there are plenty of folders for less. Volvo recommends their 4 blade 20 x 14 folder for that engine, its only about $200 less than the Autostream. I don't like the quality of the reverse thrust of folders. Also in towing tests the Volvo folder had about 4 times the resistance compared to the good feathering props (though still a small fraction of a fixed prop).

In looking further at these props though, I have observed a couple of things. One is that Volvo's literature claims for the 150S saildrive "virtually vibration free running" - that's what I'm looking for! Second, the volvo high skew prop and the Autostream in particular put the blades considerably further aft of the leg than the Variprop. The design of the hub on the Variprop places the blades only about 4 inches aft of the drive leg while operating. The Volvo prop puts them 6 inches aft. The Autostream 7 inches aft. If the problem is interaction with the drive leg, then that distance would be significant.

I have failed at trying to make the vibration sensor - my experiments with machinery around the shop indicates that the vibration from an engine is very likely to be too broadband for simple equipment to make sense out of. So I have bit the bullet and popped for a 10" Autostream - hope I didn't waste the 3 Large!