Fixed wire method

The fixed wire method is possibly the fastest way to accurately determining the speed of a target. In order to get an accurate result with the fixed wire method, it is necessary to know the length of the target accurately, and thus the target must first be identified. Once a ship has been identified own ship must either be stopped, or the bow must be pointed straight towards or away from the target, and an observation device such as a periscope or the UZO, with the central line just in front of the target. Keep a steady course, and do not move the observation device. Once the bow of the target crosses the line, start a stopwatch and stop it once the aft of the ship clears the line. Use the obtained information in the following formula to obtain the speed in knots.

$$\frac{Length*1,94}{time}=speed$$

It can be advantageous to measure the speed several times and use the average for a more accurate reading. Also note that, although theoretically possible, it is difficult to get an accurate reading at AOB far away from 90 degrees. The main reason being that it becomes difficult to tell where the forwards most and aft most point of the ship is, as they may be obscured due to the width of the ship. To a lesser extent the much slower motion across the wire makes it harder to accurately start and stop the stopwatch at the right time.

If you are sailing at a near parallel course to the target it is possible to use this method without stopping own ship. One simply adds own speed to the measured speed of the target. Note that if own ship speed is higher than the target, the target will appear to move backwards across the line. Measured speed must then be counted as negative to account for this.