set and drift
Tuesday, October 27, 2009 at 21:23 
It's all about current. The water you're floating on isn't stationary. In fact, it's frequent to encounter current that moves faster than some rivers.
The Mississippi River averages around 1.5mph, tidal speeds can easily exceed that. In the Bay of Fundy, 115 billion tons of water go in and out of the bay in 24 hours. That's a lot of water moving around, and your boat is involved in it.
A vessel has course and speed. Current is described in terms of its set and drift.
Set is the direction which the current is moving. In a harbor, the current can be "setting in" or "setting out". At sea, it might be "setting north", if the current was heading north.
Drift is the speed across the ocean bottom. The current offshore here in San Diego is currently "setting south-south-west, at 1 knot".
That also means that if you're heading north-north-east (directly against the current) and your knot meter is saying 7 knots, you're really only moving 6 knots, since one of those knots is lost to current. Likewise, if you were sailing with the current, your SOG (speed over ground) would be 8 knots.
Close to shore, currents tend to go along with the tide, although offshore currents have their own patterns independent of tides.
