Bayesian Disaster Follow-Up
DOES THE SINKING OF THIS 180' SAILING SUPERYACHT CALL INTO QUESTION INDUSTRY DESIGN AND CLASSIFICATION PRACTICES?
Following the sinking of Bayesian, a former captain of the yacht during the years 2015-2020, Stephen Edwards, published a literate and intelligent review of the yacht as he knew her when she was under his command.
Captain Edwards agrees that she was probably sunk by unchecked down-flooding initiated by her being severely heeled by very localized, freak extreme weather conditions. And in a calm, objective “voice”, he details various features of the yacht’s design and construction that might have contributed to the disaster.
Although Edwards maintains a staunch veneer of non-judgemental objectivity, he does, seemingly in passing, make a couple of truly stunning remarks — which may qualify as additional 800 lb. gorillas-in-the-room. (The first 800 lb. gorilla is the question I raised in my previous article on this topic, namely, why did a 180-foot superyacht that was reportedly divided into multiple watertight compartments, go down so quickly?)
The first of Captain Edwards’ remarks to strike me is his statement that Bayesian’s angle of vanishing stability (AVS) is only 70 to 75 degrees with her ballasted centerboard raised. And only 90 degrees fully lowered.
[Editors note: This last number is the correction of a typographical error contained in the originally-published version of this article.]
These values are significantly lower than what is generally found on smaller offshore sailing yachts (less than 22 metres LOA). The AVS for such smaller craft is commonly 120+ degrees. In fact, Bayesian’s AVS is significantly less than the AVS found on many offshore sailing yachts that are larger than 22 metres LOA, most of which have an AVS of 100 to 110 degrees. Indeed, Bayesian’s maximum AVS value is what is considered by many offshore sailors to be a rock-bottom minimum — specifically, 90 degrees from vertical.
Maintaining positive stability (i.e., the ability of the hull to right itself) to at least 90 degrees angle of heel is not an arbitrary election. Rather, it coincides with the angle of heel at which the wind ceases to exert significant heeling force on the yacht’s rig. That is why a vessel pushed by the force of the wind to 90 degrees angle of heel is said to have been “knocked down”. In such circumstances, the general expectation is that the wind alone will not put the yacht “down” any further and that, when the wind abates, assuming the yacht doesn’t down-flood and sink, she will return herself to an upright position. At least, that’s the commonly held theory and the generally accepted rationale for having an AVS of at least 90 degrees. It is also likely the reason it was maintained in the design of Bayesian, which design was struggling to achieve even that minimum angle.
[ Author’s Note: This last is confirmed by the official Stability Book produced when the plans were reviewed and approved by ABS, in which it is disclosed that the board-down AVS in light-load condition is only 83-degrees.]
Larger vessels — such as megayachts, superyachts, and cruise ships — are often designed with a significantly lower AVS than the average offshore sailing yacht of less than 22 metres LOA. However, this does not mean the larger vessels are “less stable”.
A heeled vessel’s “righting moment” (the force that drives a heeled vessel back to a vertical position) is the product of the vessel’s weight multiplied by the length of the “righting arm” (the horizontal distance from the vessels transverse center of gravity to her heeled center of buoyancy). For a given length of righting arm, the heavier the vessel, the greater the magnitude of righting moment. And a short righting arm can be offset by the absolute weight of a given vessel.
For example, it might surprise you to find out that giant cruiseships have relatively short heeled righting arms and what seem to many sailors absurdly low AVS’s — on the order of 20 degrees or even less. But because they are so massively heavy, the magnitude of their righting moment is enormous, notwithstanding their relatively short righting arm. And their relatively low AVS is deemed acceptable because their righting moments at small angles of heel are so great, they are not likely to be heeled to the limits of their AVS, even in a hurricane.
If you are not fully familiar or comfortable with the basics of vessel buoyant stability, I recommend the following previously-published brief discussion:
That said, the second of Captain Edwards’ stunning statements is that Bayesian has HVAC and engine-room air supply vents that are placed in a way that could result in major down-flooding (if not controlled) at only — wait for it — 45 degrees angle of heel — IMO, an angle sorely insufficient for a sailing vessel that regularly runs offshore in open, unprotected waters.
Now you might very well wonder who am I to say that, since the Bayesian’s design and engineering were passed by both the American Bureau of Shipping (ABS) and the British Maritime and Coastguard Agency (MCA). Well, FWIW, I did for several years in the early 2000s run Palmer Johnson Yachts, a world-class U.S. megayacht shipyard. During that period, we built more than a dozen tri-deck yachts in the 40 to 50 metre range. And during that period, we built the first U.S.-flagged yacht to be granted an MCA letter of compliance. Moreover, although I am not a degreed Naval Architect, I am a trained and experienced small craft designer, as well as an experienced boatbuilding manager who has dealt with all aspects of bringing yachts from concept to reality — including with issues of stability — over decades. As a result, I’ve developed a sense of judgement as to which risks are acceptable, which are not, and which are readily eliminable and so call for a return to the drawing board. Therefore, as I said, you can take it for what it’s worth, but it does not seem to me that having vents places in a way that could result in significant down-flooding at 45-degrees angle of heel is a really good idea in a tall-masted sloop or cutter like Bayesian — particularly since major down-flooding very quickly turns into a vicious positive-feedback loop that often leads to a fatal loss of buoyant stability.
To be sure, properly skilled and experienced operation might be reasonably anticipated to offset the latent negatives disclosed in Captain Edwards’ notes. But my experience over the years is that naval architects and engineers working in the offices of classification societies and the like tend, at times, to forget Murphy’s Law. As do others involved in yacht building, especially when large, expensive, and profitable projects are at stake and demanding, ultra-wealthy buyers and owners are focused primarily on aesthetics and creature comfort. And I predict that, in the wake of the Bayesian disaster, there will be much soul-searching and several procedural changes in a number of naval design and engineering offices, including those of the major classification societies.
— Phil Friedman
Copyright © 2024 by Phil Friedman — All Rights Reserved
Authors note: The sailing and yachting community, as well as the yacht design and construction sector, owe Captain Edwards a debt of gratitude for his integrity and professionalism in publishing his notes on his experience with Bayesian.
➡️ Read the stability notes published by former Bayesian Captain, Stephen Edwards here ⬅️
Additional Note: Late-Breaking News - As I prepared this article for publication, I learned that Peter Swanson, EIC of the Loose Cannon newsletter, has secured and published a copy of Bayesian’s official stability document. You can read it at the following link:
As usual, Phil has presented a useful review of the proper application of current yacht design, manufacturing, and use standards. Since yachting is a relatively small industry, it will be one of the last to convert from human brain limited concepts (such as righting moments) to standards based upon patterns in data recognizable only by machine intelligence. Phil is also correct in noting that some wealthy yacht owners, engaged in conspicuous consumption, will prioritize other concerns over safety standards, especially standards too complex to be understood completely by humans.
I am curious about the position of the vents. As you know Phil, much smaller sailing boats and yachts do have vents that successfully avoid flooding. I don't see why a larger yacht, with a much greater height of the superstructure above main deck, shouldn't be able to implement a safe venting.
I will say one thing, when designing Georgia we were fortunate enough to have the LRS design approval office in the Trieste. Italy. It is full of serious sailors and we're quite cooperative in calculating the compression of the deck plating thickness around the 56+ Mt mast.