EP position and how to estimate a dead reckoning position in nautical navigation and a boat log book and dr position.
Records of the course steered, distance run, times of course alterations, and the leeway experienced is used in plotting Estimated Position EP on the chart. This is done by the navigator at regular intervals of approximately half an hour to an hour, unless offshore. Through a process of determining a position through dead reckoning and refining it through calculating estimated position, a navigation position is plotted. If electronic fixing aids are available it is only by comparing an EP with a fix that errors are discovered and rectified.
The ships logbook is an important record and is kept up to date throughout the passage. Do not neglect log keeping or plotting EPs because of the expectation of a navigation fix from electronic aids.
The ships logbook is updated regularly to include distance run, course ordered and course achieved, estimated leeway, time of any course changes, and details of fixes. Barometer readings, wind direction and strength, and general comments help forecast the weather.
The ships logbook has two columns for Required Course and Actual Course as it is not always possible to steer the perfect course to an objective. When filling in the log, the helmsman is asked for an honest assessment of the average course steered.
The helmsman will be unable to steer a direct course if the destination is upwind so the course achieved is logged along with the time, new heading every time the boat tacks.
A Dead Reckoning DR position, when navigating, is plotted on the chart by drawing the course that has been steered from the last known position and measuring off the distance sailed.
If there is no tidal stream or current, this is the yacht's position at that time.
Plotting Estimated Position EP when navigating, requires applying the effects of any tidal stream or current to the Dead Reckoning DR position.
Once the first estimated position has been plotted and possible errors have been taken into account, use it to plot subsequent EP positions. Plot the water track since the last EP position then measure the distance sailed before plotting tidal set and drift.
A variety of reasons cause navigational errors in the estimated position when navigating:
It is imperative that navigation errors are allowed for when the boat sails without a confirmed position fix for any lengthy period of time.
Take into account a list of possible errors and estimate their maximum effect, focusing on the course that has been steered, distance sailed, leeway, and tidal information.
Any possible errors are pencilled in and are used to create a likely position. This approach is more realistic than assuming that the boat is at the single point suggested by the EP. For safety, shape the next course assuming that the boat is at the point closest to any nearby hazard.
Estimate possible errors in the course steered and the distance sailed recognizing a course error puts the boat either side of the track and a log error affects the distance along the track. Plot possible navigation errors to get an area of likely position.