In Search of Stability
An official measurer of the America's Cup, our contributing editor discusses the importance of stability and how to get it.
On July 15, 1992, a sudden squall swept J/24 Fleet 128 during a Wednesday evening race off Sandy Hook, New Jersey. Nine of the 12 boats racing suffered knockdowns. Three sank. Miraculously, despite the fact that a third of the total crewmembers in the fleet ended up in the water, no one drowned.
On December 27, 1991, a steadily increasing gale pounded the nine yachts participating in the seventh annual Japan-Guam Yacht Race. Two boats - the lucky ones, in retrospect -retired, one with a broken mast, the other with a badly damaged mainsail. Marine Marine, a 3g' IOR boat displacing about 11,500 pounds, lost her keel and capsized. Taka, a 47' IMS ocean racer with a measurement displacement of about 16,000 pounds, but an actual sailing displacement of about 20,000 pounds, was rolled down into an inverted position. The boat ultimately righted, filled, and sank. A total of 14 crewmembers from the two capsized boats died.
The risk of capsize is a reality that every sailor must face. Dinghy sailors accept capsize both as part of the learning process and the occasional result of sailing the boat too close to the edge. Typically, dinghy races take place in protected waters, and the boats are followed by chase boats which offer immediate assistance in an emergency.
Keelboat sailors, on the other hand, often have a little too much faith in the ability of their boats to right themselves. In the average course of sailing, every keelboat will eventually encounter a situation - a spinnaker broach, an unexpectedly big gust of wind - that results in a knockdown to 45° or more. Almost always, the boat will quickly pop back upright when the gust passes or the guy is eased. Faith in the keelboat's ability to right itself is reinforced.
Good stability for a racing sailor may be the ability to carry a #1genoa upwind in 20 knots of breeze. Stability for the cruising sailor involves a different and more serious set of questions. What happens when a boat is knocked down so far that it doesn't come back up? What if it comes back up, but is full of water and is at risk of sinking? From what degree of capsize should a boat be able to right itself?
Walking the docks at a boat show, you are bombarded by statistics from boat manufacturers: hull and rig dimensions, ballast and displacement, numbers of berths, volume of fuel and water tankage. When was the last time you saw a boat's range of positive stability listed?
Sailboat Range of Stability
Its what? A boat's range of positive stability is, put simply, the angle of heel to which the boat can be knocked down and still maintain energy to right itself, rather than having a tendency to either stay knocked down or turn turtle.
The range of positive stability is given in degrees from the vertical. A boat with a go° range of positive stability, once it is lying flat on its side in the water, is just as likely to continue to capsize as it is to come back up. Once capsized, it is likely to stay inverted until either a wave comes along that rolls it back up past go°, or its crew is able to apply leverage to the boat-standing on the keel, in a small daysailing keelboat - to roll the boat up to the point where its own positive righting moment kicks in.
Standing on the keel may be practical in a daysailer, but it clearly is not on a 35-footer. You must depend instead on the boat's inherent stability.
Remember that ultimate stability and sail-carrying ability should not be confused. At the low angles of heel normally encountered when sailing, hull form alone can be a powerful determinant of stability. A multihull is the extreme example of this. The great beam of the typical multihull gives it superb sail-carrying ability in normal conditions. But if knocked flat by a gust of wind - as anyone who has ever capsized a daysailing multihull will tell you - the average unballasted multihull will not come up of its own accord.
On the other hand, a narrow, deep, heavy boat such as an International Twelve Meter may sail upwind at a 25° angle of heel in only 10 knots of wind, but will, if its watertight integrity is maintained, right itself literally from a 180° capsize with no assistance from its crew or from wave action.
Many cruising sailors, however, would not choose to go to sea either in a multihull or a Twelve Meter. Some compromise is in order. What type of compromise makes sense?
