Originally Posted by Cadence
Don't feel alone. I'll admit my trig. is rusting with age and it's been awhile since I've had one. I would love a quick explanation.
Its a type of running fix.
You are sailing past a point of land/lighthouse/steeple/radio tower/silo, or other non moving landmark which is ahead.
Take a bearing with your trusty hand bearing compass
. Knowing your heading, and the bearing, use simple subtraction to calculate the angle off the bow.
Note your speed, or better yet your odometer reading (on your speed/log instrument).
Continue sailing along.
When the angle off the bow is DOUBLE the original angle, the distance run equals the distance off. Its an isosceles triangle....two sides have the same length. One side is the distance run, the other the distance off. They are the same. But no need to remember all that...just : when you double the angle off the bow the distance run equals distance off.
No its not magic, just trig. But its a handy trick. You don't need anything except your speed, your watch, and your compass
. Even if you aren't precise, you can get a pretty good fix without lifting a pencil. If you actually lay it off on a paper chart it will be surprisingly accurate. Especially if you can combine it with a depth
And just for you newbs who have only ever used GPS
...the most common "fix" requires taking bearings off three points. A running fix requires only one. Sometimes you only get one.