The doubling angle on the bow fix is a useful way to find your location when you only have one marker or feature to fix from. It’s less accurate than fixing your position with two points, and your knowledge of your speed, any currents and wind affects the accuracy of the exercise, but when you only have one point to fix from, it can help you get a reasonable measure of your position. It’s something you might use on a long crossing to help fix your position when passing shoals or markers.
Take a bearing to a known feature or marker, and note the bearing and angle off your bow. Call this position point “A” and start tracking your speed. If you have a grease pen, note the bearing, time and estimated speed on your kayak’s deck. When the bearing to the feature or marker doubles, the distance between you and it is equal to the distance you have traveled from point “A.”
For example, you know you’re traveling at about 3 miles per hour and at point “A” your bearing reads 135, which, because you’re traveling on a course of 90 degrees, is a 45 degree angle on your bow. At point “B” your angle reads 180 degrees, which is a 90 degree angle on your bow. If it took you 20 minutes to travel from “A” to “B,” then you know the distance traveled was 1 mile, so the distance from “B” to the marker at “C” is 1 mile.
Another way you can double the angle on the bow without using a compass is to hold your hand out in front of you. Align your thumb with the bow and your pinkie with the feature or marker. After traveling a distance that forces you to span the distance between the bow and the feature with two hands, you’ve doubled angle.
For kayak and canoe navigation purposes, it’s much more likely that you’ll fix your position using two features, because you’ll have lots of features on the map to look at, but if you don’t, then this technique is worth using.