Dead Reckoning

[Chas F]

Hi, All,

If one uses a windvane for overnight or single-handed cruising, how does one calculate his DR course?

Chas

[Jeff H]

Typically you still need to track your speed and direction as if you were steering. One way to do that is to set an alarm that goes off every 10 to 15 minutes and to record the average speed, course and time when the alarm goes off. That way you end up with a series of short legs that can be compared to the distance logged, and corrected for leeway and drift.

Jeff

[Chas F]

Jeff,

It's good to talk with you again. But course and speed readings every 10 to 15 minutes doesn't give a single-hander much sleep.

From what I understand about windvanes, as the weather backs, the boat backs with it. When the single-hander wakes from his sleep, he will look at his compass and know his heading. But how will he know how much time he sailed on each intermediate course in order to figure his current location? Or is he forced to assume a constant boat speed and constant wind/course change and calculate a big arc on the chart until he is able to make a sighting?

Are there any other options, short of using electronics, that cruisers can use to estimate a course when they get out of the bunk? How have cruisers typically handled this situation before GPS?

Thanks,
Chas

[GordMay]

Prior to GPS, BLUEWATER cruisers used Dead Reckoning to approximate their position (estimated position), and Celestial Navigation to get a little closer. Radio Direction-Finding, then LORAN gave some good nearer-shore information. Closer in, Depth Contours help determine distance off (shore).

We never knew EXACTLY where (on the open ocean) we were, until we made a visual fix on a known feature (ie: Navaid, or Geographic).

All of the above skillsets remain useful today.

Take a look at "The American Practical Navigator" ("Bowditch") On-line @
www.marineplanner.com/bowditch/bowditch.cfm
"Bowditch" as it is commonly known is the most comprehensive manual ever compiled on the art and science of navigation.

Safe navigating,
Gord

[Jeff H]

In reasonably clear weather 15 minutes is the length of time that it takes for a fast ship at speed to get from the horizon to running you over. 10 to 15 minute cat naps is considered the way to go for offshore single-handers so it is a matter of taking notes between naps. They give you a pattern that you then attempt to interpolate to the best of your ability.

Jeff

[Chas F]

Hmmm. I was going to use a radar detector and a radar with a collision alarm so that I could get restful, REM sleep on long passages. Maybe that's not going to work. Well, thanks for the input, everyone.

Chas

[exposure]

Singlehanding is a bit like rolling the dice with your life on the line. And, the closer to shore and shipping, the worse are your odds.

Off the beaten path, at sea, a noon site and DR based on 4 hour updates could be just fine. You might find you are off by 20 miles when you get a fix. Even with a noon site you get one LOP and have to use the DR. But at sea 20 miles is not a problem.

As you close land, you need to use multiple sights and keep a closer eye on the log and compass. That's what makes those stories of white knuckle landfalls so exciting! "There I was, looking for the pass, with nothing but crashing surf before me and 10 minutes sleep the last three days..."