A Concern for Stability
Following the Fastnet Race disaster of 1979, in which dozens of boats suffered major knockdowns or capsizing and 15 lives were lost, the yacht racing design and management community put a great deal of time, effort, and money into an analysis of the factors that cause boats to capsize or prevent them from capsizing. The Final Report of the Directors of the Safety from Capsizing Project, available from US Sailing, Box 209, Newport, RI 02840 ($5 plus shipping), is probably the best publication available to explain the mechanics of capsize cause and prevention in away that can be understood by the non-technical layman but appreciated by the technically oriented sailor or marine industry professional.
The basic conclusion is this: Waves, rather than just wind, are the prime cause of capsize. Waves exist in the ocean that can capsize any sailboat. Not unexpectedly, the larger the waves are, the more likely you are to encounter the one with your number on it. Statistically speaking, big waves are less common than small ones, and a boat with a greater resistance to capsize is less likely to encounter the wave that can capsize it than is a boat with less resistance to capsize.
Even if capsized, a boat with a good range of positive stability is likely to spend less time inverted, and is more likely to come up of its own accord. This may seem like small comfort, but it may well be a matter of life or death. If, for example, you're tethered and trapped in the cockpit of a capsized boat, you're less likely to drown if the boat rolls back upright almost immediately than if it stays inverted for five minutes. Once capsized, a boat with a low range of positive stability is likely to stay capsized longer before encountering the wave that rolls it back upright.
In general, boats are not really designed to be watertight in an inverted position. The longer a boat is capsized, the more water it is likely to take on through lockers, hatches, ports, and ventilators. The more water it takes on, the more likely it is to sink even when it comes back upright.
How Much Stability?
The surest way to start a fight in a barroom full of yacht designers is to state categorically that "no boat should go to sea that has a range of positive stability of less than 120° (or 110°, or 140°)." For each designer - and each boat owner - there is a different answer to what constitutes acceptable stability, and the answers are far from absolute. If a 120° range of positive stability is good, is 119° unacceptable? How about 117°? How much better is 130° than 120°?
At one extreme, the capsizing report states that a boat with a range of positive stability of 140° is unlikely to ever end up inverted, or stay there for any period of time. On the other hand, there is the long record of successful offshore passages by boats having much lower ranges of positive stability, such as the Hinckley Bermuda 40.
As a rule of thumb, we at Practical Sailor have recommended a minimum range of positive stability of 120° for serious offshore sailing. For the 1994 Newport-Bermuda Race, the Cruising Club of America has set a minimum IMS stability index (a size-adjusted range of positive stability; all other things being equal, bigger boats are less likely to be capsized than smaller ones) of 115°. This lower range of positive stability allows the participation of many keel-center boarders with proven ocean-going success, such as the Alden 44, Hinckley Sou'wester 42, and F&C 44.
Racing and cruising are two different things. The serious cruising sailor, who may spend extended periods of time at sea, will be safer with a boat with a greater range of positive stability than might be acceptable for the occasional ocean racer. Since a range of stability of 120°or more is very easy to achieve, it remains a reasonable minimum for a serious cruising boat.
Taka's IMS range of positive sta bility was 108°. She would therefore not have qualified for the 1994 Newport-Bermuda Race. In measurement trim, Taka' s ballast/displacement ratio was 42 percent, but in racing trim, the weight of crew and stores reduced her ballast/ displacement ratio to about 33 percent. While 33 percent may be a reasonable number, when coupled with her relatively shoal draft of 6', it yields a fairly high center of gravity in a conventional fin keel with a small bulb at the bottom.
Taka's maximum beam of 12' 6" was by no means excessive, but she carried that beam well aft - as do many modern boats - which probably contributed to her inverted stability.
Taka remained inverted for almost an hour. After a half hour upside down, her desperate crew opened the companionway hatch to escape. When the boat finally righted, the open companionway hatch - and perhaps an open cockpit locker and foredeck anchor well - allowed the boat to fill to the point where she was almost awash. With bilge pumps fouled and a good sea running, it took little time for the boat to sink. The six surviving crew members - one had already drowned - took to the raft.
