This is the simplest of them all and very handy for not looking to the GPS
every 5 minutes.
6 minutes is 1/10th of and hour. So in 6 minutes your travel 1/10th of your speed in distance. Not handy at all unless you convert to yds. (again, we're using 2000yds - 1 NM.)
Doing 5 kts, so it 6 minutes we'll travel 1000 yds. (5 x 2000 = 10000 / 10 = 1000.)
So to skip the math, here's the rule
Six Minute Rule:
Speed in kts * 2, then add two zeros (shift the decimal two places) and you have the yards you'll travel.
8kts will be 1600 yds in 6 min.
3 kts will be 600 yds.
4.6 kts will be 460 yds.
Even easier is if you need to keep it in miles you're just moving the decimal.
6 kts will be .6NM in 6 min
Three minute rule
is the same but don't double the number. Just shift the decimal 2 places.
5kts = 500yds in 3 min.
4.5kts = 450 yds in 3 min.
Example: You're 3.2 miles from your harbor, you're doing 5 kts. Your wife wants to know "are we there yet". What's your reply?
3.2 miles at .5 NM each 6 minutes. 3.2/.5 is about 6 and a little bit (remember we're doing quick brain calculations here) 6*6 = 36 min.... say about 38 min.
3.2 miles = 6400 yds. 5kts means 1000 yds in 6 min, 500 yds in 3 min. So 6 six mins, and a little less than 1 three min. So just under 39 min.