Bow Beam Bearings

Remember sitting in math class and wondering what use you would ever have for knowing about right triangles? Well you can use the theory of right triangles to help navigate. You may want to print out this section and add to your navigation book.

What if you wanted to know how far offshore you were and also wanted to fix your position? In order to do that all you need to do is to take a bow and beam bearing to a fixed object onshore. This is done by taking two bearings on this fixed object at different times.

The key to the bow beam bearing is to make sure that you have formed a right triangle by taking the bearings at 45º relative to the bow and again at 90º relative if the object is to starboard. If the object is to port you would take bearings of 315º relative and 270º relative.

To determine your position using a bow beam bearing as shown in the example above, let’s assume you are cruising along the coast and you site a lighthouse ahead to starboard.

  1. Take the first “bow bearing” when the light house is at 045º relative
  2. Note the time, 0900 and your boat speed, 10 kts.
  3. At 0930 you have the lighthouse dead abeam at 090º relative.
  4. Using your DST formula, calculate that you have traveled 5 nm in the 30 minutes since the “bow bearing”. (10 kts X .5 hrs = 5 nm)
  5. This distance traveled (5 nm) is equal to the distance off the light house.
  6. To plot your location, draw a 090º relative LOP on your chart to the lighthouse. Using your dividers, measure the 5 mile distance from the lighthouse along the LOP and you have your fixed position. You can check your depth sounder and the depth shown on the chart for additional verification.

Test your new-found knowledge of Bow Beam Bearings.

Using your 1210 TR training chart solve the following problem:

You are cruising about 2 nm south off the south shore of Martha’s Vineyard in the vicinity of Tisbury Great Pond on a course of 262º True. Your boat speed is 5 kts. At 0930 you observe the Radar Tower on Martha’s Vineyard at 045º relative.
What is the true bearing to the Radar Tower?
What will be the true bearing when the Radar Tower is directly abeam?
At 1001 the Radar Tower is directly abeam. Remember that the distance traveled using your DST formula equals the distance off the object at the time of the second LOP.
What was the distance traveled?
What is your 1001 position?
What is the charted water depth?

Leave a Reply

Your email address will not be published. Required fields are marked *