Ducted propellers?

[SailorHarry]

While I was in Anacortes, WA I visited with an older gentleman--who's name unfortunately escapes me at the moment--who reported that he had put a Kort nozzle on a steam launch many years ago. Perhaps Lopez Mike knows who I'm talking about. He reported that it was extremely maneuverable but the nozzle was unstable. If you took your hands off the wheel it would start going one way or the other and slam itself hard over. They had to fit some kind of damper arrangement and I got the impression it still wasn't extremely stable. Probably something about the location of the pivot point.

After hearing that I considered fitting on to the launch that I am building, but I had already gotten the aluminum plates cut for the hull and couldn't find the room in the stern to do it properly. I believe the hull would have to be designed specifically for it with lots of extra room around the prop.

[preaton]

Here is my ducted propeller. Note the "fishtail" rudder with endplate. This is needed to get the most out of the nozzle.

Cheers

Paul

[Lopez Mike]

Probably Russ Hibler. Wears a hat with the brim turned up. Has for the fifty odd years I've known him. I'll ask him about it the next time I see him. He's 99 but I expect he'll last until I ask him about it.
Mike

[boatbum]

So Paul,

Does the duct turn with the rudder or are they separate? How much of a gain did you get with the duct vs with out?

Keith

[Lopez Mike]

I had a discussion about this with Keith Sternberg yesterday and his memory is that it was, indeed, Russ Hibler who had a boat in Anacortes many years ago with a duct on the rudder. His memory was that it was a simple tube. No information at this time whether it did much of anything but attract kelp and bags. Might have help steering at large rudder deflections.
Mike

[preaton]

The duct does not turn on the prop. There is ~ 1 cm clearance between the prop tips and the nozzle. The nozzle has an airfoil cross-section and essentially provides lift. The rudder is transom - hung and is a form of "fishtail" (like a Schilling rudder but less extreme) The endplates improve the efficiency of the rudder.

As to what improvement it makes I'll let you know.