As Ian says the Bruntons Autoprop is a very good but expensive prop.
Alternatives are the Bruntons Varifold, Flexofold and Volvo. All are standard folding props that are a bit cheaper than the Autoprop however Volvo seem to have stopped making folding props for the older sail drive legs such as the MS25.
Over the years I have had 2 blade folding props from each manufacturer all on sail drives and found the Flexofold to be the best by far under power and sail than the Bruntons and Volvo.
On anything less than 38 foot I would not waste the additional cost on a 3 blade prop as long as you get the right diameter and pitch.