BOATS AND THEIR BITS SHIFTING WEIGHT AND FREE SURFACE EFFECT


Unlike the crates used in our crude demonstration, boats heel progressively and don’t achieve their maximum stability − that’s to say their maximum resistance to capsize − until heeled several degrees. The angle of maximum stability varies from design to design: as little as 10° for a catamaran, perhaps 20°−30° for a single hulled power boat and considerably more for a ballasted sailing yacht. Power boats rarely push their static stability to the limit.


1


G


G


Fig 1:12


B


Fig 1:13


Let’s imagine a boat at rest and floating level (Fig 1:12). The combined weight of the boat and everything it contains can be thought of as pressing downwards through its Centre of Gravity (G). This is resisted by the boat’s buoyancy which pushes upwards through what’s known as the Centre of Buoyancy (B). The skipper picks up his rod and moves to the gunwale to fish. His weight shifts the C of G of the whole, and the boat heels over (Fig 1:13). Note how the Centre of Buoyancy has also moved in the direction of heel so that G again exactly opposes B. The situation is once more in a state of equilibrium and will remain that way until our man changes his position. Of course, if he was to rush repeatedly from one gunwale to the other, the boat would rock alarmingly, possibly to the point where water sloshes in and partially fills the boat (Fig 1:14). The shipped water can have disastrous effects on stability, producing an extremely dangerous phenomenon known as ‘free surface effect’ − the very same that caused the death of 197 people when the Herald of Free Enterprise capsized off Zeebrugge in 1987. Once on board, the water swills from side to side, causing the boat to roll ever more wildly. Unless removed very quickly, more will be scooped up and the boat will flip or founder. To be avoided at all costs!


Fig 1:14 RYA Seamanship for Sea Anglers 11


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