This question has and is plaguing a lot of intermediate level windsurfers. It gets a lot of debate thrown at it and is often given as the reason for some maneuvers not working or the windsurfing experience being unpleasant. Let me try to shed some light on the matter.
There are mainly two reasons for which we might put the mast-foot further forward or backward. Both are related to board handling. Here they are:
1. Beginner to intermediate planing: up until the point where we are just getting the hang of planing, the mast-foot position will determine how easy it will be for is to bear away from the wind and how much effort we have to put into going on a closed haul. The further forward the mast-foot is, the further forward the whole sail will be, and with it the sails pressure point. By moving the mast foot forward we are in essence moving the sails pressure point away from the fins (or skegs) pressure point which makes it easier to bear away from the wind. This is a good trick for those who have trouble with the board turning into the wind when they get into the foot-straps.
2. Advanced (fast) planing: once we are planing at high speeds we will be at a level where we no longer have to make use of small adjustments to the mast-foot position in order to get going or feel comfortable while sailing. At that point we need to find the position which give the most speed and the most control for the conditions. When we position the mast-foot towards the back we are able to get more speed (at least according to slalom racers, I still haven’t found the physical explanation for this) whilst the further forward we position the mast foot, the more control we have over the board as the pressure of the sail is transferred into the board further forward, keeping the nose down. Therefore, on flat water the mast-foot will do better more towards te back whereas in choppy conditions, putting the mast-foot further forward will provide us with more control.
We shouldn’t forget that these are only guidelines for gaining more control over the kit when sailing. Our size will also have an impact on where we will feel the most comfortable so try out different positions for each sail size in different water conditions and see which is the one that offers you the most enjoyable windsurfing experience.
I hope this helped. Let me know your experiences with testing various mast-moot positions.