ANALYSIS: ‘Mechanical failure’ is puzzling
Posted on 23 February 2012
Written by Eric Sorensen
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.
On 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.”