The 7/8ths rule, or doubling the bearing

Here's an easy exercise, but it presumes you are holding a steady course and speed.

Pick a point on land that you are going to pass abeam.

Note the time in minutes and seconds when it is 30-degrees off the bow.

When the point is 60-degrees off the bow, note the time again.

Subtract the first time from the second time, and calculate the distance accomplished. Simplified example: An exactly ten minute run at 18 knots will cover 1/6 of 18 nautical miles or three nautical miles.

The distance run between the 30-degree reading and the 60-degree reading will equal the distance to the point on land at the position of the 60-degree reading.

To know how close the boat will pass to the point of land when it is directly abeam (90 degree relative bearing), multiply the 60-degree reading by 7/8.

That's an old tried and true piloting trick but I prefer to use my radar, much quicker and probably more accurate.

Rick

Check out some of the GPS units available today, it should answer your question.

http://www.gpsonsale.com/MarineGPS/Marine-Index.htm

Assuming you have a chart, always a good idea, use the GPS to get your lat/lon, plot your lat/lon position on the chart, and measure from there to the point of land. Old fashioned navigation 101, sort of in between Chuck's piloting trick and the present state of the art.

Distance off by radar goes back a really long way however, for those who had it onboard. There was an intermediate state of technology with the early Loran-C units that read out in "lines of position" (LOPs), instead of in Lat/Lon. To use LOPs you had to have a chart with the lines overprinted on the chart detail. Each station had its own set of lines, and with a different designator. A a minimum of two stations was required to get a fix. It all seems very backward these days but it was a huge step forward at the time (late 70s/early 80s). For once we could actually figure out exactly where we were to a reasonable degree of accuracy, even in the fog. Prior to that a lot of boats were using aircraft VOR equipment which was OK along the coast but not for too far. Loran-A was available also but the equipment was power hungry and difficult to use, especially at night with sky wave reflections.

After multiplying the distance determined at the 60-degree bearing by 7/8 and getting the distance set off at 90-degrees relative bearing, set the dividers for that distance and describe a short arc where it would seem to make sense for the boat to be located given the distance from the point ashore. Somewhere in that arc is your position. To find out exactly where, more the heading line to a point where it just touches the arc (if the heading line passes through the arc it has been moved too close to land). This "two-point" fix will be surprisingly accurate in most circumstances, and shoud prove adequate for casual coastal navigation.

Yeah, it's easier to use a GPS plotter. :-)

But thanks, Rick

Yee Ha! Now that was navigation kiddies, and we were only a mile off after sailing for 36 hours. Close enough for government work as we used to say.

As we're returning to Marblehead, we could hear the Eagle Island Shoal horn off the port beam and the harbor bell off the starboard. Hey we're golden - we know exactly where we are, took a compass bearing and headed into the harbor. Cranked the revs on the engine up a little - whoo hoo!!

Just about then this really big gray with guns and radar and sqiuds hanging off the rails. The Navy, in it's infinite wisdom, decided to arrive early for the July 4th celebration which I proceeded to have a near miss with.

Damn squids.