I've put a deposit on a 19" four blade Variprop, but it's not going on the shaft until next winter. I just wrote a message to a friend outlining what I've bought and why, which I'll paste here with the hope it helps.
My friend asked if I was intended to install the PSS shaft seal device instead of a traditional bronze stuffing box, to which I replied:
Yes, that is the "bellows" style of shaft seal. I will be installing it with this coupling system:
http://www.aquadrive.com/product_range/B10.html#1010
plus with "soft mounts" for the engine.
and the Variprop 4-blade 19" wheel
The advantages of all these expensive parts are (moving forward up the shaft from the prop)
1) more torque and better power band utilization with the Variprop 4 blade;
2) speed gain under sail with the feathering blades.
3) ability to change the prop pitch underwater with an Allen key.
4) shaft seal means a dry bilge.
5) thrust bearing/universal joint coupling allows isolation of transmission components from shaft play, reducing engine wear.
6) reduction of noise and vibration to hull
7) soft mounts further reduce vibration, allowing engine to "float" relative to shaft.
8) engine can be run at a heel without fear of sheering effects. Engine is still subject to oil distribution problems at heel, but you won't wear the tranny out as would happen with a hard coupling.
9) Alignment problems no longer a factor.
I've examined the various options, and we feel this gives us a quieter and more reliable drivetrain that makes full use of the shaft output while gaining us the real advantage in drag reduction available by using a feathering prop.