<h4>COG and SOG</h4>
The Course Over Ground (COG) is the actual course the vessel will take over the seabed and is the true bearing of the first line we draw on a CTS plot, the ground track. This is the straightest route to our destination and this is the course we want to take over the seabed. This is the line with the 2 small arrows on it. Remember COG is always expressed as a true and never as magnetic or compass bearing.
<a href=”http://freedomsailingscotland.com/wp-content/uploads/2015/12/Fig-8.4.png”><img class=”wp-image-1765 size-medium” src=”http://freedomsailingscotland.com/wp-content/uploads/2015/12/Fig-8.4-300×233.png” alt=”Fig 8.4″ width=”300″ height=”233″ /></a> Fig 8.4 – We automatically compensate for the tide when we draw the arc the boat speed line from the end of the tidal vector to where it crosses the ground track. To get SOG we measure from the origin to the arced line where it crosses the ground track.
Students get confused with working out the SOG on course to steer plots but is really quite simple. We use the compass to measure one hours worth of boat speed and ‘arc’ this distance from the end of the tidal vector to where it meets the ground track we have plotted on the chart. The exact position of the arced off boat speed line can fall before our destination, on our destination or after our destination. This is all completely normal and is depended on the tidal vector. The point where the arc crosses the ground track indicates graphically how long the boat has travelled in that hour. So if we measure fro our origin to where the boat speed arcs the ground track, this is is our speed over the ground SOG (Fig 8.4).
If the distance from our starting position or origin on the ground track to where the boat speed line crosses the ground track is 5.5 miles and this has taken 1 hour, it means that the boat was traveling at 5.5 knots. Likewise if we had used two hours of tide and the arc was 11 miles and taken two hours to travel we are still traveling at a boat speed of 5.5 knots.
Please note that when we are using the Course To Steer, the COG and SOG can only be determined from measuring along the ground track and never along the water track.