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BigWalleye · 03-21-13, 01:26 PM

First, the three-minute rule, (distance traveled in 3 minutes)/100 = speed in knots, is correct – provided the distance is measured in yards. One nautical mile equals 2025 yards. So a ship which travels 100 yards in 3 minutes travels 2000 yards (60/3*100) in one hour. This is accurate to about 1%, good enough for a firing solution.

But a nautical mile is only 1852 meters. A ship which travels 100 meters in 3 minutes travels 2000 meters in an hour, or about 1.1 knots. So you have to multiply the 3-minute travel distance in meters by 1.1 (or just add 10%) to get the speed in knots. This answer is accurate to within 2%, again good enough for a firing solution.

Now, regarding your request for a more accurate ruler, the ruler (I actually use the compass, but the scale is the same) will measure to the nearest 50 meters. This is within plus or minus 25 meters of the true value. At 10 kt, that is a 2.5% error, not bad, but if that 2% rounding error from above is in the same direction, that could be a 4.5% error in calculated speed. It’s worse for a 5 kt target speed (7%) and better for 15 kt (3%).

Now. let’s consider the effect of a 4.5% speed error. That means the true speed will be 9.5 kt rather than the 10 kt we measured. For a 44 kt torpedo fired with zero gyro angle normal to the target, the correct deflection will be 12 degrees instead of 13 degrees. (13 degrees is the correct deflection for a 10 kt target.) At 1000 meters, a 150 meter long merchant ship will subtend an angle of about 9 degrees. Plenty of room for a 1-degree error there. At 2000 meters, the ship covers 4.5 degrees, still pretty comfy. At 3000 meters, we’re down to 3 degrees, so a 1 degree error could be significant, although 2 fish should ensure one hit.

So, unless you are firing at rather long ranges, the stock measuring tools are more than adequate. And so is a 3-minute speed calculation, provided you remember to add that extra 10% when you are measuring in meters.

Gluck auf!

03-21-13, 01:26 PM	#4
BigWalleye Sea Lord Join Date: Jul 2012 Location: On the Eye-lond, mon! Posts: 1,987 Downloads: 465 Uploads: 0	First, the three-minute rule, (distance traveled in 3 minutes)/100 = speed in knots, is correct – provided the distance is measured in yards. One nautical mile equals 2025 yards. So a ship which travels 100 yards in 3 minutes travels 2000 yards (60/3100) in one hour. This is accurate to about 1%, good enough for a firing solution. But a nautical mile is only 1852 meters*. A ship which travels 100 meters in 3 minutes travels 2000 meters in an hour, or about 1.1 knots. So you have to multiply the 3-minute travel distance in meters by 1.1 (or just add 10%) to get the speed in knots. This answer is accurate to within 2%, again good enough for a firing solution. Now, regarding your request for a more accurate ruler, the ruler (I actually use the compass, but the scale is the same) will measure to the nearest 50 meters. This is within plus or minus 25 meters of the true value. At 10 kt, that is a 2.5% error, not bad, but if that 2% rounding error from above is in the same direction, that could be a 4.5% error in calculated speed. It’s worse for a 5 kt target speed (7%) and better for 15 kt (3%). Now. let’s consider the effect of a 4.5% speed error. That means the true speed will be 9.5 kt rather than the 10 kt we measured. For a 44 kt torpedo fired with zero gyro angle normal to the target, the correct deflection will be 12 degrees instead of 13 degrees. (13 degrees is the correct deflection for a 10 kt target.) At 1000 meters, a 150 meter long merchant ship will subtend an angle of about 9 degrees. Plenty of room for a 1-degree error there. At 2000 meters, the ship covers 4.5 degrees, still pretty comfy. At 3000 meters, we’re down to 3 degrees, so a 1 degree error could be significant, although 2 fish should ensure one hit. So, unless you are firing at rather long ranges, the stock measuring tools are more than adequate. And so is a 3-minute speed calculation, provided you remember to add that extra 10% when you are measuring in meters. Gluck auf!