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. 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 navigation position (DR) is plotted on the chart by drawing the course steered from the last known position and measuring off the distance sailed.
- If working in °M on the chart, correct the course steered (°C) for deviation before plotting it and if working in °T, also correct for variation.
- The Dead Reckoning navigation position is plotted after correcting for deviation and variation then applying the leeway correction to leeward of the course steered.
- On the chart, plot the corrected course steered marking it with a single arrowhead to signify that it is the water track.
- The distance sailed according to the log, is measured off from the last known position and this Dead Reckoning navigation position is marked with a cross along with a time record and log reading.
If there is no tidal stream or current, this is the yacht's position at that time.
Plotting Estimated Position requires applying the effects of any tidal stream to the DR position.
- Work out the set (direction) and drift (distance) of the tide since the last known position using the chart or tidal atlas.
- Plot the set of the tide at the DR position by drawing a line on the tide’s bearing and mark it with three arrowheads.
- Tidal sets are shown in °T, so apply the variation if chart work is done in °M.
- Measure the amount of tide along the tidal line using the dividers taking the distance from the latitude scale on the chart.
- The EP point is marked with a triangle with a dot in the middle along with a time record and log reading alongside.
Once the first estimated position has been plotted and possible errors have been taken into account, use it to plot subsequent EPs. Plot the water track since the last EP then measure the distance sailed before plotting tidal set and drift.
A variety of reasons cause navigation errors:
- The difficulty of steering an accurate course,
- Log readings in different conditions,
- Uncalibrated instruments, or
- An inexperienced helmsman.
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 navigation errors and estimate their maximum effect, focusing on course steered, distance sailed, leeway, and tidal information.
Any possible navigation 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 navigation 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.