This thread How Important Does “Feel” Matter In Ship Stability? brought up the loss of the tug Valour and the subject of the vessel stability letter.
The now inactive site Towmasters has some good posts on those subjects.
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.
This is true on deep-sea vessels as well, it’s just a matter of plugging the numbers into the computer, no calculations to speak of. A little theory helps but is not needed.