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动画演示:
The first step is very straight forward.
Use the blue << and >> buttons to see the animation and stop it at the "End" frame to see the whole picture.
Given information.
Boat speed 5.5 knots.
Tide 260°(T). 1 knot.
Leeway 4° from a westerly breeze.
Variation 7° west.
Deviation 4° east.
Start point (fix) given in the animation.
Destination (waypoint) given in the animation.
Steps.
Plot the fix and waypoint.
Ground track.
Draw the ground track line, from the start point (fix) extended through the destination (waypoint).
Two arrows on the ground track identify it on the chart.
Work out the distance.
The distance to the destination is required to work out how long the journey will take and how much tide to allow for.
With a boat speed of 5.5 knots the trip in the graphic should take around 1 hour.
It is not necessary and prone to errors trying to work out the tides and the journey time for the exact distance. Work to the nearest half hour.
Estimated boat speed 5.5 knots. 1 hour at 5.5 knots = 5.5 nautical miles. A 1 hour vector will be satisfactory for this trip.
Work out the tides.
The direction and rate of the tide has to be found for the correct hour of travel by looking at the tide tables and then the tidal diamond or stream information.
For this lesson the tide is given as1.0 knots 260°(T).
Draw the tidal vector.
The tidal vector, identified by three arrows, is drawn from the start point on a bearing of 260°(T) 1.0 miles long
Plot the water track.
Using dividers measure 5.5 miles (boat speed) from the latitude scale on the side of the chart plot an small line, on the ground track, 5.5 miles from the end of the tidal vector. This is called the intercept. Plot the water track from the end of the tidal vector to the intercept. Please wait for the animation to load
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Using the plotter the true course to steer can be found from the water track.
Leeway, if applicable, variation and deviation.
To counteract the wind the the skipper will need to steer the boat towards the wind by the amount of leeway expected given as 6° from a westerly breeze.
Variation given as 7° west will be added to the true course to obtain the magnetic course.
Deviation given as 4° east will be subtracted from the magnetic course to obtain the compass course to steer.
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