Quick question on speed calculation

[Sag75]

Originally Posted by Fader_Berg

To clearify the AOB problem a little more...

If we would look at a moving target in an AOB of 90 degrees. It would cross our field of view as fast as possible in any given speed. While if we watch it in a AOB of 45 degrees it would appear to cross it slower, or actually (sin(45) =) 0,71 as fast over the same view. But, at a fixed aim point, and as the AOB of 45 degrees also makes the ships footprint only (sin(45) =) 0,71 of its true (90 degree) size. The two factors evens out.

So a ship with an AOB of 45 degrees will appear to move 0,71 slower across our point of aim than a ship with a AOB of 90 degree. But the 0,71 smaller footprint of the 45 degree target compensates for that, and the time for both to travel their full body will be the same.

Thanks! this is the confirmation I was looking for!

@Fader_Berg

Good info. Thanks.

[Pisces]

The only bow or angle you should be worried about when doing this trick is that of your own! Have the periscope look at 0 or 180 degrees, turn until right in front of his bow and then all is good.

[Pisces]

The only bow or angle you should be worried about when doing this trick is that of your own! Have the periscope look at 0 or 180 degrees, turn until right in front of his bow and then all is good.

p.s. If you used 1.852 as conversion factor instead of 1.94...yada yada yada... then your lead will be underestimated by 5%, or about one in twenty degrees too less. If you used simply 2 you'd have over-estimated the speed by 3%. That's better and simpler.

If this is a hit or miss really depends on the size of the target and the range. 1 degree is 17.5 meters wide at 1km. So most small vessels (78.5 meters is common) are between 4 and 5 degrees wide at 1 km distance. When closer disproportionally bigger in degrees. You'll no doubt hit it, but maybe not where you intended.

[Fader_Berg]

Originally Posted by Pisces

The only bow or angle you should be worried about when doing this trick is that of your own! Have the periscope look at 0 or 180 degrees, turn until right in front of his bow and then all is good.

Was this intended as an answer to my contribution?

Originally Posted by Pisces

The only bow or angle you should be worried about when doing this trick is that of your own! Have the periscope look at 0 or 180 degrees, turn until right in front of his bow and then all is good.

p.s. If you used 1.852 as conversion factor instead of 1.94...yada yada yada... then your lead will be underestimated by 5%, or about one in twenty degrees too less. If you used simply 2 you'd have over-estimated the speed by 3%. That's better and simpler.

If this is a hit or miss really depends on the size of the target and the range. 1 degree is 17.5 meters wide at 1km. So most small vessels (78.5 meters is common) are between 4 and 5 degrees wide at 1 km distance. When closer disproportionally bigger in degrees. You'll no doubt hit it, but maybe not where you intended.

You are 100% right.
That's why torpedoes were hitting towards the stern on a regular basis.
Only getting so close allowed me to get away with it so often.

I love this place. Thanks for the assist. (I need it now and then).

[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

[Pisces]

Originally Posted by Fader_Berg

Was this intended as an answer to my contribution?

Not in particular. Just to emphasize in general that an 'angle on own-bow' can easier wreck your measurement. Snestorm asked about how accurate is accurate. Therefore I gave some ballpark figures about how much it is going to be off. No worries mate.

Thanks, everybody.
100% improvement in accuracey.
Results, and confidense, are way up following this last patrol.