Dec 7, 2011

Perfect sailing boat? - part 2: Resistance to capsize

When sailing for the first time, one is usually reassured by someone more experienced that sailing boat cannot capsize. This is true with a keelboat when it comes to a pressure of wind. However, it is the heavy seas and breaking waves that pose the threat of capsize for bigger yachts as well. According to the model tests, breaking waves of just 30 per cent of the hull length can capsize some of the yachts. Breaking waves of over 60 per cent of the hull length will probably capsize most of the yachts. However, there are differences among boats on how they resist capsize. "The ability of a yacht to recover from a breaking wave encounter depends on the hull and coachroof shape" (Adlard Coles' Heavy Weather Sailing pp. 17).

If you like to quantize things, there are different numbers that help to compare different boat designs. For stability comparisons, the STIX (Stability Index) and AVS (Angle of Vanishing Stability) are widely used. AVS is the point of heel angle beyond which the boat looses its righting moment and capsizes. When the boat is upside down, it depends on its self-righting ability if it will come back up. Wider boats in general have a larger inverted stability, so they tend to resist self-righting. For a CE category A(unlimited ocean voyages), the minimum STIX value of the boat is 32. The minimum AVS depends on the weight of the boat. For example, the minimum AVS of a 5 tons boat is 120 degrees. That means, that heeled at an angle 120 degrees the boat should still right itself and come back up.

Example of a Stability Curve (HR 310).

One problem with AVS and STIX is, that those figures may be difficult or impossible to find for older boats. This is also the case with HR 29 and many other boats, which I considered during the selection process. There are, however, some less complicated estimators, that work as a rough guide for capsize resistance screening. AVS can be estimated if you know a boat's beam, displacement, ballast and hull draft (i.e. draft excluding keel). Other formula that I used to compare different boat models was Capsize Screening Formula (CSF). Offshore sailing yacht should have CSF value less than 2.0. The smaller the value the better the boat is suitable for ocean passages. In general, heavy boats with narrow hull are able to resist capsize better, so CSF reflects this.

Other ratios that I used as tools to compare the qualities of different boats were Motion Comfort Ratio (MC), Sa/Disp (sail area to displacement) and B/D (ballast to displacement). One good program for comparing different boat models is Sail Calculator v3.53. This website has a good database of key figures for a number of different boats. You can also make comparisons between boats, which is very handy.

Website sailboatdata has basic information that is needed to calculate these figures. Ratios such as Ted Brewer´s Motion Comfort Ratio have raised some criticism because a single figure is thought to be overly simplified to predict a boat's behavior at the sea. However, I feel that this kinds of ratios can be of some value, but they should not be given too much weight. A good rule of thumb is to check, that for example AVS, MC, CSF and Ballast/displacement ratio are not below (or above) the recommendations for an offshore cruising boat.
/Antti

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It would be very great to hear your opinion or comments. Thank you in advance for commenting! -Antti & Minna