[Platapus]

I do use a separate stopwatch as it enhances my role playing. But you can get a cheap digital timer for about 5 bucks these days, or just use your watch.

Here is another quick way to get the speed.

If you know the length of the target ship in meters.
If you can get a good look at the side of the target (AoB 30-150)
If the bearing to the target is +/- 20 degrees from 0/180

1. Lock the periscope to the target. The lock function places the periscope hairline approximately in the center of your target. Close enough for our purposes.

2. Get your timer ready and when ever you are ready to time, unlock the periscope but don't move it.

3. Stop timing when the butt of your target crosses the hairline.

Speed in Knots = Length in meters/time in seconds.

This is a variation of the original, and more accurate:

Speed in knots = 1.94 x Length in meters/time in seconds routine. The locking method just gives you the speed faster and is accurate enough for 1500 meter and shorter shots.