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Your Stability Letter: Read It & Heed It.

The ongoing analysis of the sinking of the tug Valour brings us to the stability letter, and how the master and all of his licensed officers, both deck and engineering, consistently and routinely failed to comply with it. This failure was found by the Coast Guard investigator to be the primary cause of the loss of stability which ultimately led to the loss of the vessel and three lives, and very nearly more.

While there are occasionally some differences, most towing vessel stability letters are very similar to one another. What follows is a verbatim transcription of the stability letter for the tug I’m currently assigned to. I’ll use it to illustrate some important points and to clarify some terminology in an effort to improve everyone’s understanding and promote better adherence to the operating restrictions found in every tug’s stability letter.

My vessel’s letter, like all others, is specifically addressed to the Master, so it’s very clear as to where the final responsibility lies. Nevertheless, it is the duty of all crew members and especially the other officer or officers, deck and engineering alike, to assist the master in any way possible to ensure that both watertight integrity and stability are maintained constantly. Remember, the ass you save may be your own!


Master, You are responsible for maintaining this vessel in a satisfactory stability condition at all times and for following the instructions and precautions below.

A stability test witnessed by ABS Americas on behalf of the U.S. Coast Guard, was conducted on the XXXXXXX, O.N. 1234567, a sister vessel to the YYYYYYY, at Houma, La. on 17 February 2005. On the basis of that test, stability calculations have been performed. Results indicate that the stability of the YYYYYYY, as presently outfitted and equipped, is satisfactory for operation on Exposed Waters, provided that the following restrictions are observed. ***Any questions so far?***


1. ROUTE: Operation on Exposed Waters is permitted.

Note: As per 46 CFR § 170.050 (c)Exposed Waters are defined as “waters more than 20 nautical miles from the mouth of a harbor of safe refuge and other waters which the Officer in Charge, Marine Inspection (OCMI) determines to present special hazards due to weather or other circumstances.” Other routes that you may see are: Partially Protected Waters (waters within 20 nautical miles of the mouth of a harbor of safe refuge, unless determined by the OCMI to be exposed waters, or those portions of rivers, harbors, lakes, etc. which the OCMI determines not to be sheltered), Protected Waters (sheltered waters presenting no special hazards such as most rivers, harbors, lakes, etc.), and Rivers (any river, canal, or any similar body of water designated by the OCMI).

2. FREEBOARD & DRAFT: A maximum baseline draft of 12 feet 11-1/4 inches is permitted. This corresponds to a minimum freeboard of 2 feet 2-5’8 inches from the main deck measured 7 inches forward of frame 24. Trim should be minimized. ***Self-explanatory***

3. DECK CARGO: Carriage of deck cargo is not permitted. ***Self-explanatory***

4. TANKS: No more than one centerline tank or P/S tank pair of potable water, fuel oil storage, lube oil, and ballast water may be partially filled at any one time. Any cross connections between port and starboard tank pairs shall be kept closed at all times when underway.

Note: This is a frequently misunderstood part of stability letters. The restriction on having no more than one centerline or port and starboard pair of tanks slack at a time applies to each type of tank listed individually, not collectively. Your fuel oil day tanks do not count towards the fuel oil storage tank limit. Underway is defined as when your vessel is not made fast to a pier, dock, bulkhead or other structure, riding at anchor or on a mooring buoy, or aground. I will go one step further and argue that, unless you are loading fuel, water, etc. or are leveling off your tanks, the crossovers should never be left open, underway or not. You have everything to lose and nothing to gain by leaving them open, so don’t. If the engineer doesn’t like it, that’s too bad.

5. HULL OPENINGS: Any openings that could allow water to enter into the hull or deckhouse shall be kept closed when rough weather or sea conditions exist or are anticipated.

Note: Don’t wait until the last minute. Anticipate the weather and sea conditions. People frequently forget to close doors and hatches, and occasionally boats sink because of it. Plan ahead.

6. WEIGHT CHANGES: This stability letter has been issued based upon the following light ship parameters: Displacement – 389.18 Long Tons, VCG – 12.98 Feet Above The Baseline, LCG – 0.60 Feet Aft of Frame 26.

