I can't give you a reference, but I can give you the answer.
Have you ever climbed the stairs while riding up an escalator? You get to the top faster. But if you get on the up escalator and try to walk down, it takes longer to get to the top.
The same thing happens in the ocean current. Your boat is sitting on a moving surface (the water) and if you do nothing, it will be carried along at the speed and direction of the current.
If you use your engine
to move the same direction as the current, it is just like walking up the moving up escalator. Your engine
moves you forward at 5 knots while sitting on a surface that is moving 1 knot
. Your speed relative to the ground is 6 knots, though your speed relative to the water
is still only 5.
Now, suppose you run your engine to go 1 knot
against the current. You move forward at 1 knot over the water
, but the water is moving backward at 1 knot over the land. This is just like getting on an escalator and walking the opposite direction that it is moving, but at exactly the same speed. If you only look at the water (or the escalator stairs), you think you are moving, but if you look at a distant island (or something in the building) you see that you are actually standing still relative to other fixed objects.
But if you increase your speed to 5 knots, you will go forward 5 miles and backward 1 mile for every hour. The net result is that you are only moving at 4 knots relative to the ground.
Does that help?