3min 15 sec

rcflyertim · 12-09-12, 05:18 PM

Ahhh so it is a knots thing. OK so 3 min 15 sec x 4 = 13 min. If I wait 13 minutes I will miss my target! So I take it that these times are for long distance spottings?

Bubblehead729 · 12-09-12, 07:00 PM

You don't necessarily have to wait the whole 13 minutes; its just that the longer time tends to reduce the amount of error in the speed calculation. Just remember the 3:15 time interval (for metric units-3:00 for Imperial units).
So if you have a limited amount of time, just time for 3:15; if you have longer, then take multiple measurements every 3:15.

Pisces · 12-10-12, 12:35 PM

The 3 seconds difference from 13 minutes can be neglected for simplicity. It's only 0.4% off from exact (777.5 seconds, or 12m57s). Even 195 seconds (3m15s) is 0.3% from exact (194.4s) So, you give up the same amount of inaccuracy on your speed for either time-interval. This is not the reason why it is more accurate.

The idea behind taking the target motion over 4x 3m15s, instead of just 1x3m15s, is spreading the position error of the begin and endpoint over a longer time period.

Let's say your watchofficer on the bridge gives a target position report. He gives distances in steps of 100 meters, and a bearing in steps of degrees. A degree at maximum visual distance of 16km is 280 meters wide. So the beginpoint of the target motion is somewhere in such a 100m by 280m box on the map. If you are lucky it is smaller when the target is closer. But doing periscope observations is much more inaccurate anyway.

The target speed is a line connecting some point in the begin-box with the a point in the end-box, taken over the time period. There is a smallest distance connecting the inside borders of the boxes. But the speed can also be the largest distance, connecting the far edges of the boxes. The actual motion of the target is somewhere in between those lengths.

If the interval is really short (theoretical 1 second) then the position uncertainty totally envellops the target motion track. If the interval is much longer, then the position uncertainty is only a small fraction of the target motion. A 6 knot target moves 600 meters in 3m15s. Two 280m position uncertainty boxes spaced appart by 600m would suggest a speed of something between 3.2 and 8.8 knots. That's quite a range. Conclusion: 3m15s is useless considering position inaccuracy in the plots.

However, it moves 2.4 kilometer in 13 minutes. The uncertainty boxes are still of the same size, but are much further appart. Based on the uncertainty boxes, it could have a speed between 5,3 and 6.7 knots. Now you can be much more confident that it is actually moving with 6 knots! It could still be 5.5 knots, or 6.5 knots. But you certainly narrowed it down to something acceptable.

Pisces · 12-10-12, 12:48 PM

P.S. In my ramblings above I considered plotting the target motion while you are doing the end-around/overhauling/sprint-ahead at maximum range not to get detected. It's safe from a distance, and time well spent. You do not want to wait with measuring target speed at the very last moment before firing (like the notepad-method wants you to) when you are submerged.

If you must, then the fixed-wire method is a better option. But just as a check. As it also requires ID of the ship's length, and good visual on the bow and stern. It takes in the order of a minute in most cases. You better not be turning, or looking to the side with your periscope when you are moving. Or it will be wrong.

Speed is the most important value to get the torpedoes leaded onto the target. It deserves the most effort and time to get it right. And the least chance of mistakes made in a hurry.

Kalamies · 11-08-15, 01:14 PM

I've seen people use both 3min and 3min 15sec rule. Is there really any difference between these two? If there is, which is more accurate in terms of estimating speed?

Sailor Steve · 11-08-15, 01:55 PM

3 minutes is the proper time when using yards for distance. 3 minutes 15 seconds works for meters. The closer you are when you shoot the less it matters.

Aktungbby · 11-08-15, 02:11 PM

WOW a worthy thread resurrection! A new fact I was not aware of and will certainly use! Thanks; the both of you!

Kalamies · 11-08-15, 03:25 PM

Quote:

Originally Posted by Sailor Steve

3 minutes is the proper time when using yards for distance. 3 minutes 15 seconds works for meters. The closer you are when you shoot the less it matters.

Didn't know that! Thanks!

12-09-12, 07:00 PM	#2
Bubblehead729 Sailor man Join Date: Nov 2011 Location: In Port Posts: 50 Downloads: 49 Uploads: 0	You don't necessarily have to wait the whole 13 minutes; its just that the longer time tends to reduce the amount of error in the speed calculation. Just remember the 3:15 time interval (for metric units-3:00 for Imperial units). So if you have a limited amount of time, just time for 3:15; if you have longer, then take multiple measurements every 3:15. __________________ There are two types of ships in the world......submarines and targets.

