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Boat Stability

By 'boat stability' we usually mean transverse stability, which is the resistance a boat has towards heeling. If, say we assume, that a yacht heeled by the same gust of wind, we wouldn’t be at all surprised to learn that a very stable boat would heel less than one of lesser stability. In the jargon of the sea we refer to them as being ‘stiff’ and ‘tender’ respectively.

Basics of stabilitySo, how does a hull gain its stability? To understand this it helps to refer to the drawing (right) which shows a sailboat hull heeled to 15° and demonstrates the principles of what's known as 'form stability'.

G’ represents the boat’s Centre of Gravity – the point through which the downward forces due to the boat’s entire structure and contents can be said to act.

B’ is the Centre of Buoyancy – the point through which the upward forces (bestowed by buoyancy) can be said to act.

For all intents and purposes, G’s position is fixed, whatever the angle of heel. (Yes, I know it will change a bit as liquids slosh around in their tanks, cans roll around in their lockers and crew members get restless, but let’s not be too pernickety).

By contrast, B’s position is anything but fixed. The shape of a hull’s immersed volume alters drastically with shifts in heel and trim. And be thankful that it does – because this is where we get our form stability.

Now back to our drawing. The lower case ‘b’ shows B’s position when the hull was upright. At that time G and B were in vertical alignment and the hull was in a state of transverse equilibrium – i.e. zero heel. Then along came that puff of wind and down went one gunwale. What happened next?

Well really, the drawing says it all. Much to our relief, B moves to leeward of G creating a ‘couple’ – basically a lever or 'righting arm', usually known as GZ, whose purpose is to force the boat back upright again. Knowing the length of the righting arm and the weight (mass) of the boat, we can derive the ‘righting moment’ which can be expressed in a number of units: pound/feet, kilogram/metres, or more commonly these days Newton/metres.

Another significant point is ‘M’ – the metacentre. This is at the intersection between the extension of the hull’s centreline and a line drawn vertically upwards from B. The distance between G and M is known as the metacentric height (GM) and – at small angles of heel – is proportional to the boat’s stability. If GM is positive – that’s to say M is above G – the boat is stable. If it's negative the boat will heel until stability is restored – a condition known somewhat charmingly as the ‘angle of loll’.

Righting moment curveBut, back to GZ. It’s common practice for naval architects and designers to plot GZ for every heel angle between upright and capsized. It’s usually called a ‘GZ curve’ or ‘stability curve’. Whatever you call it, the curve is of huge significance.

The area contained under the positive curve above the line indicates the amount of energy available to keep a boat on her feet. More chillingly, the negative area below the line shows the energy required for a boat to right itself should it be unfortunate enough to capsize. The larger this area, the less readily a boat will return to the upright position. A GZ curve for most multihulls, for example, shows them to be just as stable inverted as they are the right way up. This is because they carry no ballast and are entirely reliant on form stability.

The case with multihulls is obvious but it's not always appreciated that many modern keelboat designs are headed that way. Although by no means so extreme, the trend towards beamy, lightweight boats, having relatively low ballast ratios, almost inevitably results in yachts which – from the stability aspect at least – are less forgiving than their predecessors.

Many would say that this is a retrograde step, but I don't agree. As discussed in the article Light Displacement - lighter means faster it's my belief that the benefits of light weight outweigh the penalties and that whether sailing a light or heavy boat, mono or multi, at the end of the day, safety comes down that good old-fashioned quality known as seamanship.


On a related subject...

Water Ballast - the advantages of carrying weight to windward

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