When discussing stability, I often return to the 2006 sinking of the tug Valour because it serves as an excellent example for towing vessel mariners.
Numerous wrong moves were made on that voyage during a stormy winter night off the coast of North Carolina. But as with all error chains that lead to accidents, there were plenty of missed opportunities that could have broken the chain that led to the tragic outcome. In this case the first and best opportunity to prevent the casualty came before the Valour ever left the dock — someone should have ensured that all of the crossover valves were closed.
Every tugboat stability letter that I’ve ever looked at has basically said the same thing: Don’t have too many tanks in a slack condition, keep doors and hatches closed, keep the bilges pumped to minimum content, etc. One of the most important is to keep any crossover valves between tanks closed whenever the vessel is underway. This helps prevent hydrostatic balancing between tanks and the loss of transverse stability, which is what eventually did in the Valour .
Note that the operative word here is underway, wherever that may be, not underway, outside of the jetties or underway, if it starts to get rough out, and certainly not underway, if you can remember and also feel like it. The definition of underway is not being made fast to a dock, pier or other structure, at anchor, riding on a mooring buoy, or aground.
The best practice is to keep the valves closed at all times, whether or not the boat is underway. When loading fuel and water, open them just long enough to level the tanks, then immediately close them. If your default setting for them is closed, it is much less likely that the valves will accidentally be left open.
It’s extremely important to remember that there was nothing abnormal about the casual approach the crew of the Valour took toward stability. Under similar circumstances, what happened to them could easily have happened to most tug mariners. Tug captains should review their practices, make any necessary changes, and ensure that their mates and engineers are broken of any bad habits.