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Assessing Stablity

Jan 1, 2003

One of the potentially more significant pieces of data available for assessing the suitability of a boat for voyaging is its stability curve (otherwise known as its GZ curve). A stability curve is developed by calculating or measuring the forces needed to heel a boat and then using accurate data describing the hull's shape and center of gravity to develop a curve that shows, among other things, the point at which the boat has its maximum resistance to heeling (the point of maximum stability) and the point at which it will roll over and turn upside down (the limit of positive stability [LPS], also known as the angle of vanishing stability [AVS] and the point of no return).

Depending on how the calculations are made, it is possible to come up with significantly different numbers for the same boat. Unfortunately, the International Measurement System (IMS) and the International Standards Organization (ISO) have different methodologies, although both are based on a lightly loaded condition and both exclude the effect of any superstructure on the calculation, so the results are likely to be reasonably close. Much bigger differences are likely between either of these methodologies and any methodology that includes the superstructure (which significantly increases the LPS/AVS) and/or substantial payload. For this reason, it is important to use the same measurement methodology when comparing boats.

Given that the European community is requiring some sort of stability testing for all new boats, we can expect most boats (both European and American) to be tested to the ISO standard in the near future, which will provide a measure of consistency.

However, a couple of caveats need to be borne in mind. On one hand, this stability test is conducted in a lightly loaded condition (minimum sailing condition - factory installed equipment on board plus an estimated crew weight), which tends to understate the stability of a boat loaded down with voyaging stores.

On the other hand, the test is done on the honor system to some extent, so it is quite possible for a builder to test a boat with, for example, a deep-draft keel and hanked-on sails, which will maximize the stability rating, and then sell the boat with a shoal-draft keel and roller furling sails, which will significantly reduce its stability. As always, for accurate comparisons it is important to find out on what basis the numbers have been derived. Beam versus stability

Irrespective of the way in which the calculations are run, a beamy, lightweight boat that relies primarily on form stability for its stiffness will reach its point of maximum stability and its limit of positive stability well before a deeper-draft, narrower-beam boat that relies more on ballast weight for stiffness. If either boat capsizes, the beamy, lightweight boat will have a greater tendency to remain upside down, as is well illustrated by some of the current crop of single-handed, round-the-world racers, which are quite stable in the inverted position.

"The way beam is used in combination with displacement and center of gravity is the crux of the stability question," wrote Olin Stephens, the famous yacht designer, in Desirable and Undesirable Characteristics of Offshore Yachts, edited by John Rousmaniere. "The worst of all combinations is large beam with a light-displacement, shoal-bodied hull having necessarily limited ballast that is too high." These characteristics will be reflected in a low AVS number.

If a boat is intended for extended voyaging where it runs the risk of getting caught in extreme conditions, it should have an AVS of at least 120°; a number of experienced cruiser/writers recommend 130°. The figure of 120° is chosen because if such a boat is inverted, in theory, another wave will right it in about two minutes, which is the longest most people can hold their breath. With an AVS of 100°, the boat will theoretically remain inverted for five minutes; at 140° the inversion time is minimal. For coastal voyaging, an AVS as low as 115° is acceptable.

Our Pacific Seacraft 40 has not yet been measured under the IMS or ISO rules, but it has been measured with the superstructure and cockpit included in the calculations (a more realistic assessment). This produces a point of maximum stability of 65° and ·VS of 143°, both numbers being on the high end for modern boats. Although these numbers would be lower using the IMS and ISO methodologies, the AVS would still be well above 130°. Stability ratio

Another interesting way of comparing boats is to take the area under the positive portion of the stability curve, which represents the amount of energy necessary to capsize the boat, and divide this by the area under the negative part of the curve, which represents the energy required to return an inverted boat to the point at which it will right itself.

The ratio of these two areas - the stability ratio - is a measure of the relative stability of the boat, both upright and capsized. The higher the number, the better. On a voyaging boat, it should be at least 3.0, and preferably higher; the farther offshore the boat goes, the higher the ratio should be. I don't have the ratio for the Pacific Seacraft 40, but my guess is it is above 10.0.

Stability curves and ratios are useful as a guide for selecting offshore boats, but they need to be put in perspective. The curves are based on a static calculation and take no account for the dynamic forces at work in conditions of heavy breaking seas when a knockdown is most likely to occur. According to one school of thought, a boat with relatively low freeboard and a deep keel has significantly less wave-loading area than one with high freeboard, and is therefore less likely to get rolled. According to another school of thought, a lightweight boat with high freeboard and a shallow keel is more likely to skid sideways before the wave, dissipating the wave's energy and so forestalling a capsize!

As noted, the stability curve and ratio are usually based on some form of light ship displacement. The addition of voyaging stores and gear has a significant effect, since weight somewhat above or anywhere below the boat's center of gravity tends to increase its stability; whereas, added weight well above the center of gravity decreases stability.