Finding the Numbers
The biggest repository of stability information in this country is the US Sailing Association, formerly USYRU. For all boats measured under the International Measurement System, a hydrostatic analysis is performed as part of the creation of the IMS rating certi?cate, and the stability index is a key portion of the measurer's hydrostatic analysis.
The IMS hydrostatic analysis is the result of years of empirical testing and mathematical modeling. While far from perfect (it does not, for example, calculate the potential addition to range of stability of deck camber or deck structures, nor the detraction from stability of cockpit volume), the IMS stability index is the most readily available measure of ultimate stability for most modern cruisers/racers.
Hundreds of popular production sailboat models have been measured under the system, and not just boats that you would consider to be racers. Virtually every model built by C&C, Nautor, Pearson, Little Harbor, Beneteau, Baltic, J/Boats, Cal, Irwin, and most other well-known builders is represented in the IMS ?les.
Performance Characteristics Pro?le of the North American IMS Fleet contains performance and hydrostatic data for 650 models of production sailboats that have been measured under IMS, as well as data for about 400 custom boats. The book is a gold mine of information for any sailor who loves numbers.
IMS File Caveats
One thing you quickly discover from analyzing the IMS ?les is that supposedly identical boats may have substantially different ranges of positive stability. Even one-design boats such as the J/ 35 show boat-to-boat differences in range of positive stability of almost 20°. This is, in fact, the most extreme example in the ?les. More typically, the variance within a class will be 10° or less, with very tightly controlled classes such as the McCurdy-designed Navy 44s showing a difference of less than 3° between all boats.
In cases of extreme variation, common sense suggests that large divergence from the class mean is the result of either measurement error or unusual loading at the time of measurement, rather than ?aws in the calculation.
How Loading Affects Boat Stability
The range of stability of any boat can be impacted - positively or negatively - by loading. It is pretty clear that if you take 400 pounds off the bottom of a 5,000-pound keel and place that weight on the deck, you will have a negative impact on both the boat’s ability to carry sail and its range of positive stability.
More subtle changes can also have measurable impact. An anchor carried in a bow roller will detract from stability, while the same anchor carried in the bilge will add to it. Less obviously, perhaps, a radar scanner mounted 10' above deck level on a Questus backstay mount will have less negative impact on stability than one mounted 20' off the deck at the
Spreaders. Taken further, canned goods stored under settees may add slightly to stability, while the same cans stowed in lockers above the galley may detract from it.
In general, the lighter the boat, the more impact your loading will have both on the boat’s ability to carry sail, and on its range of positive stability. This doesn’t mean that you can load a heavy boat any way you want, but it does mean that you must pay particular attention in lighter boats.
It does you no good whatsoever to have a bilge full of anchors and chain that end up on the leeward settee in a knockdown, or worse yet, on the overhead in a capsize. Heavy items that are not properly secured are extremely dangerous both for their negative impact on stability and their potential to cause injury and damage in a knockdown.
Likewise, it does no good to have a 130° range of positive stability if you have cockpit lockers that flop open, companionway dropboards that fall out, and deck hatches that leak like sieves in a 100° knockdown. You may not have much control over your boat's basic hydrostatic characteristics, but you have almost complete control over the details that can make it fill up with water in the event of a capsize.
A boat with complete watertight integrity, well-secured heavy objects below, and only a 110° range of positive stability, is probably a safer boat offshore than a boat with a 120° range of positive stability whose engine and batteries come loose in a knockdown, or whose cockpit lockers flop open.
Keel Type, Keel Hype
A boat's keel shape has no inherent impact on its range of positive stability. A deep fin keel may be harder to keep attached to a boat than a traditional full-length keel, but the deeper keel may well yield a better range of positive stability than a shallow full-length keel.
