Home Dispatches ANALYSIS: ‘Mechanical failure’ is puzzling

ANALYSIS: ‘Mechanical failure’ is puzzling

Although no one knows how Yogi sank, attributing it to “mechanical failure” is curious. As a first defense against sinking, a well-found vessel relies on compartmentation, which divides it into many watertight sections by watertight bulkheads.

These bulkheads should have no penetrations below the point to which the vessel would settle when one, two or three compartments are completely flooded, depending on whether it is a one-, two- or three-compartment ship. A two-compartment ship, for example, is designed to stay afloat with adequate reserve buoyancy and stability in a damaged condition with two contiguous compartments completely flooded.

ship330On three-compartment Navy ships, a 15-degree “V” is drawn with its apex 4 feet above the damaged waterline, and the bulkhead must be watertight, with no penetrations, below this “V line.” If Yogi was built to this standard, with no penetrations below her V lines, it is difficult to understand how a single hull breach or mechanical failure anywhere in the hull could have sunk it unless a single breach somehow compromised two spaces and Yogi was built to a one-compartment standard.

What I find troubling while walking the docks at boat shows is the huge tender “garages” on these big yachts, with watertight doors leading forward to the machinery space or accommodations, depending on what is directly forward of the garage. If these doors are left open, progressive flooding is possible — or actually inevitable — under the right conditions. In a well-found vessel, these doors would not exist; you would have to climb up above the V-line level at each bulkhead, and then back down, to go from space to space.

Equally troubling are the hull windows that in recent yacht designs are getting both larger and closer to the waterline. Putting a window in a hull anywhere should be undertaken with extreme caution, but to put it in the lower half of the hull, in terms of freeboard, close to the static waterline, is at the very least inviting trouble. Risk may be mitigated by recessing the windows back from the hull sides, and by using thick glass, but it’s still a window near the waterline, and a break in structural continuity. Putting windows down low in a hull almost certainly compromises seaworthiness in favor of a great outside view from the lower deck accommodations. The hull is there first and foremost to keep the ocean out of the vessel, and intentionally introducing a series of weak points made of glass that compromise this vital function creates a vessel that I personally would have little confidence in out in heavy weather. While many owners would not know what a serious compromise to structural integrity such windows may represent, the vessels’ designers get paid to know better.

It seems to me that designers are tempting fate with designs such as this, just as the disproportionately outsized superstructures on modern cruise ships push dynamic stability to the margins, particularly when the vessel is damaged. The most seaworthy ships and boats tend to have large hulls and small superstructures, attributes that lower the center of gravity and decrease sail area, which in turn increases dynamic stability and survivability in heavy weather.

I emphasize dynamic over static stability because a modern over-decked cruise ship’s superstructure increases heel and roll due to the effect of wind and sea far more than a static stability test alone would indicate.

Click here for video footage and a report on the sinking.

Soundings technical writer Eric Sorensen is a consultant to boat- and shipbuilders, boat owners, and to the government. He was founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.”

Comments (2) Comments are closed
2 Sunday, 04 March 2012 03:08
Bob Seldon
It does not appear that Mr. Sorensen is at all interested in waiting for the "facts" - he is enjoying (at others expense) his 15 minutes of fame pontificating and speculating. For Yogi to stay afloat after whatever disabled her, for 8 hours in gale force winds is quite an accomplishment.
As John above wisely suggested, wait for the real and pertinent facts to come in before you judge her builder, designed or crew.
Bob Seldon
Retired Yacht Broker of 31 years
1 Friday, 02 March 2012 05:11
John DeCaro
I truly feel that before people speculate further regarding this incident that we should all wait until the all of the facts are found and disclosed. This endless speculation and conjure is not productive, informative and can be at times miss leading.
A few facts; Yogi was built to ABS and MCA LY2 which requires only single compartment flooding survivability. This is the standard required by all of the major class societies in the world today with regards to this class of vessel. In fact that MCA LY2 code is a recent up grade on the codes that had been in use for quite a number of years. Two compartment survivability is only found in much larger commercial and solas passenger vessels. As far as I can determine only navy vessels require 3 compartment flooding survivability.
Your comment “If Yogi was built to this standard, with no penetrations below her V lines, it is difficult to understand how a single hull breach or mechanical failure anywhere in the hull could have sunk it unless a single breach somehow compromised two spaces and Yogi was built to a one-compartment standard.” Is miss leading in a number of ways, first no vessel in Yogi’s class is built to that standard, in fact no vessel in the world that I know of is built “with no penetrations below her V water line. All vessels have them, they are required by class to be water tight and do not affect the water tight integrity of the bulkhead.
With regards to water tight doors, it is also completely impractical to build a YACHT with no water tight doors between compartments, in fact all commercial vessels no mater how large and navy vessels of all types have water doors. To run up and down between compartments required by class even in single flood requirements would make the vessels impractical.
Regarding port holes in yacht hulls and “what a serious compromise to structural integrity such windows may represent” and there position on the hull. This is again a item that all class societies have strict rules regarding, ports have to be a set distance above the water line and can only be of a set size in relationship to the hull, the glass strength has to meet or exceed the strength of the hull AND THEY ARE ALL REQUIRED TO HAVE STEEL DEAD LIGHTS, which are covers that are closed in rough weather or on long trips. Class would not allow the port lights if the were a “compromise to the structural integrity”
Regarding “emphasize dynamic over static stability”, All class societies require commercial vessels and passenger vessels to meet stability requirements, the stability is calculated in both intact and damaged conditions, one, two or three compartments flooded depending on which class the vessel is built to. When the stability is calculated to class requirements for both intact and damaged conditions various wind and sea states are factored in “IMO severe wind and rolling criterion”. I believe this is what you are revering to when you use the term Dynamic stability.
Modern yachts built to Class requirements and MCA codes meet all international standards for, flooding, water tight doors, port lights and a huge number of other safety requirements. Yachts built to these standards are, truly well founded vessels.
Designs “know better” then most others the difficulties of creating a vessel that meets all of these requirements and is something a yacht owner would want to build and/or buy. A vessel with a large hull, no port lights and a very small house, compartments without water tight doors structured for 3 compartment flooding, would indeed reduce the potential for a owner to experience a sinking at sea, mainly because he would never built or be on such a vessel.
Yogi in fact took over 8 hours to sink in rough conditions and all of her crew got of safely, when we do learn the complete facts I will speculate that it will prove that she was a well founded yacht. I do look forward to learning the truth regarding this event.
In closing I will refer to the quote below.
Encyclopædia Britannica
To build a passenger ship that would survive all possible floodings is impractical, since the required fine subdivision would preclude effective use of the interior space.
John DeCaro
All Ocean Yachts
fbtwit yt

Great Gear,