DIY self tacking staysail

[Daydream believer]

Makes no sense. How could the loose car somehow pull itself away from the force with no force on it? Do a force vector diagram and you'll see, the horizontal companant has to be outboard.
A curved track allows for moving the block inboard which the dyneema won't but that's no great hardship.

With due respect I feel that you misunderstand.
If one had a straight track the shortest distance would be on the centre line of the boat. So if one sheeted in the sail tight then the sail would automatically align with the centre of the boat. In light wind that would be useless. The more weight one put on the dynema the more it would bend towards the sail & the more it would force the sail to the centre. To move the sail outwards it would be applying extra tension to the sheet & that would not work.
The curved track maintains the same tension as the sail passes through the tack.

If i am wrong tell me why virtually all the cruisers & most dinghies with ST sails that are on the market have a curved track?

Do you honestly think that if a straight track worked they would go to the expense of a curved one?

 

[GHA]

If you've already got the sail in question, it might be worth looking at the angles the sheet makes when it's set nicely. Then you can work out if the line unning from A to B in the diagram will pull the 'carriage' inboard more than the line from B to the sail pulls it outboard.

Not how it works unfortunately to get a good set . With the normal car on a track setup you have adjustment of both the tension and the up/down angle of the sheet at the clew. Self tacking there's just adjustment of tension, the car will be in the same position as the car would be if all the way forward on the old track.
If using a line forward even without drawing up the vectors just visualising it seems fairly certain the forces from the clew will outweigh the forces from the line forward. Probably easier just to use the existing sheets anyway.
Not that it will matter that much, not like it's a racing boat and will just take moments to swap to a low friction ring to have adjustment of both the angle at the clew and tension same as a track.
Probably...

 

[lw395]

With a selftacker, you can adjust the effective vertical sheeting angle, i.e. the foot tension:leach tension balance by either moving the tack up or down a little or moving the sheet to a different hole on a clew-board. Dinghies tend to do this before the race starts.