To find the amount of "slip" that your boat is exhibiting, use a protractor that is shaped like a "D." These protractors have a flat edge (the left side of the D) and a hemispherical angle gauge.

At the center of the flat edge, a hole is usually found. Tie fishing line into that hole. Drag that line, with any sort of drag weight (split shot sinker) from the protractor. Place the flat edge of the protractor against your rear crossbar and trail the drag weight behind.

The fishing line will align itself with a degree angle on the protactor while you sail the boat. This reading of degrees, where the line crosses the protractor scale, will be the difference in the direction that the boat is aimed, and the direction that the boat is actually moving.

(OR... compare your compass reading with your GPS reading.)

GARY