Bearing Methods for Speed
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"A to Z" Use of Tools in this File
DOWNLOAD FILE TO YOUR HARD DRIVE - FILE-DOWNLOAD AS-EXCEL
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Use of the tools in this file begins when first sighting the enemy.
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I. On first sighting, achieve constant bearing to compute target speed:
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Quickly eyeball AOB and turn the boat to roughly match the enemy's course.
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Ensure the TDC is set to accept bearings from the UZO.
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Pick one ship and observe through the UZO how its bearing changes, using a feature such as the funnel as a guide.
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Adjust speed in such a way that the bearing only slightly moves.
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(hint: use the "both engines 10 slower" EOT setting to achieve speeds between the main EOT settings)
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Adjust course in small increments (preferably away from target) and observe the bearing change after each adjustment
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until the bearing line doesn't move off that spot on the target for at least a couple minutes. This exercise may take 10-15 min
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but the information gleaned pays dividends.
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At that constant bearing, have the TDC operator read off the exact bearing to the 10th degree and input in tab 2 Speed Calculation
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section as Constant Bearing.
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Estimate as closely as possible the AOB at this bearing and have it entered into the TDC, and input into tab 2 Speed Calculation
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section as Original AOB.
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Read calculated target speed from tab 2 Speed Calculation section and have entered into TDC for reference.
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Enter own course into tab 2 Speed Calculation section and make note of the resulting target course. We will use this for comparison
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later in overtaking the target.
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II. Begin overtaking the target and plot to refine course/speed estimate. If already ahead of target, skip this section:
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Accelerate to desired speed and turn to begin overtaking the target.
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Once speed stabilizes, enter it in the Range Calculation section of tab 1 as Own Spd.
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Enter estimated target speed from tab 2 in the Range Calculation section of tab 1 as Tgt Spd.
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Find a reference point on the target ship and start the timer, having TDC read off the exact bearing and
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entering it into Range Calculation section, 1st Brg. Mark own position on map.
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After exactly 1 minute, recenter on reference point on ship and have TDC read off exact bearing and enter
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into Range Calculation section, 2nd Brg.
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Have TDC read off AOB at 2nd bearing, and enter into Range Calculation section, AOB @ b2.
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Read target range as calculated in meters at bottom of Range Calculation.
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Enter own course into Range Calculation section, Own Course, and use this target range and the 1ST true bearing
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calculated below it to start a plot, using own marked position on map. Plot the target's course from this point, marking off
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where he'd be at intervals of 5 minutes (or any desired interval) along his course based on the speed estimate.
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Observe the target at these time intervals, estimating AOB and comparing observed bearing and rough
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range to the plotted points to further refine target course/speed.
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Alternatively, you could also forego the Range Calculation section and simply perform a plot using ranges derived from
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AOB and target length. See the Plot tab - fill in the section for the correponding observation. The reason for keeping a
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record of previous observations is that, if and when you estimate a more-accurate AOB, you will need to go back and
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readjust the AOBs of the previous observations by the same amount, thus truing up your plot (because the ranges will
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change).
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Use the AOB estimated at intervals to further refine the enemy course estimate using the Attack Disk section
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of tab 2 (Course from AOB). This section has a "Difference" field which compares the new course to the one originally
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estimated. Adjust the Original AOB in the Speed Calculation section of tab 2 by the amount of the difference so the difference
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zeroes out. This "trues up" the initial speed estimate from the constant bearing method for the new AOB.
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If so desired, use this more refined course to adjust the AOB in the Range Calculation section to "true up"
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the initial plot point and adjust the plot for the change in the speed estimate, going back and forth
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in this manner further refining course and speed, if time allows.
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DO NOT FORGET TO UPDATE THE AOB IN THE TDC FOR ANY NEW ESTIMATES THROUGHOUT!
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III.Check/determine target speed while closing with target:
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Having achieved a position far enough ahead where the approach can begin, turn toward and start approach.
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As target becomes more clear, continue refining the AOB (and thus target course) using the Attack Disk section in
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tab 2 (Course from AOB), adjusting the Original AOB in the tab 2 Speed Calculation to further refine speed.
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When close enough to see the masttips clearly, obtain a range by entering mast height, scope marks and magnification
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in the Speed Calculation section of tab 1. Alternatively, if you've plotted his track using the 0 AOB method in the tutorial,
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read range off where the bearing intersects his track.
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Pick a ship and a reference point on that ship and perform the "1-minute" timing exercise as before, noting exact bearings
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and entering these along with own speed and AOB at 2nd bearing in the appropriate fields in the Speed Calculation section of
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tab 1 (adjust AOB as necessary if appears different from observation).
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Read off calculated target speed and enter into TDC (along with any new AOB estimate).
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As a triple-check (or double-check in case plot wasn't done), you can do the "fixed-wire" method of checking speed. Turn
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bow-onto target and time how long it takes for the target to cross the wire (irrespective of AOB), then calculate speed:
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Target length / seconds x 1.94
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The disadvantage here is the need to either stop or turn bow-on, which can be very impractical during an approach.
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IV. Shoot while minimizing gyro angle while turning:
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Every subsimmer has that "aha" moment eventually in their time while hunting high and low (some will get that joke, some won't)
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when they realize that, the smaller the gyro angle is at the time you shoot (i.e. the less the torpedo has to turn), the less range matters
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as a TDC input. At a 0 gyro angle, range has no impact to the solution at all. And so, historically, commanders strove to shoot at as close
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to a 0 gyro angle as possible to eliminate range as a factor. It is still a good idea to at least input a rough estimate of range into the TDC
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