How to plot a course to steer on a nautical chart

Okay so you’re getting ready to go sailing, your passage plan is done and it’s time to plot your course to steer, where do you start? Well of course these days you have GPS to provide your Course Over Ground (COG), however it’s essential to ensure you can work out where you are, and where you’re heading, should your GPS or other electronics fail.

You will need:

  • Up to date paper charts
  • Pencils and erasers
  • The boat log book
  • Course plotter
  • Almanac
  • Dividers

How to plot a course to steer on a nautical chart

  • Draw your ground track from start point (A) through and beyond your end point (B).
  • Measure the distance of your ground track from A to B using your dividers, measure this against the scale on your chart to determine the distance.
  • Estimate the time it will take your boat to travel from A to B, rounding the time to the nearest hour or half hour (T).
  • Check the times of high (HW) and low water (LW) on tide tables to establish a tidal ladder, which you can use to estimate your passage time, plus or minus HW.
  • Establish whether tide is running on springs, neaps or in between and use a tidal diamond to find the speed and direction of tidal flow. If you’re in between neaps and springs, remember to use the range and computation of rates chart to find the speed at which the tide is running.
  • Plot your tide stream from A to C.
  • Use your boat speed to establish the distance you will travel in Time, use your dividers to strike the distance from C to cross the ground track at D.
  • Note: you should never just join up C to B.
  • C to D shows your water track and your True course to steer.  
  • The True course to steer must then be corrected for leeway followed by variation and deviation (see our other blog).
  • Remember to note down your course to steer instructions in your logbook once you have communicated it to the helm.
  • If you need a course to steer over many hours then be sure to add each hour of tide to your course to steer.

Remember: sailors use magnetic headings when using a compass so when we are working with True degrees on a chart we need to convert this to a magnetic course to steer for our helm.

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Useful things to consider

It is a good idea to do your course to steer for the first leg of your passage before leaving the marina, based on your expected time of departure, tide and leeway. That way you don’t have to rush downstairs and do it once you get out of the marina.

Go sailing

Now it’s time to start sailing! Once you have departed the marina keep an eye on boat speed, marine weather conditions, tide and other factors that may mean you need to adjust your estimated arrival points at each waypoint. For example if you’re sailing at eight knots rather than five, due to an inaccurate wind forecast, you will reach your first waypoint faster and need to be ready to alter course early or adjust your course to steer. Always keep time of your route and be ready to adjust as needed.

About savvy navvy.

Created by sailors for sailors, savvy navvy is a startup working to build the best sailing app for sailors worldwide. Instead of using multiple apps, you can now do everything in one place. This is the first sailing navigation app to combine real-time marine weather and chart data to provide a comprehensive sailing route planner suited to your boat specifications.

We are working to develop and release new features all the time. Currently our sailing app provides your Course Over Ground (COG), once we implement tidal calculations we will then work to bring you a course to steer in the app for future updates along with marina recommendations, rain avoidance and even greater chart coverage.

Try our marine navigation app for free with demo sailing weather.

Happy sailing!

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