The higher the added weight on the boat, the more deleterious the effect. Such things as a radar antenna sited high on the mast, roller reefing headsails, an outboard motor stowed on the rail, and dinghies, anchors, ground tackle, and all the other gear commonly placed well above the waterline, all have a significant, negative impact on the numbers. We have all these things on our boat.

Peter Bruce, in the fifth edition of Adlard Coles' Heavy Weather Sailing, reports that the addition of in-mast furling and a roller-reefing headsail to a 28.5-foot production voyaging yacht reduced its AVS/LPS from 127° to 96°. This is a potentially life-threatening reduction in stability. Although this is an extreme case (the reduction in AVS caused by similar gear on a larger boat of significantly heavier displacement is more likely to be on the order of 3 to 4 percent), voyaging sailors need to be aware of the effect that additional weight can have on stability, especially weight high up, and then make sure that any given boat can handle the load without a serious loss of performance or stability. Every effort must be made to keep heavy weights low in the boat. Capsize screening value andSTIX number

After the disastrous 1979 Fastnet race, in which numerous boats were repeatedly rolled and 15 people lost their lives, a long, hard look was taken at the stability issue. Many yacht designers acknowledged that the International Offshore Rule (IOR), which dominated yacht design in the 1970s and 80s, under which many of the participating boats were designed, was actually promoting the development of unsafe boats (non-IOR boats built before 1975 survived the race with few problems).

A great deal of work was put into developing a simple formula that would weed out the worst excesses resulting from attempts to beat the rule. The formula that was developed is known as the capsize screening formula. It is intended to assess both "the risk of being unduly, easily capsized and the risk of sticking in the inverted position for an extended period of time," according to the Final Report of the Directors, USYRU/SNAME Joint Committee on Safety from Capsizing.

The capsize screening value for any boat is found by dividing the cube root of the boat's displacement volume into its maximum beam (Bmax). The higher the resulting number is than a value of 2.0, the greater the chance that the boat will be unduly prone to capsize; if it is below 2.0, it should be safe offshore.

It should be noted, however, that since the capsize screening value is a function of displacement and beam, any two boats with the same displacement and beam will have the same capsize screening value. This is so even if, for example, one boat has a heavily ballasted, deep-fin keel, while the other has a centerboard and internal ballast, in which case the former will in fact be much more stable.

At press time, the ISO was working (and has been for eight years) on a more sophisticated stability index (STIX), which takes into account a greater number of variables. Until this work is completed, the existing capsize screening value, despite its shortcomings, is a useful indicator of stability.

Looking at our Pacific Seacraft 40: it has a half-load displacement of 26,830 lbs (light ship + 3,750 lbs), which is 419 cubic feet. The cube root of 419 is 7.48; Bmax is 12.42 feet. The capsize screening value is 12.42/7.48 = 1.66, which is well below the target of 2.0, confirming the boat's high degree of capsize resistance. If we use the light ship displacement to work the numbers (this is the worst-case scenario in terms of the capsize screening value) we get a value of 1.75, which is still well below the target of 2.0.

Once the STIX standard is completed, it should provide a more comprehensive means of comparing boats than the current capsize screening value (although this is by no means certain - the standards-writing process is both political and controversial). However, earlier drafts of the STIX standard gave a score of 30 or more, which resulted in an A rating - the rating for oceangoing boats - to boats with an AVS as low as 95, in spite of the fact that these boats were clearly not suitable for extended ocean voyaging. After a well-publicized sinking in a Bay of Biscay gale of an A-rated boat with an AVS of 110, the STIX score for an A rating has been raised to 32, and there is discussion of raising the AVS to 120. Minimum numbers

Given these facts, and bearing in mind that longer boats inherently score higher on the STIX scale than shorter boats, regardless of the number that the STIX committee finally determines is appropriate for an A rating, it may make sense to set a minimum score of 35 for oceangoing voyaging, or maybe even 40, and to progressively raise this for boats more than 40 feet in length. For coastal voyaging, a minimum STIX number of 30 will give a greater degree of security than the current 23. It, too, should be increased for boats more than 40-feet in length.

A final word of caution is in order. Calculating a STIX number is a complicated (and hence, expensive) process. Boatbuilders have the option of entering worst-case default numbers into the formula at various points. Any builder who can do this and still come up with a STIX number that exceeds 32, and thus get a CE A rating, may well choose this route in order to keep down costs. But in this case, the resulting STIX number is likely to be well below what it would be if the full calculation were to be made. As with all numeric parameters, when comparing STIX numbers it is important to find out how they have been derived.

If the conditions get nasty enough, any boat can be rolled. At such a time, the survival of the crew is going to be significantly affected by how fast the boat will right itself. Only boats that are likely to recover in a minute or two should be considered for blue-water voyaging. With this in mind, I look for an AVS/LPS of 120 or higher, a capsize screening value of 2.0 or higher, and a STIX rating well above 32.

This article is an excerpt from Contributing Editor Nigel Calder's latest book, Nigel Calder's Cruising Handbook, published by International Marine/McGraw Hill.


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