A running fix (often referred to as a “run fix” or “RFix”) is a navigational technique used to determine a ship’s position by using two or more lines of position (LOPs) taken at different times and advancing or retiring one or more of them to the time of another, thus obtaining a fix. Here’s how to perform a running fix:

1. Obtain the First Line of Position (LOP1):
   • Take a bearing to a known navigational aid (like a lighthouse, buoy, or other identifiable landmark).
   • Plot this bearing on your chart from the navigational aid. This creates a line on which you know your ship lies; however, without another LOP, you don’t know where along this line you are.
2. Steer a Known Course for a Known Time:
   • Note the time when you took the first bearing.
   • Continue on a steady course and speed.
3. Obtain the Second Line of Position (LOP2):
   • After a certain period (the longer the time between LOPs, the more accurate the fix, but the greater the chance of other factors like drift affecting the result), take another bearing to the same navigational aid or a different one.
   • Record the time of this second bearing.
   • Plot this second bearing on your chart.
4. Advance the First Line of Position (LOP1) to the Time of the Second Bearing:
   • Using your known speed and course, calculate your estimated position at the time of the second bearing.
   • Transfer (or “advance”) the first LOP (LOP1) to this estimated position. The advanced LOP1 and the second LOP (LOP2) will intersect, and this intersection is your running fix.
   • This is done by marking a point on the first LOP for your estimated distance traveled and drawing a parallel line to the first LOP from this point. This parallel line is the advanced LOP1.
5. Mark the Fix on the Chart:
   • Where the advanced LOP1 intersects with LOP2 is your running fix (RFix). This point represents your estimated position at the time of the second bearing.
   • Label this point with the time of the second bearing to keep track of when the fix was obtained.
6. Continue Navigation:
   • With your running fix established, continue to monitor your position using other navigational techniques and regularly update your position.

Remember, the accuracy of a running fix can be affected by several factors, including:

• Changes in speed or course between the two bearings.
• Errors in estimating speed or distance traveled.
• Set and drift due to currents.
• Compass or observational errors when taking bearings.

As with any navigational technique, it’s good practice to use multiple methods to confirm your position whenever possible.