Optical sight. If the ball used as a mark is of known diameter then its apparent diameter will yield it's distance. The scale on a protractor will work.

It doesn't matter whether you're full speed or not, as long as everyone is making the same comparison. If you sail the course in ten minutes and I sail it in eleven, clearly you sailed better.

All you need is a course of agreed length, observed wind speed and elapsed time. The point is to give people of lower skill level an opportunity to quantify their results and try to determine where their weaknesses are. If I do three ten minute races in an hour, I can then stop and try to figure how to improve my performance. At the end of a regular regatta weekend I can't remember anything at all, it's just a blur. Always has been.