[Svonne]

Three Minute Rule

The Three Minute Rule (which, more accurately, should be called the Three Minute Fifteen Second Rule) is a simple way of determining your target speed

Introduction

By measuring the distance a vessel has moved over an interval of three minutes and fifteen seconds, it is possible to easily determine the speed. Of course, any measure of time and distance can be used to determine a target's speed, but the Three Minute Rule has the convenient advantage of equating a tenth of a kilometer to one knot of speed.

For example:

If over 3 minutes, 15 seconds the target moves 400m (0.4km), then the target speed is 4 knots.
If over 3 minutes, 15 seconds the target moves 1500m (1.5km), then the target speed is 15 knots.

In essence, take the distance travelled in that time in km, multiply by ten and that is the speed in knots.

The math behind it:

1 knot = 1.852 kilometers per hour
3 minutes 15 seconds = 0.0542 hours
.0542 hours * 1.852 kilometers per hour = 0.1004 kilometers =~ 1 knot

Example

This example assumes that the No map contact update realism setting is not enabled; that is, map updates should be turned on.

The U-boat captain has spotted a small tanker at about 45 degrees off the U-boat's bow. Immediately he starts a plot to determine the target's course and speed.

Zooming in on the navigation map (F5), a mark (Mark 2 in the example) is plotted exactly against the bow of the target.




Making the initial mark for the plot


Immediately after making the mark, the view is switched to the Attack Periscope (F3) and the Stopwatch is started (outlined in red) by clicking once on its face.

Starting the stopwatch to begin the count

The caption then lowers the periscope to avoid detection, and waits until 3 around minutes. During this time, if you have already identified the target, you can set up your torpedo pistol and depth information ready for the shot and maneuver the U-Boat to a more favourable angle if necessary.

Just after 3 minutes have elapsed, the captain returns to, and raises, the periscope in preparation for the next mark. Note the display on the stopwatch.

Raising the periscope just before 3 minutes and 15 seconds has elapsed

At exactly 3:15, a second point on the map is made, showing the precise point of the target's bow at that time. It can be helpful to switch back to the navigation map a few seconds before 3:15, and manually count the remaining time to ensure that the mark is made as close to exactly 3:15 as possible.

Making the second mark for the plot

Just before lowering the periscope once more, the captain may take a bearing to the target, to assist in calculating AOB. He now has all of the information required to calculate the target speed and course.

Back on the navigation map, use the Ruler tool to draw a line between the two points we have marked. If you wish to also determine the course, ensure that the extra compass display is turned on by clicking the '?' at the top-right of the screen if necessary.

In the example, the distance between the points is 0.7km. Due to us timing over exactly 3 minutes and 15 seconds, we can simply multiply by ten to calculate the speed in knots. In this case therefore, the speed is 0.7 x 10 = 7 knots. The target course can be determined by using the compass display. Always look at the point where the "tail" of the vessel crosses the bearing ring. In this case, the target is heading down the page, therefore the "tail" is crossing at 180°.

We have successfully calculated our target course and speed: 180° at 7 kts.

Determining the speed and course