Plotting a course over ground tidal nav boat plotting navigation course and plotting a course to steer worked example. Tide symbol and tidal streams as well as set and drift plotting calculations.
Navigating is setting a course to steer from departure point to destination and is known as shaping a course, and is a vital skill for safe navigation offshore. This section gives an example on how to plot a sailing course on a chart allowing for tidal effects and leeway.
When chart plotting a sailing course and positions, navigators use navigation chart plotting symbols doing chartwork in order to save space and avoid confusion. There are standard [ navigation chart plotting symbols ] but the navigator can devise his own if he wishes, as long as everyone else doing chart work on your boat understands the chartwork symbols and uses them consistently. Time is usually written using 24-hour-clock notation (e.g. 14.15) and should include the relevant time zone (e.g. 14.15GMT).
When shaping a sailing course, plot a line on the chart joining the departure point (A) and the destination (B). The navigation chart symbol is signified by marking two arrowheads on the line and called the [ Ground Track ] and is the course to be followed, over the ground, to the destination. Make a check to see that the course over ground does not pass over or near hazards or restricted areas.
An example of calculating the course to steer, first measure the length of the track using the dividers and transfer the length of the line to the latitude scale at the side of the chart. Divide this distance by the anticipated speed gives the expected duration of the passage and an estimated time of arrival (ETA).
The ground track marked on the chart is the course to steer, discounting the effects of tidal stream or leeway. The ground track direction is measured on the chart by using a parallel ruler or plotter to measure direction with the distance and bearing written alongside the line.
To allow for the effects of any tidal stream, obtain the set and drift of the stream from your chart or tidal atlas. [ Measure a line ] from point (A) along the direction of the tide's set using the tide's drift during the next hour to point (C). Mark the plotted line with 3 arrowheads .
Open dividers for the distance expected to sail in the next hour and from (C) intersect the Ground Track at (D) and plot. This gives the [ water track ], signified by a single arrowhead, denoting the course to follow through the water.
The boat moves relative to the seabed, along the ground track, while pointing in the direction of the water track. Not taking leeway into account, the bearing of water track is the course to steer. Measure its bearing using a plotter or parallel rules, and convert to °C prior to giving the course direction to the helmsman.
If sailing with the wind forwards of the beam, make allowance for leeway. Calculate the boat's leeway angle by taking a bearing on the boat’s wake using a hand bearing compass. Compare this bearing with the reciprocal of the compass heading by adding or subtracting 180° to or from the compass heading and this difference is the leeway angle.
Use these rules to apply leeway.
Adjust the water track to windward by the leeway angle which is typically between five and ten degrees. It varies according to hull shape, rig type and weather factors.
The adjustment is usually made by calculation rather than being drawn on the chart, but pencil in the wind direction adjusting the water track towards it to avoid confusion.
After allowing for leeway in order to plot the course, apply the corrections for variation and deviation to determine the compass course to steer in °C then give the course direction to the helmsman.
Many situations when plotting a course to steer, requires an allowance for a tidal stream and drift. A tidal stream parallel to the ground track does not affect the course, but affects the speed achieved over the ground. An example:
When there are no hazards near the track, use a simple [ plotting process ] for changing tides. When making tidal stream calculations: