Lateral Speed Method

Efshapo · 05-06-21, 09:07 AM

I just noticed that you had actually already replied this same answer on my video. And I also already thought you were talking about the constant bearing method, I'm so sorry.

Thanks again for you obvious expertise!

derstosstrupp · 05-06-21, 09:10 AM

Quote:

Originally Posted by Efshapo

I just noticed that you had actually already replied this same answer on my video. And I also already thought you were talking about the constant bearing method, I'm so sorry.

Thanks again for you obvious expertise!

No worries at all! German is famous for loooionnnnngggg words and the words for these methods look similar at a glance.

Efshapo · 05-06-21, 09:31 AM

BTW, there's another huge advantage in using the centiradians scale instead of turning the periscope, which is that you can use several chronos at the same time.

Of course, you can do it like I did in my video, measuring real and lateral speeds simultaneously. But you can also multi-check your measurements! I recently started to use one chronometer to measure lateral speed from -4 to +4 centirad, as in the video, but also from -5 to +3, and -3 to +5, which gives me 3 time values! I can then check that they are similar, and take the median one, or an average. I then do the same for real speed, using -1, 0 and +1 marks. It's a real gain in accuracy!

mobucks · 05-13-21, 08:03 AM

If you really did figure this out on your own, I tip my hat to you. Very interesting! Thanks!

ljqcn101 · 05-15-21, 01:24 AM

Pretty quick and useful method. Thanks.

Efshapo · 05-15-21, 06:26 PM

Quote:

Originally Posted by ljqcn101

Pretty quick and useful method. Thanks.

Thank you for your feedback!

Quote:

Originally Posted by mobucks

If you really did figure this out on your own, I tip my hat to you. Very interesting! Thanks!

I did, thank you!
I also developed a TDC simulator in Excel to measure the error injected by the False AOB 90 method.

VonHoffman · 05-05-22, 11:14 PM

Interesting technique.

I use something similar. What you're doing is instead of measuring bearing angle change by rotating the periscope, you are letting the target pass the more accurate graduations on the scope itself to yield a more accurate measurement.

The only issue I see is that your own ship needs to be stationary.

What I do is stop my boat, and time how long it takes for the entire visible length to pass the centerline of the scope. Regardless of AoB I know the target speed.

Example:

Target is directly abeam (90 degrees) and the target is 30 meters long, if it takes 10 seconds for the bow to stern distance to pass the centerline of my scope, then the target is travelling at ~ 6 kts.

If the same target is at relative bearing 120, and the AoB of the target is 30 degrees, how long does it take the ship to cross the centerline of the scope? IT STILL TAKES 10 SECONDS!

It appears to be moving slower, but the overall length of the ship still moves 30 meters in 10 seconds! As I'm measuring it crossing bow to stern across a fixed point, I can still get a good fix on its speed. Also, you can use any magnification you like to get a good view for the measurement as you only require the ship to pass a fixed mark - the center of the scope.

To get accurate AoB, I can then cheat:

If I know its range (let's say 10 hm), I know that a 30 m long ship should take 3 centiradians if it is at 90 degrees AoB. If I only see 1 centiradian (or 4 centiradians at 6x), its AoB is asin(1/3) = 20 degrees.

To parallel its course and remain in the same relative position:

180 - 120 = 60 off my stern
90 - 60 - 20 = 10 degree AoB less than I should see
I need to turn left 10 degrees and run at 6 kts to maintain relative position.

If I was already parallel to his course and he's 60 degrees off my stern I should have an AoB of 90 - 60 = 30 degrees. In the example it's not, so I must correct his relative bearing for the AoB seen.

SnipersLaww · 02-20-25, 11:37 AM

Quote:

Originally Posted by Efshapo

Thank you for your feedback!

I did, thank you!
I also developed a TDC simulator in Excel to measure the error injected by the False AOB 90 method.

man.. i would love to be able to have a graph/display like that in my spreadsheet calculator that ive been building. im still learning the program and formulas and such though so just completing the calculators alone has been rough but its nearly mostly done. a graph display is just a bit too advanced for me yet.

05-15-21, 01:24 AM	#5
ljqcn101 Sailor man Join Date: Jan 2013 Location: Hong Kong Posts: 50 Downloads: 78 Uploads: 3	Pretty quick and useful method. Thanks. __________________ Automatic TMA calculator (metric system) Automatic TMA line detector and solver for DW/SC/688i Automatic TMA line detector and solver for maneuvering target (new)

05-05-22, 11:14 PM	#7
VonHoffman Seaman Join Date: Oct 2019 Location: Near crush depth. Maybe. Posts: 33 Downloads: 6 Uploads: 0	Interesting technique. I use something similar. What you're doing is instead of measuring bearing angle change by rotating the periscope, you are letting the target pass the more accurate graduations on the scope itself to yield a more accurate measurement. The only issue I see is that your own ship needs to be stationary. What I do is stop my boat, and time how long it takes for the entire visible length to pass the centerline of the scope. Regardless of AoB I know the target speed. Example: Target is directly abeam (90 degrees) and the target is 30 meters long, if it takes 10 seconds for the bow to stern distance to pass the centerline of my scope, then the target is travelling at ~ 6 kts. If the same target is at relative bearing 120, and the AoB of the target is 30 degrees, how long does it take the ship to cross the centerline of the scope? IT STILL TAKES 10 SECONDS! It appears to be moving slower, but the overall length of the ship still moves 30 meters in 10 seconds! As I'm measuring it crossing bow to stern across a fixed point, I can still get a good fix on its speed. Also, you can use any magnification you like to get a good view for the measurement as you only require the ship to pass a fixed mark - the center of the scope. To get accurate AoB, I can then cheat: If I know its range (let's say 10 hm), I know that a 30 m long ship should take 3 centiradians if it is at 90 degrees AoB. If I only see 1 centiradian (or 4 centiradians at 6x), its AoB is asin(1/3) = 20 degrees. To parallel its course and remain in the same relative position: 180 - 120 = 60 off my stern 90 - 60 - 20 = 10 degree AoB less than I should see I need to turn left 10 degrees and run at 6 kts to maintain relative position. If I was already parallel to his course and he's 60 degrees off my stern I should have an AoB of 90 - 60 = 30 degrees. In the example it's not, so I must correct his relative bearing for the AoB seen. Last edited by VonHoffman; 05-06-22 at 04:12 PM.

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05-06-21, 09:07 AM	#1
Efshapo Watch Join Date: May 2021 Location: France Posts: 30 Downloads: 3 Uploads: 0	I just noticed that you had actually already replied this same answer on my video. And I also already thought you were talking about the constant bearing method, I'm so sorry. Thanks again for you obvious expertise!

05-06-21, 09:31 AM	#3
Efshapo Watch Join Date: May 2021 Location: France Posts: 30 Downloads: 3 Uploads: 0	BTW, there's another huge advantage in using the centiradians scale instead of turning the periscope, which is that you can use several chronos at the same time. Of course, you can do it like I did in my video, measuring real and lateral speeds simultaneously. But you can also multi-check your measurements! I recently started to use one chronometer to measure lateral speed from -4 to +4 centirad, as in the video, but also from -5 to +3, and -3 to +5, which gives me 3 time values! I can then check that they are similar, and take the median one, or an average. I then do the same for real speed, using -1, 0 and +1 marks. It's a real gain in accuracy!

05-13-21, 08:03 AM	#4
mobucks Seasoned Skipper Join Date: Jan 2010 Posts: 713 Downloads: 209 Uploads: 0	If you really did figure this out on your own, I tip my hat to you. Very interesting! Thanks!