RE: iSonic 115 tuning (for Ian)
Right... As JJ has correctly noted, the usual cause of the front foot slip is an imbalance in the weight/load distribution betwwen the two feet (even if at the time it may not initially feel like that).
The typical telltale is the footslip trend increasing as the wind/power comes on (especially if the rider remains on the one sailing angle- for example beam reach B&F). In this case, the main "change" has been the increase in rearfoot pressureas the rider sheets harder on the backhand to compensate the extra sail load- but more importantly if the sail draft stability moves back under the increased load, the balance of the rig also moves aft and the whole trim has become not as balanced/centred.
Adding extra downhaul (beach trim) helps lock the draft forward and low, while adding extra outhaul (both beach trim or more importantly actively using a variable outhaul on the water) will help a lot in keeping the rig as balanced as possible for the conditions.
This is particularly important in lighter (big7-9m) conditions and with mid to larger slalom boards where the rider may experience a wider range of dynamic loads on the sail as they go from marginal conditions (light-moderate loads when the board is subplaning or at planing threshold ) to quite hi apparent wind loads when fully planing (in not too many more actual knots windspeed). In these conditions, the water can often be quite flat (minimal wind chop) which also allows for quite fast (c/w rougher conditions) open water speeds, so the total dynamic apparent (vector of the actual wind plus board speed) loads up the rig a lot more than sailing "slow" in the same actual windspeed.
On the water, modern slalom gear is more capable than just about any other style of equipment for changes in sailing angle (pinching hi upwind vs beam reach B&F vs broad downwind/speed) - also with resultant changes in overall apparent wind sail loads, and these changes also obviously effect the optimum sail trim - another compelling vote towards fitting and using a really effective variable outhaul in "big" slalom conditions more than about any form of sailing (except FW).
Noting also that often in modern 7-8+ m slalom conditions, the actual wind may be "light" or quite powered" for the same equipment, so a corresponding range of sail tune is more important (and more likely) than, say, in more always powered conditions when using a more standard (one trim fits all) sail tune may be more applicable.
This really be came apparent when the HS released, as a lot of people from other styles of windsurfing suddenly experienced this subtle but real difference of (relatively) high speeds in light/er) wind conditions.
Yes, you will also get a change/improvement by varying the other stance factors, (and ultimately some refinement of those may also be ideal) but predominantly (if your stance was initially correct/optimum) you are then dealing with the results (imbalanced stance) rather than curing the cause (imbalanced rig). In most cases, getting the ideal balance in the rig for the conditions will help improve your sailing and range quite a lot - and at least with the primary issue of the rig under control, the rider is then at the best position to fine tune their stance.
Rider height and leg length will ultimately factor into the best effective stance (strap pitch / boom height /harness length + hook height etc, )but again the level of variance based on leg length will be relatively minor in overall trim unless the rider is exceptionally tall (or short) or of very unusual leg length vs overall height.
Hope this helps solve the problems and gets you flying ~
.. across the water rather than into it !
Cheers ~ Ian