Any alteration resulting in a change in these parameters will invalidate this stability letter. No fixed ballast or other such weights may be added, removed, altered, and/or relocated without the authorization and supervision of the cognizant OCMI. There is no permanent ballast aboard this vessel.

Note: Any additions or subtractions of vessel equipment or alterations to the hull or superstructure constitute a “change” that invalidates the letter. A new stability test, or at the very least a stability review and recalculation by a qualified naval architect, is then required before a revised stability letter can be issued.

7. BILGES: The vessel’s bilges and voids shall be kept pumped to minimum content at all times consistent with pollution prevention requirements.

Note: To keep the bilges pumped to minimum content at all times while simultaneously being “consistent with pollution prevention requirements” means that you need adequate slop tank capacity to  safely and legally store it in until it can be properly disposed of ashore. It also means that tug crews need proper shoreside support so that they can get their slop tanks regularly pumped out without a lot of hassle. A carefully planned out “clean bilge” system in the engine room, utilizing drip pans under all machinery and hose connections and / or troughs under the shaft thru-hull fittings to catch the necessary leakage around the shaft packing, would eliminate a lot of unnecessary costs and the logistical problems caused by proper oily waste disposal. The upcoming towing vessel inspection regulations would be a good time and place to address this.

8. FREEING PORTS: Deck freeing ports shall be maintained operable and completely unobstructed at all times.

Note: Freeing port is the technical term for a scupper. If you ship water on deck at a rate faster than it can drain back overboard through the scuppers you will be adding significant weight on deck, thereby raising your vertical center of gravity, and adding substantial free-surface forces as the water sloshes back and forth the full width of the tug between the bulwarks. In addition, many tugs have flush hatches on their back decks. If these hatches are continuously under water and they leak any more than a trace amount you may begin slowly flooding the rudder room or empty ballast tanks without knowing it, reducing your buoyancy and adding even more free surface area. Scuppers that are icing over must be continuously cleared, with a fire axe if necessary.

9. LIST: You should make every effort to determine the cause of any list of the vessel before taking corrective action.

Note: This should be self-evident, but its importance cannot be over-emphasized. Had the crew of the Valour carefully checked all of their crossover valves when they first noticed that they were starting to develop a list for no readily apparent reason (a logical first step to take under the circumstances) and simply closed them, rather than blindly pumping ballast, they might have corrected the problem in time to ride out the storm with just that slight list and otherwise have been none the worse for wear. Any attempt to correct a list without first knowing why you’re listing is likely to make matters worse, often drastically worse, so don’t do it.

This stability letter shall be posted under glass or other suitable transparent material in the pilothouse of the vessel so that all pages are visible.

Note: I would suggest that a large-print version of it should also be posted prominently in the engine room, with the most important parts highlighted, at the bilge / ballast pump controls. Tug engineers, licensed or not, must understand the importance of stability too. In fact, this should be an all-hands project.

I know, it’s a terribly boring subject. But if you stayed with me this far I hope that you’ve now seen that there’s absolutely no reason to be confused or intimidated by the basic practice of complying with your tug’s stability letter. There’s no complicated mathematical equations or calculations required. No math at all, for that matter. If your thru-hulls, hatches and doors don’t leak, and you can bring yourself to follow the operating restrictions, (especially those relating to the tanks and weight changes) then you should have no trouble at all keeping your tug safe and stable. What happened to the Valour should never happen again.

For those who wish to dive deeper into the Coast Guard’s stability regulations, click here and poke around to your heart’s content in 46 CFR – Parts 170 through 174. Government Warning: use of this product may cause headache, dizzyness, constipation, or a vague feeling of dread.

Also see the posts Tug Valour Investigation ReportClose The Crossovers! , Practical Stability For Tugs and Damaged Stability For Barges for more on this subject.

#FreeSurfaceEffect #CorrectingAList #ProtectedWaters #FreeingPorts #SlackTanks #Rivers #ExposedWaters #Scuppers #towingvesselstability #stabilityletter #Bilges