12-10-12, 12:35 PM	#3
Pisces Silent Hunter Join Date: Dec 2004 Location: AN9771 Posts: 4,904 Downloads: 304 Uploads: 0	The 3 seconds difference from 13 minutes can be neglected for simplicity. It's only 0.4% off from exact (777.5 seconds, or 12m57s). Even 195 seconds (3m15s) is 0.3% from exact (194.4s) So, you give up the same amount of inaccuracy on your speed for either time-interval. This is not the reason why it is more accurate. The idea behind taking the target motion over 4x 3m15s, instead of just 1x3m15s, is spreading the position error of the begin and endpoint over a longer time period. Let's say your watchofficer on the bridge gives a target position report. He gives distances in steps of 100 meters, and a bearing in steps of degrees. A degree at maximum visual distance of 16km is 280 meters wide. So the beginpoint of the target motion is somewhere in such a 100m by 280m box on the map. If you are lucky it is smaller when the target is closer. But doing periscope observations is much more inaccurate anyway. The target speed is a line connecting some point in the begin-box with the a point in the end-box, taken over the time period. There is a smallest distance connecting the inside borders of the boxes. But the speed can also be the largest distance, connecting the far edges of the boxes. The actual motion of the target is somewhere in between those lengths. If the interval is really short (theoretical 1 second) then the position uncertainty totally envellops the target motion track. If the interval is much longer, then the position uncertainty is only a small fraction of the target motion. A 6 knot target moves 600 meters in 3m15s. Two 280m position uncertainty boxes spaced appart by 600m would suggest a speed of something between 3.2 and 8.8 knots. That's quite a range. Conclusion: 3m15s is useless considering position inaccuracy in the plots. However, it moves 2.4 kilometer in 13 minutes. The uncertainty boxes are still of the same size, but are much further appart. Based on the uncertainty boxes, it could have a speed between 5,3 and 6.7 knots. Now you can be much more confident that it is actually moving with 6 knots! It could still be 5.5 knots, or 6.5 knots. But you certainly narrowed it down to something acceptable. Last edited by Pisces; 10-12-23 at 05:29 PM.

11-08-15, 01:55 PM	#6
Sailor Steve Eternal Patrol Join Date: Nov 2002 Location: High in the mountains of Utah Posts: 50,369 Downloads: 745 Uploads: 249	3 minutes is the proper time when using yards for distance. 3 minutes 15 seconds works for meters. The closer you are when you shoot the less it matters. __________________ “Never do anything you can't take back.” —Rocky Russo

11-08-15, 02:11 PM	#7
Aktungbby Gefallen Engel U-666 Join Date: Jul 2013 Location: On a tilted, overheated, overpopulated spinning mudball on Collision course with Andromeda Galaxy Posts: 30,058 Downloads: 24 Uploads: 0	WOW a worthy thread resurrection! A new fact I was not aware of and will certainly use! Thanks; the both of you! __________________ "Only two things are infinite; The Universe and human squirrelyness?!!

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12-09-12, 05:18 PM	#1
rcflyertim Seaman Join Date: Oct 2012 Posts: 34 Downloads: 74 Uploads: 0	Ahhh so it is a knots thing. OK so 3 min 15 sec x 4 = 13 min. If I wait 13 minutes I will miss my target! So I take it that these times are for long distance spottings?

12-10-12, 12:48 PM	#4
Pisces Silent Hunter Join Date: Dec 2004 Location: AN9771 Posts: 4,904 Downloads: 304 Uploads: 0	P.S. In my ramblings above I considered plotting the target motion while you are doing the end-around/overhauling/sprint-ahead at maximum range not to get detected. It's safe from a distance, and time well spent. You do not want to wait with measuring target speed at the very last moment before firing (like the notepad-method wants you to) when you are submerged. If you must, then the fixed-wire method is a better option. But just as a check. As it also requires ID of the ship's length, and good visual on the bow and stern. It takes in the order of a minute in most cases. You better not be turning, or looking to the side with your periscope when you are moving. Or it will be wrong. Speed is the most important value to get the torpedoes leaded onto the target. It deserves the most effort and time to get it right. And the least chance of mistakes made in a hurry.

11-08-15, 01:14 PM	#5
Kalamies Watch Join Date: Nov 2015 Posts: 24 Downloads: 33 Uploads: 0	I've seen people use both 3min and 3min 15sec rule. Is there really any difference between these two? If there is, which is more accurate in terms of estimating speed?