Eric,
Be sure and tell the person that the 4:1 mechanical advantage pulley system that goes between the jib car and jib clew is a necessary part of the self tacking jib system when a straight jib traveller track is used. Also a jib batten low in the jib sail spanning the luff to the jib clew position is necessary.
The higher the mechanical advantage of jib clew pulley system is, the more outboard the car will position itself. The lower this mechanical advantage, the more inboard the car will position itself along the track. No jib clew pulley system and the jib car will position itself right in front of the mast.
The stiffer the jib bottom batten, the more outboard the jib car will position itself. The softer the bottom batten, the more inboard the jib car will position itself.
These are the tuning levers for the self tacking jib system with the straight track. Once selected, the jib car in/out position and jib leech twist are automatically set and coordinated for a given jib sheet tension/wind strength. Only one control line is necessary; one jib sheet and one cleat.
Also with this system, sailing to windward for example, when a puff is encountered, the increased sail pressure does two things: 1. It pushes the jib car outboard opening the slot slightly. (The relative wind angle on both sails has increased in the puff.) 2. The jib is sheeted down harder by the outward movement of the jib car and this increases jib leech tension which keeps the jib leech from blowing open and wasting the extra energy in the puff.
The perfect circle jib tracks where the track radius equals the foot of the jib radius does not do these things. The jib car is free to run out along this ideal radius and therefore another control line, pulleys and cleat are necessary to position the jib car and this position fixed, no modulation during the puffs and lulls. This is a slower boat speed arrangement of the self tacking jib system during wind speed transients.
One other comment based on experience: The self tacking jib system and jib sail plan form on ARC products resulted in no boat speed loss even though the jib sail area went down. The jib luff length went up approximately 2.5ft with the addition of the "pelican stricker" tube also 2.5ft long.
The forward part of the jib, the luff, is where the forward push comes from in a jib sail. The aft part of a sail pushes sideways and makes the daggerboard have to generate lift equal and opposite to the sail side force. In doing so the daggerboard makes induced drag. The aft part of a sail is necessary to support the front part of the sail, but in the front 1/3rd of a sail is where the majority of the sail forward driving force comes from. Long luff sails are faster; long luffs lead to higher sail aspect ratios. Higher sail aspect ratios, jibs in this case, are faster. On the ARC boats we found this to be the case because we made such a large increase in jib luff length, 2.5ft even though the total jib sail area went down relative to the old style jib with the tack at the top of the forestay bridle and some mast overlapp in the jib leech area.
Bill