Calculating ships speed

[keith_uk]

To calculate a ships speed, i place a mark on the map where the ship is, then set the stop watch going for one minute or more, then mark its position again.
I then measure the distance in yards the ship covered divided by the number of minutes its taken X 60 (seconds) =, then divide that figure by 1760 (which is how many yards are in a mile and that gives me the speed in mph.

Is there a way to calculate the speed in knots? I was wondering if a few seconds less than 60 seconds might work? if so, how many seconds?

Thanks,

Keith.