The differences between EP and CTS

The differences between EP and CTS

There are some essential differences between an Estimated Position and a Course To Steer that are worth mentioning.

The starting position

The chartwork symbols are conventions allowing others to understand your plotting, so they should be used appropriately at all times. The starting position for EP and CTS are similar.

Estimates Position (EP) Course to Steer (CTS)
Starting Position A confirmed position of any type (fix)

A previous estimated position

A confirmed position of any type (fix)

A previous estimated position

Remember to mark the starting position with:

  • The correct symbol.
  • The time when the vessel was at the position.
  • The log reading when the vessel was at the position.

From the starting position

Estimates Position (EP) Course to Steer (CTS)
From the starting position Plot the water track, i.e. the heading of the vessel corrected for leeway for the distance run. This provides a dead reckoning (DR) position. Plot the proposed ground track, i.e. from the starting position to, and beyond, the desired destination.

Plot the tidal stream

Remember to mark the plotted lines with:

  • One arrow for the water track on the EP.
  • Two arrows for the ground track on the CTS.
  • Three arrows for the tidal stream on the CTS.

The differences

 

Estimated Position (EP) Course to Steer (CTS)
Plot the tidal stream at the end of the water track, the DR position, to estimate where you actually ended up with the tide taken into account.

Plot a line from the starting position to the EP. This is the ground track (COG) and shows where the vessel actually travelled in relation to the hazards around it.

Draw a wind arrow near the water track. With an EP, leeway is applied first before any lines are plotted

The effect of leeway on a boat is to blow the vessel away from the wind. The wind arrow helps you recognise clearly whether the water track bearing needs leeway added to, or subtracted from, it.

 

From the end of the tidal stream(s) “ARC off” boat speed to see how far you’ll travel along the ground track when both tide and vessel motion are taken into account.

Draw a line from the end of the tidal vectors to where the ARC intersects the proposed ground track to determine the course to steer.

Draw a wind arrow near the water track. For a CTS it is applied after the last line has been plotted.

Steer the boat into the wind to counteract the effects of leeway. The wind arrow helps you recognise clearly whether the CTS bearing needs leeway added to, or subtracted from, it.

Summary

  • With an EP you are recording what has happened, i.e. it is historic, so first you sailed the course and while doing so the vessel was affected by the tidal stream. First plot the course sailed, then plot the tidal stream experienced.
  • With a CTS you are planning what will happen, i.e. it is predictive, so first you see where you want to go and then you need to determine how to counteract any tide that will prevent you getting there.
  • In the end of course assessments it is critical that you demonstrate you can differentiate between an estimated position and a course to steer. These are points of principle and if you get them wrong, e.g. put the tide at the wrong end of the plot or connect the end of the tide to the destination (i.e. you don’t use the ARC’ing method we’ve taught) in a course to steer plot, your paper is unlikely to pass. .