wave action and crew position also have their effects.

Although the whole subject might seem complicated, the basics are really very simple. Firstly, at low angles of heel a boat with a wide beam will be more stable than a narrower one. Look at the two crates shown below as 1 and 2. Assuming they weigh the same, it’s easy to appreciate why 2 – the wider one – is the more stable. Now let’s compare 3 with 4. They are both of the same size and in the same position but 4 weighs twice as much as 3 and would obviously be the most difficult to turn over – i.e. the most stable.

So weight is as important as width – basically, the more you have the better. Is that the end of it? Not quite. Now look at 5 and 6. The crates are the same size and weight but their contents are concentrated at different heights. No prizes for guessing that the one with its contents at floor level (6) runs less risk of being knocked over than top-heavy 5. That’s simply because 6’s Centre of Gravity is lower.
Of course, boats float in a fluid rather than stand on solid surfaces but the principles are similar. We can summarise it like this: beamy boats with lots of weight carried low will always be more stable than lighter, skinnier ones with weight carried high.

Now let’s take a look at the dynamics afloat. The sectional drawing below to the left shows a hull floating upright. The Centre of Gravity (G) is a fixed point, regardless of heel, whose mass is acting directly downwards. We also have what’s known as the Centre of Buoyancy (B) which can be thought of as the geometric centre of the immersed portion of the hull. It acts directly upwards, supporting the boat.

The hull to the right is heeled to 30°. G is fixed so stays where it is but B moves outboard as the inclined hull shape changes. Now with G acting downwards and B acting upwards we have a ‘righting arm’ that resists the heeling and works to bring the boat upright again. In boat design terms this righting arm is known as GZ.

The length of GZ (and the righting effects it brings) varies with angles of heel and can be plotted on a graph commonly known as a ‘GZ Curve’ (see below). The curve shown here is for a typical modern sailboat, but can vary greatly with individual design specifics. Starting at zero with the boat upright, GZ peaks at about 60° (for our specimen boat) before decreasing as the boat rolls further. At about 140° of heel the righting effects of GZ become zero once more – a point termed the Angle of Vanishing Stability (AVS) and not something any of us want to experience because your sailboat will then capsize.

The area enclosed by the curve above the zero GZ line represents the amount of positive stability you have, while that below the line shows the inverted stability. The narrower designs of yesteryear had very little inverted stability and would spring back to their feet relatively quickly. The same can’t be said of some of today’s designs which, in their quest to maximise accommodation space below, are worryingly stable upside down.

There’s a powerful message here for all sailors – especially those sailing offshore where rescue may not be close to hand. Every extra ounce you put aloft will increase your inverted stability by raising the overall centre of gravity. So watch out for such things as mast-mounted radar scanners, radar reflectors, roller reefing (particularly retro-fitted behind-the-mast) gears and the like. As described in another article, stability increases with boat size by the power of four, which means that smaller boats are especially vulnerable. If you remember that a 60 footer will be somewhere around sixteen (2x2x2x2=16) times more stable than a 30 footer (explained in the Scaling Factors article) you will understand that what might be practical on the larger vessel could seriously destabilise a smaller one.


Sea Books