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Originally Posted by Nisgeis
From a static point, a ship steaming East at 5 knots at a range of 5,000 yards will have the same bearing change rate as a ship steaming East at 10 knots, at a range of 10,000 yards. This is a problem if you spend some time observing your target whilst running a parallel course.
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:hmm: I think that's the point of adjusting speed.
Say you judged your target to be 5,000 yards away. But it's a bigger ship and actually 10,000 yards away. Your approach officer plots the correct bearing but at the estimated range of 5,000 yards ... then a second plot gives him a speed of 5 knots.
Plugging the data into the TDC and engaging PK, the actual target will cross each successive bearing sooner than indicated on the TDC. So you increase target speed. Eventually, you'll cancel out the error generated by incorrect target distance ... although the geometry in the TDC won't necessarily look anything like the real approach.
A good way to test this is by plotting sonar. With map contacts on, mark the tip of one sonar line ... then do it again after about three minutes. Measure length of the sonar line and take an educated guess at target speed. Then insert range, bearing, AOB (target heading) and speed into the TDC. Turn on PK and track target bearing via sonar. If the target runs ahead of PK bearing, increase target speed. If it runs behind, decrease speed.
One good thing about this technique is that you can change target speed on the TDC without messing up any of the other data (changing range and bearing always changes target heading ... setting it to whatever heading corresponds to last-set AOB).
The bad thing is you can find yourself chasing the speed dial. If your target bearing is advancing slower than the TDC, you need to memorize a good speed, then dial down to near-zero and wait for the target to catch up ... then set the new speed. If your target bearing is advancing faster than the TDC, you need to insert a speed higher than required and let the TDC catch up ... then set the correct target speed. (Instead, you could, of course, adjust bearing directly ... but that would then throw off your estimated range and target heading.)