Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec
Increasing safety
(vi) the presence of a mast decreases the time to recover from inversion considerably – it is thought because it helps to 'trip' the boat back again when hit by breaking waves. The re-righting probability is more dependent on the height of mast retained and wave characteristics than vertical centre of gravity.
The last finding suggests that real benefit to re-righting ability could be obtained by strengthening the lower mast, thus improving the likelihood of mast retention after an inversion.
5. RE-RIGHTING CRITERIA
The following criteria have been proposed in respect of minimum angle of vanishing stability.
5.1 UK – WOLFSON / MCA
Figure 3: Critical Wave Height for Capsizing/Re-righting versus Angle of Vanishing Stability (taken from [11])
4.2 AUSTRALIA 4.2 (a) Experimental Work
Since about 1998 considerable further work into this subject has
since been undertaken at the Australian
Maritime College, for example see [12, 13]. Testing took place with a single breaking wave and in steep irregular waves using three different hull forms, each representative of typical 40 foot (12m) offshore sailing yachts. The tests included investigation of the effects of a coachroof, the presence of a mast, and the effect of internal flood water.
4.2 (b) Findings The main findings can be summarised as follows: (i) testing in
a single (ii) the wave height (iii) breaking needed wave
In 1990 the Marine Directorate of the UK Department of Transport published a code of practice for sail training ships under 24m length, the stability requirements for which were based on research conducted by the Wolfson Unit [14]. These requirements are shown graphically in Figure 4, which also shows data relating to stability casualties.
Yachts Casualties Rolled & survived
180 Categories 0 & 1 160 140 generates
assessments comparable to testing in irregular waves, but takes much less time.
to re-right a vessel
decreases with increasing AVS (where this has been derived rigorously).
the value of AVS derived from IMS data is consistently significantly less than that calculated rigorously.
(iv) lowering the VCG results in a smaller wave height being necessary to obtain re-righting.
(v) increasing AVS by up to 10° either by adding a coachroof or by assuming some level of internal flood water does not invariably decrease the time spent inverted. Indeed it can be increased.
100 80 60 40 120
Category 2 Category 3
5
10
152025 Length Overall - metres
Figure 4: UK Minimum AVS for Sail Training Vessels (updated version of that in [14], courtesy of the author)
©2011: The Royal Institution of Naval Architects B-99
30
Range of Stability - degrees
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