Coaxial propellers?

They ve been around forever, first on torpedoes.

Volvo was the first to bring the technology to recreational boats with their duo prop stern drive, in the late 80s. Mercruiser followed a few years later with the bravo 3 drive and all its corrosion and reliability issues...

Then a few years back they used similar technology aimed at larger inboard boats with the IPS pods, although the props were mounted facing forward. Merc followed with the Zeus...

The smaller counter rotating props are more efficient and allows the use of smaller props. I m sure you can find technical references on line covering the hydrodynamics .

Years ago when duo prop came out I had a small 25 footer single stern drive. I upgraded the drive lower unit to DP and the difference was huge with much betters acceleration and planning

The added cost and complexity of shafting, seals, and reduction gearing very quickly outpaces the relatively minor benefits that might accrue to a recreational vessel using contra-rotating props.

There are (or were) exceptions:

Not just Russian. Here is a link to a good explanation of why they are used in helicopters. http://www.gyrodynehelicopters.com/coaxial_benefits.htm

Except for outboards (pseudo-pods and IOs included) where it allows for smaller diameter props for a given power output, the technique doesn't offer much to counter the added complexity and life cycle costs.

I have owned three DuoProp equipped small boats. They have all been outstanding performers for their HP and that includes fuel mileage advantages. The last of the three is a 22' Donzi Classic with a 385 HP fuel injected BBC that will run the owners age on GPS.......(74.8). The boat runs like it is on rails at any speed and so far, the drives have all been bullet proof. I understand that the DPX drive on the Donzi was about a $3000.00 option over the black drive in 1996 which may account for the very few so equipped. I highly recommend the Volvo drive for small boat propulsion.

The Russians do like their counter-rotating props, don't think? The Tu-95 has been the nuke carrier of choice since the 1950s and it's powered by four of the most powerful propeller engines in the world. I can't even begin to imagine the maintenance on those.

The TU-114 civil airliner version set the speed record for propeller driven aircraft. It reached Mach .78 (.73 in some articles) or 540mph in level flight.

The noise level produced by the supersonic propellers induced genetic defects in people and plants under the flight path. (just joking but the sound pressure of that engine/prop combination was reported to be recorded by the subsea surveillance systems used to detect submarines.

http://www.aerospaceweb.org/aircraft/jetliner/tu114/

You ever heard of a dual prop pod raising fish?????

Old fishing stories about the sounds of 3 or 4 bladed propellers, Detroit or Cat engines, Bertram or Hatteras.
Every fisher person says their boat makes the right sounds thru the water to raise and catch fish.
That is all great and I guess they catch allot of fish.

Above, comments on how loud an airplanes dual props are.

Then I realized, I have never heard of anybody with DP or coaxial pod stating they catch allot of fish or any at all..

Eight props with four blades each, all breaking the sound barrier together...

Would that be 32 simultaneous sonic booms? I'm not sure how this would be calculated.

Huckins assembled and delivered (early 2015) a custom 45+ foot SF with Zeus pods.
Fairly fast, no big fish reports yet.