Quick question on speed calculation

[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

[Pisces]

Originally Posted by Phoenix3000

Hi Predavolk,

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

OK here's what you need to do:

...(snipped)

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.

.... (snipped)
Cheers,

Px3000

I am not sure what you mean by this. Is the target supposed to be on your side (90/270) or are you to be on the target's side (in other words AOB=90 or 270)?. If the later your are correct (for the reason Platapus described below), but the former is not to be recommended because your own speed (even if small) distorts the measurement in the worst possible way. Put the target just in front of your bow or stern and you'll minimise the effect of your speed.

[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