On a trim and stable ship, these two forces are equal and cancel each other out along the centerline; but all this changes when a boat gets shoved over onto her side. Instead of being lined up, the two forces are now laterally offset. The center of gravity stays where it is, but the center of buoyancy migrates to the submerged side, where proportionally more air has been forced below the waterline. With gravity pushing down at the center and buoyancy pushing up from the submerged side, the ship pivots on her center and returns to an even keel. The more the ship heels, the farther apart the two forces act and the more leverage the center of buoyancy has. To greatly simplify, the lateral distance between the two forces is called the righting arm, and the torque they generate is called the righting moment. Boats want a big righting moment. They want something that will right them from extreme angles of heel.
The righting moment has three main implications. First of all, the wider the ship, the more stable she is. (More air is submerged as she heels over, so the righting arm is that much longer.) The opposite is also true: The taller the ship, the more likely she is to capsize. The high center of gravity reduces what is called the metacentric height, which determines the length of the righting arm. The lower the metacentric height, the less leverage there is with which to overcome the downward force of gravity. Finally, there always comes a point where the boat can no longer right herself. Logically, this would happen when her decks have gone past vertical and the center of gravity falls outside the center of buoyancy—the "zero-moment" point. But in reality, boats get into trouble a lot sooner than that. Depending on the design, an angle of about sixty or seventy degrees starts to put a vessel's lee gunwales underwater. That means there's greenwater on deck, and the righting moment has that much more weight to overcome. The boat may eventually recover, but she's spending more and more time underwater. The deck is subject to the full fury of the waves and a hatch might come loose, a bulkhead might fail, a door might burst open because someone forgot to dog it down. Now she's not just sailing, she's sinking.
The problem with a steel boat is that the crisis curve starts out gradually and quickly becomes exponential. The more trouble she's in, the more trouble she's likely to get in, and the less capable she is of getting out of it, which is an acceleration of catastrophe that is almost impossible to reverse. With the boat's bilge partially flooded, she sits lower in the water and takes more and more prolonged rolls. Longer rolls mean less steerage; lower buoyancy means more damage. If there's enough damage, flooding may overwhelm the pumps and short out the engine or gag its air intakes. With the engine gone, the boat has no steerageway at all and turns broadside to the seas. Broadsides exposes her to the full force of the breaking waves, and eventually a part of her deck or wheelhouse lets go. After that, downflooding starts to occur.
Downflooding is the catastrophic influx of ocean water into the hold. It's a sort of death rattle at sea, the nearly vertical last leg of an exponential curve. In Portland, Maine, the Coast Guard Office of Marine Safety has a video clip of a fishing boat downflooding off the coast of Nova Scotia. The boat was rammed amidship by another boat in the fog, and the video starts with the ramming boat backing full-screw astern. It's all over in twenty seconds: the crippled vessel settles in her stern, rears bow-up, and then sinks. She goes down so fast that it looks as if she's getting yanked under by some huge hand. The last few moments of the film show the crew diving off the upended bow and trying to swim to the other boat fifty feet away. Half of them make it, half of them don't. They're sucked down by the vacuum of a large steel boat making for the deep.
Very few boats ever get to that point, of course. They might take water in the hold or lose their antennas or windows, but that's it. The result, fortunately, is that their stability limits are rarely tested in a real-life situation. The only way to know the stability profile for each boat is to perform a standard dockside test on her. A 5,ooo-pound weight is put on deck, ten feet off the centerline, and the resulting angle of heel is run through a standard formula that gives the righting moment. So many things can affect the stability of a boat, though, that even the Coast Guard considers these tests to be of limited value. Load a few tons of gear onto the deck, take a little water in her bilge, shift from longlining to dragging to gillnetting, and the dynamics of the ship change completely. As a result, stability tests are mandatory only for vessels over seventy-nine feet. At deck height, the Andrea Gail measures seventy-two.