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Five minute briefing 

Waypoints or dividers - what’s the best course to steer?

What method should we use to calculate the route between two places allowing for stream? Do we need dividers and a plotting instrument to set out a Course to Steer (CTS) or can we let the GPS or a Chart plotter do the work and use a Bearing to Waypoint (BTW)? Let’s start by having a look at the differences between CTS and BTW.

A Course to Steer (CTS) is a course between two points constructed by taking into account the vessels speed, the stream and leeway.

A Bearing To Waypoint (BTW) is a bearing gained by inserting a Latitude and Longitude position into a GPS or Chart plotter as a waypoint and using the resultant calculated bearing from your present position.

The obvious difference is that the CTS allows for stream and leeway, whereas the BTW usually does not.

Therefore whilst the boat may still get to the waypoint in a cross stream it will not travel along the shortest route, the course will constantly change, it will take longer and the final track may pass over unforeseen dangers.

But all is not lost as there is a way to allow for stream and leeway and still use a waypoint. If the Course Over Ground (COG) information from the GPS is used it will help keep the boat on course.

The COG takes into account factors affecting the boat such as stream and leeway and just states what is actually happening. If the boat is heading North and the wind and stream are coming from the West, the COG will be towards the North East. Therefore COG simply tells the truth.

So to allow for stream and leeway using a GPS, once the boat is underway and steering towards the waypoint, adjust the course so the BTW and COG match up. The boat will then travel along the BTw and this will also be it’s groung track allowing for stream and leeway. This real-time CTS takes into account the actual streams and leeway on the day and not those predicted in the tidal stream atlas. This method is a useful over short periods or where the stream rate and leeway are not going to change appreciably.

When using COG and BTW, it is a healthy practice to plot your COG to ensure it does not cross over any dangers that are lurking between your position and the waypoint.

It is even simpler when using a chart plotter if the vessel heading markers are activated. Simply adjust your heading (red line) to get the desired Course over Ground (green line).

However, every navigational method has its time and place and there are times when matching the COG to BTW will not be effective. For instance, when crossing the English Channel with its changing rates and directions of stream, every time the stream changes, so would your compass course.

So over short distances or when the stream rate and leeway stay roughly the same the GPS can be used to get the information so long as the Course over Ground is matched to the Bearing to Waypoint.

But when allowing for differing streams it is best to calculate a Course to Steer manually.

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