plot a course to steer on a nautical chart

How to plot a course to steer on a nautical chart

Simplifying Sailing Navigation
Definition of course to steer
Plan a course to steer
How to plot a course to steer on a nautical chart
General advice for plotting a course to steer

Simplifying sailing navigation

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.

plotting a course to steer on a chart

What is the definition of course to steer?

A course to steer is roughly defined as the estimated course that a boat should steer in order to arrive at a specific waypoint, taking into consideration the effects that wind and tide will have on the vessel. 

In other words, simply aiming straight for your destination without having calculated a course to steer means you may not arrive where you expected to. This could be due to the tide pushing you off course and/or the force of the wind pushing you downwind (also known as leeway).

Understanding course to steer is a key component of safe boating navigation

definition of course to steer
The main theme behind our conversation today has been around unravelling some of the mystique behind trips out into the maritime and coastal environment and the more tools that people are able to pick up and use to help them understand that environment the better. Tools like the savvy navvy app, and the information provided through the application is unbelievably useful to help people on that journey.

Guy Addington, RNLI

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What do I need to plan a course to steer?

In order to accurately plan a course to steer you will need:

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

Alternatively, to save time you could try using savvy navvy ;)

course to steer - plan a CTS

The course to steer formula: How to plot a course to steer on a nautical chart

  1. Draw your ground track from start point (A) through and beyond your end point (B).
  2. 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.
  3. 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).
  4. 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.
  5. 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.
  6. Plot your tide stream from A to C.
  7. 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.
  8. Note: you should never just join up C to B.
  9. C to D shows your water track and your True course to steer.  
  10. The True course to steer must then be corrected for leeway followed by variation and deviation.
  11. Remember to note down your course to steer instructions in your logbook once you have communicated it to the helm.
  12. 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.

course to steer on savvy navvy

General advice for plotting a course to steer

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.

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 track of your route and be ready to adjust as needed.

You can always use the savvy navvy as your GPS chartplotter app

savvy navvy, the boating app that brings all essential marine information together in one place. Featuring global charts, wind and weather forecasts, tidal graphs, GPS Tracking, automatic weather routing, and marina and anchorage information. It’s like Google Maps for boats.

Inspired to get on the water? Start your free trial today.