Overall, adhering to most of the operating restrictions contained in your vessel’s stability letter is pretty simple and straightforward. It can take some time to educate and train your crew, and some people, including captains, will never “buy in” to the concept and take it seriously. That’s life. But I would be remiss if I didn’t address the practical realities and difficulties faced by many of us when trying to comply with a certain aspect of it. Specifically, shortcomings and oversights in the design and layout of your vessel’s tank system can make strict, by-the-book compliance with your letter much easier said than done. What do you do? You do the best you can while trying to comply with at least the spirit, if not the letter, of the slack-tank and minimized-trim requirements.
“But this contradicts what you wrote in the last post!”, you might say. That is true. But the primary purpose of that post was to describe how to read, understand and comply with your stability letter in the strictest sense possible. When the realities of the imperfect world that we work in intrude upon our best attempts at “perfect world” regulatory compliance then we sometimes must get creative to stay safe. The mix-and-match approach may be the only way to accomplish it.
Here’s a possible solution: go over your stability letter carefully and total up the maximum number of tanks in all categories, be they centerline tanks or port & starboard tank pairs, that you’re allowed to have slack (partially-filled) at one time. To keep from making a mistake I prefer to use the term units, with either a centerline tank or port & starboard tank pair each counting as one unit. Then see if you can find a way to not exceed that maximum number of units while still utilizing the tanks you need to operate the boat normally. It should be possible to do this in virtually all cases. Yes, this may mean being in technical violation from the standpoint of having more than one centerline or port & starboard pair of a specific category of tank (fuel, ballast water, potable water, etc.) slack at one time. But it is the cumulative amount of free-surface forces in all of your tanks, compartments and on deck that act to destabilize a vessel. You can generally offset a slack-tank overage in one type of tank by giving up an allowable slack centerline tank or port & starboard tank pair of some other type. It’s the total number that really counts.
And here follows a practical example of using this solution. My tug has no aft ballast tanks, so keeping the boat properly trimmed as we gradually burn off fuel would be impossible without drawing from both forward and aft fuel storage tanks at the same time. This, of course, puts us in violation of our letter. But because we never make use of our forward pair of ballast tanks, which we’re allowed to both use and have slack if we want, they are the functional equivalent of voids. So we still don’t exceed the maximum number of slack-tanks for all categories that is allowed by our stability letter. We just substitute one allowable slack-tank type for another and, in so doing, keep our tug stable while still allowing it to function as it must.
We don’t design and build the tugs and towboats, we just drive ’em. Because the slack-tank restrictions found in the typical tug’s stability letter can be generic in nature, they often don’t provide the clear-cut answers that would reconcile the competing demands of maintaining adequate stability while still meeting our practical operational needs. Until such time as this changes you’ve got to work with what you’ve got.