Long, traditional keels are absolutely no guarantee of a good range of stability, particularly if combined with such characteristics as unusually high freeboard, wider than normal beam, shoal draft, and a low ballast/ displacement ratio.
In general, when sisterships are offered with different keel types, the configuration with the lowest center of gravity will yield the best range of positive stability. When designing keel and keel/centerboard versions of the same boat, it is common practice to increase the weight of ballast in the centerboarder. As long as the ballast can be added low enough to have a positive impact on the boat's vertical center of gravity, this can be an effective way to maintain stability. The trade-off is that the heavier boat will be slower in lighter wind ranges if the sail plans are the same.
In the C&C 40, a typical 1980s cruiser/racer, with the standard keel and rig configuration, has an average IMS stability index of 125°. The centerboard version with the same rig achieves a similar average range of positive stability by adding about 2,000 pounds of ballast to the keel. That 2,000 pounds of displacement costs the centerboarder about nine seconds per mile in speed in winds of about 10 knots, compared to her deeper-keeled, lighter sister, but the heavier center boarder retains a good range of positive stability for offshore sailing.
Unfortunately, some characteristics that have significant negative impact on a boat's range of positive stability have become incorporated in the modern racer/cruiser. If you're going to have a lot of interior volume in a light displacement boat, you can only achieve this in a given length by increasing freeboard, beam, or both. All other things being equal, the boat with more beam will show greater inverted stability than a narrower boat. The wider boat may be initially more stable right-side up, but she's also more stable upside down.
In order to reduce displacement, some boats remove ballast. It's much simpler - and cheaper - to reduce displacement from 15,000 pounds to
14,000 pounds by taking out 1,000 pounds of ballast than it is to remove 1,000 pounds of wood or fiberglass from the hull or interior. If that 1,000 pounds happens to be taken off the bottom of the keel, it can have a major impact on the boat's range of positive stability.
If you're going to reduce displacement by removing ballast from the keel, you'd better get that stability back by designing a new keel with a lower center of gravity.
We would be particularly wary of a boat with a combination of shoal draft, a low ballast/displacement ratio, a lot of beam, and nice, thick teak decks. We would be equally wary of the so-called fast cruiser that looks like a 1982 IOR boat with a nice interior, particularly if it has a lot of freeboard and has been turned into a cruiser by reducing draft from 8' to 5' and adding a lot of heavy interior joinerwork.
At the same time, many older production IOR cruiser/racers have excellent ranges of positive stability. The S&S Swan 44 has a stability index of about 135°, the Mull-designed Ranger 37 an index of 132°. Modern "traditional" cruisers, too, can have a good range of positive stability. The Lord Nelson 41 has a stability index of 129°, while the Island Packet 38 checks in at 130°.
The Bottom Line
One of the most important characteristics of a serious offshore cruiser is a good range of positive stability. If you only intend to sail in protected waters in good weather, your boat's ultimate stability might seem to be of little concern. But as the experience of J/24 Fleet 128 shows, even normally benign waters can at times prove dangerous.
If you're considering offshore sailing, your boat's range of positive stability should be of as much concern to you as the structural integrity of the hull and rig. With any luck, US Sailing's Performance Characteristics Profile will tell you a lot about the boat you own or want to buy. If your boat or a sister to her doesn't appear in the IMS file, you should consider having your boat measured to the IMS just for the stability and performance information the certificate provides, even if you have a pure cruiser that will never race.
Remember, however, that a boat with a 130° range of positive stability is no substitute for good seamanship and careful preparation. And it never will be.
Six of Taka' s seven crew survived to get into the life raft when she sank on December 29, 1991. Eleven patrol boats and 52 aircraft combed the race course for 18 days, looking for the raft. Despite the extensive search, her raft was not found. The air search was called off on January 16.
On January 25, 1992, the British freighter Maersk Cypress spotted a life raft 126 miles south of the Bonin Islands, of which Iwo Jima is best known to Americans. In the raft was a lone survivor from Taka.