COG, SOG, VMG, Knots, Compass direction etc.

CAUTION - BIT BORING - Read last line if you can't be bothered...!

To understand what's been going on the last few days I first need to start with a few definitions:
SOG - speed over ground. Computer simply calculates how fast boat is going  using GPS coordinates.
COG - course over ground - same, but calculates angle travelled instead of speed. 
VMG - Velocity Made Good - this is distance covered in the right direction. We take distance to Cayenne and compare it every 24hrs. 
Compass Course - this is the direction the boat is pointing in and what the infamous autohelm uses to steer. It's confirmed by the computer's independent compass.
Nautical miles - about 1850m (compared to 1600m in a "normal" mile). 
Knots - nautical miles per hour. 

So we've had a very frustrating few days on board with wind (and waves) blowing nicely in the right direction, good SOG but only disappointment each day when we find the VMG is just 60 miles. We finally realised there's some strong and variable currents at play that are pushing us off course and so reducing our VMG. 

To give an example - a boat can point south (180 degrees), have a SOG of 3 knots yet actually be traveling west due to unseen and unfelt currents - only looking at the COG will reveal that fact. So the direction the boat is pointing can be irrelevant. In fact the only relevance is the direction the boat is facing is the direction the forward propulsion of the oars will have. So in the previous example, if we are rowing south, the COG will be south west due to the currents vs oars. 

Now that we've worked this out we can track our COG closely and ensure all the miles are in the right direction which dramatically improves VMG. In the last 24hrs we jumped from 60 to 75 and on track for more of the same today. 

Lost? Sorry! Simply put, we should now arrive next Sunday in around 48 days total. That is very, very good news!!!