Determining the Ship’s Position Using Relative Bearings

Determining the Ship’s Position Using Relative Bearings Relative A relative bearing refers to a bearing taken on an object relative to the Bearings ship’s heading. They are measured from 000° through 360°. Example: If a ship is on course 090° T and a bearing taker shoots light "A", 020° relative, this means that light "A" is 20° to the right of the ship’s head. To convert relative bearings to true bearings, apply the formula SH + RB = (subtract 360 from T if over 360°). SH is ship’s head, RB is relative bearing, and T is the true bearing. When to Use Relative Bearings In almost all cases, relative bearing navigation will be used when a casualty occurs to the gyrocompass. There are several methods available for use to find the ship’s position using relative bearings. In this text, we will cover only the preferred method. Complete information on using relative bearings can be found in Dutton's. Procedure Use the following table to use relative bearings to fix the ship’s position. Rule: The helmsman must mark the ship’s head each time a round of bearings are taken; the bearing recorder must start a new column to record ship’s head data. Step Action 1. Direct bearing takers to shift to relative bearings using the outer ring of the pelorus; shoot a round of bearings. 2. Align the PMP ruler to the ships head. Example: Cse 200° T 3. With a sharp white grease pencil, mark compass deviation from the deviation tables onto the PMP scale. Remember + W - E. 4. 5. Example: For 5° W deviation, mark 205° on PMP scale. 205° is the Compass Cse the helmsman must steer. When you recommend new courses, use magnetic courses indicated by the grease pencil mark. Now mark the 180° and 0° on the PMP scale with the grease pencil. Use these marks to align the relative bearings. Plot the round of bearings using the 180° and 0° grease pencil marks. Figure 8-11 on the following page shows a PMP that is set up for relative bearing navigation. 8-22