Because the nautical mile is about 2000 yards in length, it is also more or less 6000 feet. Conveniently, there are 60 minutes in one hour, which means that in one minute, a boat travelling at one knot will have moved ahead by 100 feet. This gets better, because at any speed, you only have to multiply the number of knots by one hundred and that’s the distance you cover in a minute. If you lose a crew member overboard and hit the MOB button on the GPS, the casualty will be drifting away from the fix at 100ft multiplied by the tidal stream in knots every minute. If you haven’t got him back in 6 minutes with the tide given as 2 knots, he’s 1200 feet – almost a quarter-mile – from the fix already.