If you need another way of finding the speed of the other boats, or just want to have fun (!?) while waiting for the ship to get into your sights; then you can use the "Sound Trainer" mod (downloaded at
http://subsim.com/subsim_files/patches2005.html). Its not really a mod, but download the zip and read the document in it. It has a table of the most common ships and their "sound" at different speeds. So if you unsure of the speed, just get the RPMs and check the table, and voila you have the exact speed or from it you can determine what type of ship it could be.
Ship + RPM => Speed
Speed + RPM => type of ship
Its not necessary but some kind of fun :P