Working out a course to steer across the tide 5.

When plotting on the chart to find the correct course to steer, the best method of scaling down large diagrams is to do the following.

Instead of using a fraction of an hour as a scale, it is very simple to create a scale by using the measures on a ruler or navigation plotter.

The navigator just selects the best scale on the plotter (usually inches or centimetres) that will fit on the size of chart. Then when measuring distances, uses 1 inch as 1 M, or 1cm as 1M as appropriate.

If the tidal atlas is written up before starting the passage, all that is necessary is to find the correct page for the time in question, measure the direction of the tidal stream directly off the page, decide on the rate (dependant upon the tidal range for that tide), then transfer the plotter to the chart to draw the tidal vector, and measure off the distance with the ruler on the side of the plotter.

This approach will enable the correct course to be calculated very quickly, as it is not necessary to keep changing from the plotter to the dividers. With practice a course can be calculated in about 1 minute.

Advantages
Disadvantages
This approach is very quick to use as it is not necessary to change instruments. The plotter can be used for both angles and distances. 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.
Of the plotting techniques this is the fastest method and when practised requires very little effort to perform. It can be used for any period of time, not just for short passages.  

 

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