Compass And Sextant Bearings

In the past two installments of Bye, Bye Birdie we have used onboard electronic tools to derive our position in the absence of GPS.

- Robert Reeder

For this column, we will get back to the basics of paper charts and conventional tools and some navigation fundamentals which are often glossed over in books written for larger vessels—visual bearing fixes.

What follows are three different methods of obtaining a fix from visual bearings to three objects on shore. For these examples I used three prominent landmarks around downtown Seattle. For expedience I took these sights from the small jetty park at Duwamish Head in West Seattle. As it is not possible to determine in any meaningful way what the magnetic deviation will be for a hand bearing compass on a small boat, I will also not account for the magnetic deviation imparted from the concrete and steel construction of the jetty. As we will see, this is quite significant, and reasonably simulates the magnetic deviation you would encounter on board your own yacht.

My three objects for both fixes are the Smith Tower, the Space Needle, and the middle tower of the three television towers on Queen Anne Hill.

Visual Fix with a Hand-Bearing Compass

For the first fix we will use a standard “hockey puck” style hand bearing compass. It is important to note that many books on navigation (and most USCG licensing exams) presume that the navigator is using an alidade mounted on the ship’s steering compass to obtain visual bearings. While this arrangement allows for excellent information on the magnetic deviation affecting the visual bearings, rarely are steering compasses on small boats mounted in such a way as to make this possible.