Antiacus, to answer your confusion about the 3:15 rule of thumb...
You mark your contact on the map. Start stopwatch and count 3 minutes 15 seconds. Immediately mark your contact's new position on the map. Get out the ruler tool and measure the distance between the first and second marks (or bearing lines if you used those instead).
The distance measured will be anywhere from 0.1-0.9 km (usually). Let's say you got a result of 0.5 km from a Large Merchant.
To convert this into knots you simply move the decimal one place to the right. Therefore, 0.5 km become 5 kts!
Of course, things such as the contact "zig-zagging" will make measuring speed this way prone to an error where you will tend to underestimate. In these cases, add 1 kt to your speed measurement of the target. Underestimating a contact's speed is the most common cause for torpedoes missing to the rear of the target.
Obviously, the most accurate way to judge a contact's speed is to parallel her course and match speed while you're at 90 deg. AOB.
The reason for counting 3:15 instead of 3:00 even is due to the difference between the measure of a nautical mile and a kilometer.
1 minute @ 1 kt a ship will travel 102 ft. or 31 meters (I think SH3 rounds this up to 33 meters. Someone correct me if I"m wrong.)
However, the smallest unit used on the SH3 nav map is 100 meters or 0.1 km.
1 nm = 1.15 mi. = 1.85 km
You likely already knew a lot of this from your USCG days, but math and physics in SH3 aren't always the same as in the real world.
Hope this helps you sink enough tonnage for you first 100k patrol on full manual!
