Staysails are the smaller jib on a cutter. They are mounted to the inner forestay, which is the stay inboard of the headstay and attaches only partway up the mast. When tacking a cutter, you have the jib and staysail to sheet on each tack. This can become quite a chore when short tacking through narrow waterways.
To reduce the number of sheets that need to be adjusted, I have made the staysail self tacking. This lets me short tack up a narrow waterway, while only having to adjust the sheets of the jib (if I'm flying it). If it is a very narrow waterway, I will lower the jib and fly just the main and staysail, allowing me to short tack without having to adjust a single sheet. This lets me direct all of my focus to avoiding obstacles and other boat traffic while working to windward.
The reduced sail area does lower my speed, which is a good idea when sailing in crowded and cramped waters. If we were to collide with an obstacle, our slower speed will give us more time to react while lessening the damage of the collision.
Self tacking staysails are nothing new, but most require very complicated systems and hardware to work properly. There are a few requirements that must be met for a headsail to self tack:
The sail can not overlap the mast
The sheets must be free and clear of any deck hardware
The sail needs a clew block to allow for trimming twist from the sail (optional but very helpful)
If the sail overlaps the mast, any self tacking hardware would hit the mast on each tack. It simply would not work.
The sheets need to be free and clear of any deck hardware to avoid them getting hung up on said hardware. Also, laying on hardware can cause stress points that will lead to chafe on the line.