Contra-rotating props

[Simon.O.]

aka Duo-prop. Two props L/R inline on one shaft assembly.

Has anyone seen a model with this setep. I would love to make one
Regardless of the mechanics involved, what are the advantages of such a prop setup.
Limitations ??
Effeciency ??

General discussion on Duo-prop applications.

[Meniscus]

This has been touched on here: http://forums.offshoreelectrics.com/showthread.php?t=6188&highlight=counter-rotating

The military uses these systems in their fast boats. Especially their smaller, higher power to weight ratio boats.

[crabstick]

main advantage is no prop torque I understand

[785boats]

As chance would have it Simon I'm trying to build some small contra rotating setups for a couple of torpedoes on the latest boat I've just finished. Here's a link to the setup using scalextric parts. Scroll halfway down the page.
http://www.pt-boat.com/torpedo/torpedo.html
The same principal can be enlarged & beefed up for a F/E drive line.
I've also seen where 4 bevel gears of the same size can be used with the same setup.
Here's a couple of pics. A 55' Thornycroft M.C.B. from WWI. They're the torpedoes on the back.
Cheers.
Paul.