On our way back around the Scilly Isles (03:00, in the dark, no moon, the watch all a bit sleepy) we had a rather close encounter which came about from not paying close enough attention to the discrepancy between our course and our course over ground (COG). Our helm was steering 100° on the compass, but our course over ground was 060 degrees (a big difference!). This meant we were tracking much further north than we intended, and straight towards some rocks. Thankfully we corrected in good time, but it served as an important reminder that “direction” has many different meanings when sailing, all of which must be paid close attention to and care taken to using the right terms at the right time. “What’s our direction, helmy?” is not a good question to ask on a boat.
So what terms are used, and what do they mean?
You know this already, so just to re-cap. Directions are referred to as degrees of rotation in a circle, counting 360° clockwise from north, so:
- North (N): 0° = 360°
- East (E): 90°
- South (S): 180°
- West (W): 270°
Course: The direction the boat is intended to be steered.
Heading: The direction the boat is actually pointing. This is very seldom exactly the same as the course (either the helm is a bit wobbly, or you’re pushed off course by a wave for example).
Course over ground (COG): The direction the boat is actually moving over the Earth’s surface. The factors which cause a discrepancy between course and COG are wind and tide. In our case, there was a local tidal race as we rounded the southern tip of the island. COG is sometimes also called Ground Track, Course Made Good (CMG), Track Angle, or simply Track.
Bearing: The direction from your location to any distant point given in degrees from North. For example, if you point a hand bearing compass at a distant headland and the compass reads 056°, then the bearing to the headland is 056°.Directions should always be written and spoken as three distinct figures. Why? Many small boat compasses are marked at ten degree intervals with the final zeros left off (to save space). Ask a helm to steer “thirty” and there’s a chance they’ll actually steer 300° (to the 30 mark) rather than the 030° you intended. When spoken, “thirty” becomes zero-three-zero.
What happened to us?
Our helm was steering 100° on the compass. His actual course over ground however, due to the strong tide, was 060°. The chartplotter, plus the course line on our iPhone apps was showing 060°. The moral of this story: always watch your compass course, and your COG carefully and note any tidal races on your charts. We jibed to clear the rocks, but what we could have done earlier is steer a COG of 100°, so compass-wise, we’d have been steering closer to 160° until we’d cleared the headland.