RE: Mast track position vs sail size
Did you mean to post this twice?
If not, we can remove the first post.
OK, on to your problems with getting the S-Type 115 planing easily.
First, it's a bit narrower than you Xantos 295 (overall) but a bigger "issue" here is the difference in rockerline.
You are going to have to move much further back on the S-Type 115 to get the nose up so it can plane.
If you still have the Xantos, take a straight edge and place it on the bottom (or stretch a string tightly down the center of the board to make a straight line).
Notice that the planing surfaces (basically flat in the fore and aft direction) on your Xantos extend well forward of the mast track.
Now do the same test on your new S-Type 115.
Notice that the flat surfaces only extend forward from the tail of the board to somewhere under the front footstraps, and then the board starts to rapidly "curve away from straight.
So here's the physics of this issue.
You are accustomed to sailing a board with a long planing flat, so you can stand up near the mast foot (I used to have a Xantos 295 so I know about this) sheet in some, and work your way onto a plane BEFORE you start to move back on your board.
When you try this on the S-Type, it does not work as long as you keep your weight forward of the front footstraps.
Because there's an area I call the "rocker transition" where the shape (rockerline) in the bottom of your board "transitions" from flat at the back to an increasing curvature in front of the front footstraps.
So, as long as you keep your weight forward, the "rocker transition" stays down, the nose of your board stays down, and your plaing flat (from the front footstraps to the tail of the board) remain "inclined" down at the front.
You are pushing alot of water with the rocker transition, and there is no lift (from an inclined UP at the front) from your planing surface.
So, your board remains "stuck like glue as there's no lift to get the board over its bow wave (now stuck just in front of the rocker transition)
So, to remedy this, you need to start moving back on your board sooner (NOT faster, esp. on the S-Types) to get the nose up and change the pitch (fore and aft) trim of your planing surface from negative (down at the front) to positive (UP at the front).
This will "unstick" your board, help prevent a lot of catapaulting (due to the board not wanting to plane off easily), and get you up to speed very quickly.
As far as where to place the mast foot, the "knowledgable guy" at the shop is not giving you very good advice.
Your own instinctive " more forward for larger sails", "further back for smaller sails" is pretty good, but you need to experiment with this.
There's usually a "sweet spot" on most boards, but that does not mean that you will always put the mast base right at that point.
For each sail size, start with the mast foot at the back of the track, and go out and sail your board.
If the board is "tailwalking" and pretty much impossible to control, them move the mast foot forward a cm or 2 (3/8-3/4") at a time until you just have marginal control.
This will normally be the fastest mast foot postion for that sail size, with your rig, your stance, your boom height, and the fin you are using.
If you find the board spins out alot, move the mast foot forward a little more. Moving the pressure from the mast foot further away from the resistance of the fin can help sometimes with spin out, but it settles and slows your board.
I have quite a bit of time on the S-Type 115 and I found it to be my board of choice for 7.5-5.5 m2 in slightly rougher conditions. If the water was pretty flat I'd use the Isonic 115. If it was a little choppy and I wanted to chop hop a little the S-Type was better.
Again, moving your mast foot too far forward can cause a very rough ride.
You need to find the pitch trim, which mast foot positioning dees effect where your board is fast and free. Too far back and the nose rides too high and you may experience "tailwalking".
Too far forward, and you will "glue" your board to the water with insufficient pitch trim and the board will slam into every piece of chop and then rebound.
So you need to find the "blend" of too high (out of control) and too low
(limiited control due to slamming and rebounding with too much pitch oscillation).
Again, I think the best solution to your issues is to move your weight back on the board, progressively, as your speed increases, striving to keep the board accelerating.
Hope this helps,