2 props 1 shaft?

[ace028]

I saw a ski boat the other day, and it had two props on it. They were on the same shaft, like in a row. Now i'm just throwing this out for discussion, and it may have been asked before, but what advantage would this have and the disadvantage?

My guess without researching it is that the forward one is where most of the thrust is produced and the second produces minimal thrust from the churned water in the stream of the front prop? Just my guess. Also why don't the big boats use a streamlined prop nut? they seem to have a flare at the end of the prop.

Could we use this on our boats? Would it cause unpredictable handling?

[Rex R]

the idea of using more than one prop per shaft is not exactly new...the turbinia used more than 1 per shaft, but they had a good distance between props.

[DISAR]

There is the concept of Contra rotating propellers, two propellers on same shaft close to each other rotating in opposite directions. It is quite complex mechanism but the propulsion efficiency is increased. These are thin propellers and do not look like normal ones.

[DISAR]

Some links for contra rotating propellers (applied in aircraft, small crafts maybe will be used in large ships in the future)

http://www.wartsila.com/Wartsila/global/docs/en/ship_power/media_publications/marine_news/2004_1/hydrodynamic.pdf

http://www.freepatentsonline.com/3938464.html

[Fluid]

Counter-rotating props on a common shaft are old news - they have been used on naval torpedoes for nearly 100 years - and those props looked pretty normal. Power transfer is increased without going to a larger diameter, and prop torque is greatly reduced.

The flair on the end of the prop shaft is probably the exhaust exit. Many OBs exit the engine exhaust behind the prop for reduced noise. Some also exit the exhaust through the prop to reduce drag. A surfacing prop would not benefit from a streamlined nut, the water is so aerated there would be no advantage. BTW, low performance boats with submerged props often do use a streamlined nut - so do naval torpedoes.


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[ace028]

I'm sure they have been doing it for years, just haven't really noticed it before, and wonder why we don't try it on rc, and the gains and losses of it. The boat I saw, I don't believe they were counter rotating individually, looked like they were just bolted up together inline.

[ace028]

here is a pic of what i'm talking about, looking at it, it does look like they spin in opposite directions. these don't have the flair I was talking about.

Maybe we can have something like this made for rc, and see how well it might work?

[Fluid]

Don't count on it for a performance R/C application any time soon. Too complex, too fragile, too expensive. We can use a larger diameter prop to improve our performance so we don' t need it - the full scale boats that need props like this can't increase prop diameter so they use other means.

The props in your photo were designed for through-the-hub exhaust.




Mark 6 torpedo designed in 1908. Photo 1914.


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[Diegoboy]

Take a look at this! <----------------------
I knew I remembered seeing one of them some time ago. Took a while to re-find it though.







Time to bone up on your Japanese.

[Diegoboy]

One more pic.

[skidooguymxz550]

counter rotating props are for handling, they keep the boat tracking straight with no propwalk. we have a bravo 3x on the 1:1 and it's great especially backing into docks.