Annex 3A to the 1979 FASTNET RACE ENQUIRY

UNIVERSITY OF SOUTHAMPTON.

HIGHFIELD

SOUTHANIPTON.

ENGLAND

S09 5NH

TELEPHONE (0700) 555995

WOLFSON UNIT

for MARINIE TECHNOLOGY

AND INDUSTRIAL DYNAMICS

November 1979

Report No.431

ROYAL YACHTING ASSOCIATION

Stability Conditions on

Contessa 32 and 1976 Half Tonner

INTRODUCTION

The following report describes an investigation into the statical stability of a Contessa 32 and a Half Tonner designed in 1976. (The designer fee/s that the Ha/f Tonner is representative of yachts of her size and type designed at that time).

A programme of work was set out in a proposal issued by the Wolfson Unit on 18.10.79 and was agreed by Cdr. W. Anderson, coordinator of the Fastnet Race Inquiry, in his letter of 26.10.79.

THE YACHTS CONCERNED

The yachts selected for the investigation were a Half Tonner, and a Contessa 32.

Both yachts took part in the 1979 Fastnet Race.

PREPARATION OF HYDROSTATIC AND STATICAL STABILITY DATA

Lines plans of the two yachts, together with drawings of their deck, coachroof and cockpit arrangements were supplied by their respective designer and builders. Suitable data were lifted from these drawings adequately to define the vessels for the Department of Trade approved computer programs used to carry out the calculations.

Hydrostatic calculations were performed to obtain values for Displacement, LCB, VCB and BM for each yacht floating at its measured waterline.

A value for the righting moment at one degree of heel was supplied on the Rating certificate in each case, and with this a value of GM was calculated using the equation:

RIGHTING MOMENT= DISPLACEMENT x GM Sin X

A value for the centre of gravity height was then yielded by the equation:

VCG=BM+VCB-GM

A summary of the results of these calculations is presented in the table below:

Free trimming (GZ) curves were then calculated for the yachts, for both intact and flooded conditions. The intact GZ curves are compared in the diagrams here. The GZ curves for the yachts experiencing two stages of flooding are compared in the diagrams here(HT) and here(CO), together with their intact curve.

DISCUSSION OF RESULTS

Examination of the GZ curves for the yachts in their intact state reveals the following main points:

1. The initial stability of the yachts is similar, i.e. the slopes of their GZ curves at zero heel angle are similar. In fact the Contess a 32 is initially slightly more stable with a GM of 3.1ft compared to the Half Tonner's GM of 2.78h.

2. The Contessa 32has a greater maximum GZ value. This is largely due to the Contessa's low centre of gravity location and large coachroof. The latter is the cause of the hump in the GZ curve which appears after 70° heel.

3. The Contessa 32has a greater range of positive stability. The point of vanishing stability occurs at 156° compared with 117° for the Half Tonner. When a vessel heels past its point of vanishing stability it will become stable in the inverted position. Its stability, whilst upside down, will depend upon the slope of the GZ curve at 180° . The Contessa 32 would be less likely to remain upside down after a capsize since the slope of its GZ curve at 180° is low and it need only be rolled through 24° in order to regain upright stability.

4. The energy absorbed by a yacht from a sudden gust of wind is represented by the area under its GZ curve multiplied by its displacement. The Contessa 32, with a greater displacement, and a greater area under its GZ curve at any given angle, can absorb more energy than the Half Tonner. It cannot be assumed however that the Contessa would survive a gust capable of capsizing the Half Tonner, since the work done by the wind on the yacht is dependent on the sail plan and hull windage. As we have confined ourselves to an examination of the hulls, we can draw no conclusions on this point. The effect of flooding on the two yachts is very similar (see here) in that the angle of vanishing stability of the flooded boat is increased in both cases examined, which implies it will be less likely to remaìn inverted should a capsize occur.

It is likely that a capsized yacht will experience flooding, and as sinkage continues it will become increasingly easy for a wave or gust of wind to roll the boat back into a stable, upright position, since the area under the negative part of the GZ curve is decreasing.

In interpreting these data it must be remembered that the results are dependent on the following assumptions:

1. The VCG derived from the Rating certificate represents an accurate assessment of the vessel's centre of gravity.

2. When flooding, the flood water uniformly permeates the underwater space by 95%.

3. The aluminium mast is free flooding.

4. The displacement calculated using data contained in the Rating certificate correctly represents the sailing trim of the vessel, eg. no crew were aboard.