Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec 3.1(b) Experimental Work


A total of nine models were constructed, being of three basic designs each with wider and narrower beam geometrically-similar variants – see Figure 1. One set were of the older, long keel, heavy displacement type, the other two sets being of the modern canoe body with separate fin keel type. One of these latter sets had a higher freeboard than is generally typical of this type of design.


These models were then tested in a towing tank in two ways, in both cases in steep breaking waves generated by focussing wave trains of differing length. The intensity of the breaking crest was obtained by measuring the drag on a tethered sphere, which peaks when a wave breaks. Tests were conducted with the models free-running under radio control, laying a-hull beam-on to the waves, and impelled by a catapult device.


Full details of the experimental set-up and results are given in [3], and an overview of this work is given in chapter 2 of [2]. A discussion of the dynamics of capsize in breaking waves is given in chapter 14 of [4].


3.1(c) Findings


The main findings can be summarised as follows: (i) if the wave is not breaking,


then regardless of steepness there is no danger of capsize (inversion).


(ii) if the model was beam-on to the breaking wave then either it was rolled through 360° or it was knocked down after which it would either return to the upright or float upside down.


(iii) if the model approaches the wave head-on then it might pass straight through the crest, but even if only slightly oblique it was swept beam-on and behaved as (ii) above.


(iv) if the model was stern-on to the breaking wave then it would either surf straight ahead, or it would broach into a beam-on position, behaving as (ii) above.


(v) the narrow models had improved resistance to capsize when beam-on to the seas, inverted less frequently compared to the wider variants, and always self-righted. Their high value of AVS is believed to be the reason.


(vi) down-wave, the wide and intermediate beam


models had a tendency to nose-dive and sometimes pitchpole. The full lateral plane and balanced ends of the long keel design made it less liable to broach and capsize.


(vii) those models that had an AVS of less than about


150° to 160° can be left floating upside down after encountering a breaking wave.


(viii) a 40% change in roll inertia indicated no discernable change in response to the waves (but see 3.2(c)(i) based on more extensive model testing)


(ix) a 20% increase in freeboard did not influence the propensity to capsize, but the higher freeboard version remained inverted on more occasions.


These and subsequent tests have indicated that the risk of inversion by a breaking wave first occurs when the height of the wave exceeds the beam of the vessel. If a wave is large enough relative to the beam, then all vessels may be knocked down. The concern therefore is how the yacht will behave afterwards, ie: whether it will recover promptly or remain inverted for a significant period of time.


Subsequent work has revealed that fitting a roller furling main and headsail instead of a conventional bermudan rig may reduce the AVS by about 30°, thus having a major influence on the ability of the yacht to re-right.


3.2 USYRU / SNAME RESEARCH 3.2 (a) Introduction


In parallel with the work being undertaken in the UK, the USYRU and SNAME agreed to collaborate on a voluntary programme of research aimed at a better understanding of the causes of sailing yacht capsize in waves [5]. The work comprised four elements:


 model testing   


oceanographic & weather studies mathematical analysis


analysis of capsize incident data


This paper only considers those aspects directly related to the stability behaviour of yachts.


3.2(b) Experimental Work


Early testing concentrated on developing a means to generate a repeatable breaking wave, and on testing with a single model to gain experience of the techniques involved. These tests showed a marked similarity to the single piece of full-scale data available: a videotape of an actual capsize.


Several series of tests were subsequently undertaken to identify sensitivity to important variables such as beam, freeboard, VCG, roll moment of inertia, appendages, etc. These tests are reported in [6] and [7].


3.2(c) Findings The main findings can be summarised as follows:


©2011: The Royal Institution of Naval Architects


B-97


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