Seamanship_Secrets_185_Tips_-_Techniques_for_Better_Navigation-_Cruise_Planning-_and_Boat_Handling_Under_Power_or_Sail_(Re)_e..

72 seamanship secrets

There are five essential ingredients for hassle-free dead reckoning:

1. Known position. All navigation begins from a known position, whether
   a buoy, a jetty, or any other means of positive identifi cation.


2. Trackline. A trackline, labeled TR, extends from the known position in the
   desired direction of travel. Plot the TR boldly to make it stand out from
   other plotted lines. We hope our boat stays on the TR, but forces such as
   wind and current, or our own steering errors, can move the boat off the TR.
   The connection between your boat and its TR changes all the time.
   At any given time, your boat is in two of six relationships to its trackline:

on TR

to the right of TR

to the left of TR

moving at its predicted speed

moving faster than expected

moving slower than expected

Once you have established your boat’s relationship to its trackline, you can

decide on a course of action. In some cases, being off the TR isn’t at all

critical, and the boat can continue in safety. At other times, being off the

TR by a few yards might place the boat and crew in danger.

3. Steering course (C). Th e steering course, or courseline, is the course steered
   by the helmsman. Sometimes wind, current, or steering error can move a
   boat off its trackline. Th is means you might very well have to steer a diff erent
   course to return to the trackline. In Chapter 5 we’ll fi nd out how to tell this
   and how to correct for it. If the TR is 90 degrees but you are steering
   85 degrees to compensate for a south-setting current, you plot two lines on
   the chart. Draw the trackline boldly and label it TR—090oM. Th en draw the
   85-degree courseline with a lighter solid or dashed line and label it C—085oM.


4. DR positions require time, speed, and distance. From the last known
   position, plot dead reckoning positions along the trackline (or tracklines)
   being steered. If you are steering the TR, plot DRs along the trackline.
   Otherwise, plot them along the steering course.

Time: Pick a time interval based on your proximity to danger or

uncertainty of position. Within a mile or two of the coast, use a

frequency of every 15 or 30 minutes. Off -soundings (when you are

off shore), use an interval of an hour or longer.

Speed: DR speed is usually taken from speed tables, a knotmeter, or a knot

log. Vessels under power can use a speed/RPM table (see Chapter 3). Vessels

under sail should use the knotmeter or knot log and take an average

of readings. As mentioned in Chapter 3, the speed these methods