Steve,

I think the low down of it goes something like this :

Straight rail selftacker

-1- Easy to fit
-2- Not that much cheaper than curved track
-3- With properly jib sail design can be made just as good as a curved selftacker for ONE trim point ONLY !
-4- For all other trim points the behaviour of the straight track deteriorates relative to curved track.
-5- Design must be first time right or you have increasing difficulty to get it just right
-6- Increasing purchase btween car and clew will solve the centering problem of car when jib sheet is lead forwward to tack of jib. (mattia does not suffer from this)


Curved track

-1- Not much harder to fit
-2- Only a little more expensive than curved track; when you add all up it may be less than 10% more expensive on total amount invested. Note that getting a curved selftacking set up is not cheaper than getting two ratcheting block with cleats and beckets. We are not talking huge sums of money here.
-3- With proper alignment of track with jib the sheeting along multiple trim points can be further optimized. Especially handy when you STILL need to find the optimal angles and positions.
-4- Design must be in the ball park and the curved traveller allows you to find optimal points easily by just trying.
-5- Purchase between car and clew can be reduced or eliminated and all purchase can be had in a cascading system between jib tack and car. This works better and is saves money on blocks.


Think of a straight selftacking rail as a jib setup that is sheeted of a single point of the mainbeam. This single point may be on a adjustment rail. If you have experience with that kin of system you will know how the straight rail acts.

By moving the sheeting point outward your change the sheeting angle of the jib and thus put more pressure on the foot and less on the leech. That is the reason why old jib systems had long distances between block and clew. This minimizes the negative effects of running a block outwards (or inwards) passed its design point. With selftackers however, the distance between car and clew is very small and therefor the straight rail selftackers show serious changes in sheeting angle when moveing the car past its optimal design point.

Now the old mainbeam sheeting point system could be designed to work (very) well with regard to one sheeting point, but less with others. What sailor did was optimize this system for upwind sailing in medium conditions and forget about the negative effects on other courses and other conditions where the optimal sheeting is really somewhere else. Of course with a spi you have the spi downwind and can largely forget about the jib also you don't sail many reaches then. With respect to windconditions, this is less important negative effect. In both strong winds and light winds you want the slot to open and sheeting out will do that with respect to the leech, even when the sheeting point is fixed. Running the sheeting point out does that even more. Furthermore in heavy air control is way more important can optimal trim and in light airs strategy (finding the wind and currents) is more important than optimal trim. So this allowed for a decent compromise to work.

That is why I said of Bills straight track that is will work well enough for most sailing despite the fact that I passionately disagree with his claims that it is better than curved tracks. Curved tracks simply are better especially when looking at multiple sheeting angles. I think the choice between the two falls out between recreational sailing occasional racing and more serious racing.

This this help

Wouter