Quote:
Originally Posted by maillemaker
If I were in a convoy, then, I would be changing my speed about every 5 minutes, to easily thwart this.
And, of course, this assumes that the sub, having watched its target sail by, can catch up with it again.
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Most merchants have a top speed significantly under that of a U-boat's max speed, and if you're crossing the Atlantic, you can't afford to waste fuel through continuous evasive maneuvers. Moreover, I made an error in that first post - it is by no means necessary to do fixed line from 90 degrees. You can do it from in front of the merchant/convoy, very, very small angles.
This is possible because the angle at which it is moving
does not affect the speed at which it crosses the line. There is no reason whatsoever to wait for a 90 degree AOB to do the fixed line method. If you don't want to catch up to it, take the AOB from in front, as it is coming TOWARDS you. You can take it as many times as you like and you'll still have several minutes before the ship passes you.
This is tough for some people to grasp, but it is true. You can do fixed line at 10 degrees AOB just fine, which means you can sit in front of it. If it starts speeding up, you can turn yourself and take the reading again. You can do this as many times as necessary, and you can do it while closing distance. Against a lone merchant, there is absolutely nothing that can be done to stop you. You can do fixed line right up until you're 300 metres away, and then a torpedo at fast speed is going to hit it pretty much no matter what anyway. It is by far and away the most accurate and reliable and immediate means of getting the speed. I can't recall the last time I outright missed a shot - usually if a ship doesn't sink, it's because the torpedo was a dud. Fixed line was also a favourite method of real-life U-boat commanders, for the simple reason that it works extremely well. Give it a shot, you'll be pleasantly surprised!
Speeding up and slowing down would work much better to thwart other methods - in the plotting example, if the ship moved slow/fast/slow/fast, you would wind up with an inaccurate reading based on the "average" speed. If it goes between 4 knots and 9 knots and you wind up figuring on 7.5 when it's actually gone back to 4 or 9, you'll miss, and you'll miss huge.
Since fixed wire method takes mere seconds instead of 3 minutes and 15 seconds, in order to thwart it within visual range, ships would have to be ordering a change in speed at least every minute or two. So no, speed changes every 5 minutes would not help you. Additionally, the fuel inefficiency caused by this would be unbelievable and wholly impractical (imagine driving across your continent going between 40km/h and 90km/h every 60 seconds), which is why this was not a commonly employed strategy in the actual Battle of the Atlantic. I can take a speed reading and fire a torpedo within 15 seconds of each other, if not shorter. So if you're a merchant doing 4 knots when I do my speed reading, you'd better hope you've throttled way up within 15 seconds (and since it takes a while for the speed to be gained anyway, you're probably still toast!).
Also, keep in mind that convoys can only go as fast as the slowest ship's fuel efficiency allows, and sometimes, that was very slow indeed (3 knots cross Atlantic was not uncommon on SC convoys).
Try the fixed line method out, and try it from a few angles - take multiple readings on one ship from multiple angles without getting spotted, and you'll see that it crosses the line in the same number of seconds if it is coming towards you, going away from you, or sailing right across your 000.