I love this thread.
 

some one needs to make a chart of the various ways one can get target information like speed, AOB, range, class and the like , and the benifits and limitations of each as it applies to the game. (ship mast heights being wrong) and various things like that. casual gamers need to find the simplest ways to get torps on target, and serious sim folks need a tool box of tips and tricks and skills to build a firing solution from limited information and often under the time crunch of the much shorter ranges that running the numbers often results in. it can be done and nothing is as satisfying as getting it all right and seeing the results of doing it "right". and proper id of a ship gives you a wealth of information to work with, if you know how to use it. but when I am playing as often as not I am ready to shoot but cant tell if the ship I am ready to sink is friendly or hostile till I see the flag on the mast.
is it sad that I often have a valid solution with out having a clue of what class ship I am hunting. ( often lone merchants on the open sea useing passive sonar hunting ).

Ships are big. So if you are close to her and rather short on time for proper calculation (very bad weather or dark night or whatever) - guess 10 knots for merchants and 15 knots for warships, for destroyer in attack run guess 30 knots. It will do 85% of time.