What usually works for me to quickly identify a merch at distance is looking at;
a. the smokestack - it's location on the hull and it's size/shape
b. the masts - shape, including rigging-shape/angle if visible
For example; if the ship clearly has the smokestack aft or rear end of the ship that narrows things down considerably and I'll quickly scan the ID book for those types of ships. If that smokestack is tall and skinny too then that narrows things down even better. You get the idea.
Occasionally I'll take a calculated guess at the type of merch (because it's too far away) just to get the TDC spun up and calculating so to speak and refine the info once I make a positive ID.
What makes things difficult for me is the sometimes "fluctuating" masts at distance (a function of the graphics being rendered I guess) and making sure the stadimeter is placed on the same spot at each reading. This is a problem at longer ranges and I'll get a speed reading of 18knots then a later reading will give a ridiculous speed of 40knots. I've learned to be patient, wait until the ship gets closer and the masts are more defined which permits better stadimeter readings. Additionally noting where the masts line up with the graduated marks on your periscope also help placing the stadimeter each time.
Clear as mud right!??
