Contra-rotating variable pitch propellers - GIF

[Altair]

Contra-rotating variable pitch propellers.




Previously:

Wallace Foundry propeller - photo
Casting a giant propellor GIF
Propeller feathering - GIF

[Frank S]

I can't help but wonder what the acceptable percentage of cavitation is when at speed

[Toolmaker51]

This along lines of the Volvo Penta drive? I'm sure a lot of design and testing went into the product, and they appear on fair sized sportfisher craft. Haven't noticed any on small boats or larger semi-yachts. Has to be a range of displacement, power and hull speed getting on plane.
Interesting that front prop is 4 bladed, the CCW is 3. Positively, turning same way would cavitate instantly, maybe not appreciably propel.

[Frank S]

I noticed as well that the pitch angle in the forward position is not nearly as much as when reverse pitch is applied, this may be due to the secondary prop now becoming the primary. Another observation is even at full forward pitch the blades have less than half the pitch angle of a standard fixed blade prop and far less that any racing prop, like it was designed more for power and maneuverability control than speed.
I made a 20" 4 blade prop for one of my work barges that I had a pile driver mounted on to be powered by the 350 Chevy work motor that powered everything on the pile driver. at first I tried to replicate the pitch of a 200 HP Mercrusier with a single rudder I found that all I was doing was churning water due to the bulk of the flat bottom barge. later I reduced the pitch to about half the angle and added a 2nd rudder Forward motion was still sluggish as expected but the tandem rudders could slew the barge from side to side at will