Quick question on speed calculation

[Paul Riley]

Originally Posted by Phoenix3000

Hi Predavolk,

This method is very reliable (well, for me anyway).

OK here's what you need to do:

1. Identify the ship first to obtain its length - very important. For this example lets say its 200 metres.

2. Be at full-stop ideally (or no more than 1 kt) and try to be side-on to the vessel as much as possible - it doesn't work from straight ahead, but 90 degrees + or - 50 degrees is usually fine.

3. Turn your periscope so the vertical line is ahead of the bow of the approaching ship. Don't move the periscope any more - very important.

4. Now wait until the ship's bow reaches the vertical line and start a stopwatch.

5. Its safe to lower the periscope whilst you are waiting for the ship to cross the peri's path, but don't turn it left or right.

6. OK, as the ship passes the vertical line stop the stopwatch the moment the stern passes the line. For this example lets say it took 55 seconds.

7. Now the mathematics....

Take the figure of 1.94, multiply it by ships length (200) then divide by the time to cross the line in seconds (55).

So we have 1.94 x 200 / 55

= 7.05 knots

There you go!

Hope that helps.

Cheers,

Px3000

Easy,effective,jobs a good un