Somehow I did a nice reply here and it never posted.
I'll try again...........
"Could you or somebody else also comment on the recommended mast position on the slot? "
For you, at your 172 lbs./78 Kg., and at your skill level, I'd start out with the mast foot in the center or slightly behind center for general sailing.
Check out this link where Tiesda You gives some tuning tips for the Rio:
It explains the different suggested mast foot positions and shows the markings on your
"If I understand correctly ( but I might be wrong!) the more backward it is, ( compatibly with the size of the sail, of course, because larger sails would require a forward compensation, from what I understand) the easier will be the glide forward: the weight being more towards the rear, the front part will raise and reduce the contact of the widest part of the board with the water, thus decreasing the drag ... What are your suggestions on this?"
Not necessarily so!
Bigger sails do not mean further ahead mast foot positions most of the time.
There are 2 "real world" situations where you find the mast foot all the way forward.
Formula racers put their mast foot nearly all the way forward because they get better upwind performance with their huge 9.5-12.5 m2 rigs this way.
Longboard racers (with an on the fly adjustable mast track) move the mast foot all the way forward when racing in light to moderate winds to get the maximum waterline length and maximum lee rail engagement to help their upwind performance.
For recreational sailors (not racers) putting the mast foot all the way forward is nearly always a bad idea unless you are a super heavyweight.
"Your PLYWOOD metaphor makes your point very well and this is where I did go wrong, as I was outhauling the sail real tight, but sure there be must lots of windsurfers in the lumber trade, because when I look around I see a lot of those superflat sails, as taut as a violin string ! Some people must do windsurfing with the overwhelming consideration that the flatter the sail is , the easier it is to uphaul it from the water, because if they were to tune the sail with more draft , they fear they would have to break their back uphauling a 5 ft diam ,water filled and water-tight ...container each time ! Of course I am exaggerating, but I am curious to know how it feels to uphaul a 10.5 with a sizeable draft in it ! There must be enough water inside to trap a dolphin !"
Next time you go sailing, watch as you uphaul your sail.
There is no "5 ft diam ,water filled and water-tight ...container" because as you begin to pull the mast out of the water, all the water in the sail runs off the leech, and all the water in the luff sleeve runs out the top and bottom of the sleeve.
Our sails are now made completely from "non-absorbing" plastic materials so a sail really gains almost no weight when wet.
Double luff racing sails have large "drains" at the top and bottom of the sleeve, (Aeroforce Designs actually had little flapper valves to let the water out along the back of the sleeve).
Use an "Easy Uphaul" to allow you to put the weight of the rig on your harness until you get most of it clear of the water.
So, not the problem that you were thinking it was.
Hope this helps,