The GPS Broke. Now What?
Besides a good compass, a depth sounder is also useful; if it reads much higher or lower than the soundings near your estimated position on the chart, you're probably not where you think you are. Look out for landmarks and try to identify them, then compare rough bearings on the chart with your observations; check any buoys to see if current is sweeping you off course or if waves and wind are setting you to leeward. A quick and easy way to figure your position when you're within sight of a known landmark is to calculate a running fix.
To do this, take a bearing on a known object. Then, a certain time later, take another bearing on the same object. Here's how it's done in practice.
On a recent sail out of Newport, Rhode Island, I took a bearing on Point Judith light at 1233 and drew a line on the chart. That bearing line is called a "line of position." I knew that we were somewhere on that LOP, but I didn't know how far we were from the lighthouse.
Twenty minutes later, I took another bearing on the Point Judith light and labeled it 1253.
In those 20 minutes, we steered a course of 270 degrees magnetic and were traveling at 6 knots, making 1 nautical mile every 10 minutes. I then lined up the parallel rules on 270 degrees magnetic (using the inner circle of the compass rose on the chart) and moved them over to our lines of position. At 6 knots, we covered 2 miles, so I adjusted the dividers to 2 miles and moved the parallel rules along the bearings until the distance between the lines of position measured 2 miles; I drew another line there. Where the line intersected the 1253 LOP, I drew a circle and labeled that as our position at 1253.
There was a chance of error here: We could've been set one way or another by current or made leeway, our speed could also have been affected by current, or we could've had an inaccurate knotmeter. But if my bearings were good, we now had a reliable starting point: I knew for sure that we were somewhere on that second LOP at 1253 and somewhere on the first LOP at 1233. By adjusting the distance on the dividers and course on the parallel rules to correspond with the variables I'd observed, I could see what my margin of error is and take that into account in my subsequent course.