CHAPTER 10
This section of the manual will consider the main sources of danger created by open-water heavy weather, and list the steps which may be taken by the crews of both motor boats and sailing craft to mitigate them. Since many factors straddle the divide between the two disciplines, readers whose interests may lie in one area are encouraged to consider the whole section.
Power
Open ocean storm survival for power craft: Most of today’s power yachts are constrained by the range of their fuel tanks. Given this inherent limitation on how far they can steam from a safe haven, they generally have sufficient speed available to outrun truly severe weather so long as forecasts are regularly monitored and sensibly interpreted.
Power craft undertaking prolonged ocean passages are few. Usually they are comparatively large. The super- high-speed variety invariably carry specialised crew who are trained and experienced to cope with their particular circumstances. Skippers of slower vessels, especially those operating solely in the displacement mode, would do well to study the sections of this manual dealing with sailing boat storm survival. The principles of stability, capsize and shunning a beam-on situation are very similar. For all those who aspire only to well-planned shorter passages, matters should never go beyond the essentials of power-boat handling in rough water (as described on pages 88 to 91). However, it cannot be over-stressed that anyone undertaking extended voyages to the limits of the boat’s fuel endurance should have total confidence in the maintenance and reliability of the engines – in particular the fuel system and the cleanliness of the tanks. Seaworthy covers for large windows are an absolute must. So long as the power keeps on coming, the water stays outside the boat and the weather has been anticipated, a safe arrival should be assured.
Sail
By far the greatest hazard for sailing craft in heavy weather offshore is that of being knocked down or even capsized by a high, steep sea. Occasionally, boats are pitch-poled (thrown end-over-end), but here the question of stability hardly arises, since the boat is literally picked up and tossed onto her deck. This is not to say that steps cannot be taken to avoid it, but it is a rare event that comes about
in only the most extreme circumstances. Most knock- downs are the result of being caught beam-on to a sea, and it is this condition that is conventionally considered when assessing capsize potential. An understanding of the essential theory is a great help to a skipper when deciding how best to handle a given situation and a specific boat.
Static Stability
When a yacht is floating upright, her centre of buoyancy (‘B’) is midships, as is her centre of gravity (‘G’). When she heels, her immersed shape alters and B moves outboard relative to the centreline, away from G which, because it is made up of all the immovable weight in the boat, does not shift at all. We will call the vertical position of shifted centre of buoyancy ‘Z’. While this is pushing up under the forces of floatation (see the illustration), G is being pulled downwards by gravity. The effect is that the boat tries to come upright. If she is rolling freely she will do so. If she is under the influence of some outside force, such as her sails, she will reach a point of equilibrium whereby she remains at that angle because the force of the wind equals her capacity to self-right.
The horizontal distance between G and Z (‘GZ’) is a major contributing factor to capsize resistance. As the hull heels progressively, its length increases up to a maximum, decreasing once more as heel angle rises beyond normal sailing angles. At some point, the shifting centre of buoyancy will actually pass ‘under’ the centre of gravity and out the other side. The boat is now in dire peril because the buoyancy is still pushing up while the centre of gravity continues to pull down. These values relate to the global gravitational system rather than to any ideas the boat may have about ‘this side up’, and the result is ‘negative GZ’. The boat is trying to invert and stay there.
HEEL ANGLE Upright stability AVS Inverted stability MANUAL OF SEAMANSHIP | 101
Storm Survival
RIGHTING MOMENT (GZ)
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