What does the 3-minute rule help determine?

The 3-minute rule is a practical tool used in navigation to estimate the speed of a vessel in knots. This rule derives from the fact that if you measure the distance traveled over a period of three minutes, you can quickly convert that distance into knots, which is a measure of speed.

To apply the 3-minute rule, you determine how far the vessel travels in that three-minute timeframe and then multiply the distance in nautical miles by 20. This multiplication reflects the fact that three minutes is one-twelfth of an hour, thereby providing a direct calculation of speed in knots (nautical miles per hour).

For instance, if a vessel travels 0.3 nautical miles in three minutes, multiplying 0.3 by 20 results in a speed of 6 knots. This handy rule aids mariners by allowing them to gauge their speed without needing complex calculations or instruments. Understanding this rule is crucial for effective navigation and for making timely decisions based on a vessel’s speed.