Working out a course to steer across the tide 4.

If you use the conventional approach to drawing a course to steer to allow for the tide, one of the complications you may encounter is that the drawing is too large to fit on the chart or too large for easy use of the plotting instruments.

The solution is to scale the drawing down. One of the means of doing this is to divide all the figures by 10.

The result is that you draw a diagram for 6-minute intervals. This means that a boat speed of 5kn, becomes a distance of 0.5M, and a tidal stream of 2.3kn, 0.23M. Obviously, the only maths required is to move the decimal place one digit to the right.

The diagram is then constructed in the usual way but for only six minutes. Of course, it is still valid for the next hour. The resulting triangle would be exactly the same shape as if it had been drawn with the distances travelled in 1 hour.

In practice any time period can be used to draw the diagram, provided the same period of time is allowed for one each side of the triangle. However, the 6 minute method is probably the fastest and least likely to cause basic maths errors.

Advantages
Disadvantages
This is very similar to what most people will have learnt in the classroom, if they have taken a theory course. Any technique should be practised before it is really needed. Practice this until you can correctly construct the diagram quickly and accurately.
It works over any distance, but is especially useful when plotting near to the edges of the chart or on large-scale charts. The navigator needs to work at the chart table.
For some people, the visual impression of the diagram can help to show what is happening to the vessel. It is not a very accurate technique, it will only give you an idea of the correct course to the destination and takes no account of errors in the tidal data.

 

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