Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec (i) a high roll moment of inertia reduces the
vulnerability to capsize. This parameter is strongly influenced by the mast(s) – see Figure 2.
(ii) increasing beam strongly increases vulnerability to capsize.
(iii) once inverted, some models remained so for an appreciable time. An inverted yacht with an AVS of 140° or more is unlikely to remain inverted for long. As AVS is reduced below this figure, so the time spent inverted is progressively increased.
(iv) variation in keel profile shape has little effect.
small drogues performed better than a single large one.
The work on drogues has since been extended in the UK, see [10].
4. 4.1
SUBSEQUENT INVESTIGATIONS JAPAN
4.1 (a) Incident
In December 1991 during the Japan – Guam Race, the yacht 'Taka' (AVS = 114°) capsized and remained inverted for over an hour, only re-righting after very substantial amounts of water had flooded into the boat. Six of the seven man crew died. An account is given in chapter 3 of [2]. An investigation was begun by the Nippon Ocean Racing Club, which included 1/10th scale model tests commissioned by the Japan Craft Inspection Organisation [11]. Three model types were tested: an IOR racer, an offshore cruiser and a traditional long-keel type.
4.1 (b) Findings The main findings can be summarised as follows: (i) a breaking wave is necessary to cause capsize.
Figure 2: Effect of Roll Moment of Inertia (taken from [6])
3.3 DONALD JORDAN'S RESEARCH
Immediately post-Fastnet '79, retired Connecticut, USA, consulting engineer Donald Jordan, undertook his own amateur model
testing with modest
facilities. His insightful findings, published in [8, 9], broadly corroborate those described above, concluding that:
(i) it is not the height of a breaking wave that causes a boat to capsize when hit, but the mass and velocity of the wave crest.
(ii) larger boats are less vulnerable, in part because of their greater mass and roll moment of inertia.
(iii) the presence of a mast, because of its effect on the moment of inertia, significantly increases the resistance to capsize.
(iv) reducing the keel profile area reduces the
vulnerability to capsize (but see 3.2(c)(iv) above). (v) greater freeboard increases capsize vulnerability.
(vi) an appropriately-designed drogue or sea anchor streamed from the stern greatly reduces the probability of knockdown and inversion. A series of
B-98 resources and
(ii) with AVS of 130° or more, the boats were not capsized with the biggest wave used (40%L).
(iii) with AVS of 130° the wave necessary to re-right the boat needs to be about half the height required to cause capsize.
(iv) with AVS of 100° the wave necessary to re-right the boat needs to be the same height as that required to cause capsize.
(v) the IOR racer was capsized by smaller waves than the older type boats. The IOR type is prone to capsize and reluctant to re-right. The opposite is the long keel boat that has a modest initial stability but is not capsized easily and re-rights easily.
(vi) AVS is a good parameter from which to judge the re-righting ability of a boat.
(vii) without a mast the models were much more prone to capsizing.
The ease of self-righting clearly increases when the wave height to re-right is very much less than that required to cause an inversion – see Figure 3.
Other work conducted about this time showed that trim by the bow improves the re-righting ability.
©2011: The Royal Institution of Naval Architect
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