If it's a convoy reported by BDU - you already know it's speed.
If not I usually using RAOFB:
Other methods:
1. Place marks on map at same periods of time and calculate speed yourself. I hope everyone knows that
speed = distance / time.
2. Use
U-Jags Chrono http://www.subsim.com/radioroom/show...4&postcount=54
3. If you submerged sonarman can say how fast ship moving. If I have no time and/or no opportunity to calculate speed, I set 5 knots for slow moving targets and 10 for medium speed. If ship is big (like cimmaron tanker) and distance is less than 1000 meters - it usually enough to hit.
4. Ask XO for estimated speed. You should set correct distance and AOB before measurements. Best results will be if you are stopped. But this method never worked good for me for some reason.
P.S. It's all working with TDW UI mod. Don't really know about stock game.