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Which way do the props turn?

R/C Submarine modelers

Which way do the props turn?

Postby Jack B » Sat Feb 25, 2012 1:48 am

So I just want to be sure the props on a gato both turn out not inward Right?
So why is that?
Thanks
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Re: Which way do the props turn?

Postby Sub culture » Sat Feb 25, 2012 4:55 am

Twin prop boats turn better when the props rotate outwards rather than inwards. I'm sure someone better versed on propeller theory could explain why.
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Re: Which way do the props turn?

Postby StuartL » Sat Feb 25, 2012 5:03 am

I'm intrigued about this, the only difference I can think of regarding the top side of the prop vs the bottom is that there's lower water pressure at the top. Otherwise 'outwards' vs 'inwards' seems interchangeable for a submerged prop...

I'd love to hear more about this :)
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Re: Which way do the props turn?

Postby StuartL » Sat Feb 25, 2012 5:06 am

I've just realised, after a quick google, that the effects of swirl, eddy currents etc on the hull are critical. I haven't yet found the answer, though, lots of differing opinions online.
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Re: Which way do the props turn?

Postby wlambing » Sat Feb 25, 2012 10:11 am

US twin-screw boats turned outward. Port screw CCW, Starboard CW.

B^)
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Re: Which way do the props turn?

Postby Robert F. » Sat Feb 25, 2012 4:03 pm

As a rule, propellors on multi-propellor[ u]seagoing[/u] vessels turnoutward when moving the ship forward. Having the propellors turn outward prevents the hydrodynamic forces created by them from reinforcing each other, which might cause damage to the ship and/or the props (cavitation for example).

But why should they turn in opposite directions? Well, a turning propellor, as we all know, causes momentum, affecting the ship 's course unless this is corrected with the rudder. (Sailing a one-propped model backwards shows this effect most clearly).
This effect would be much stronger and possibly even impossible to correct with all propellors turning in the same direction. With the propellors turning in opposite directions, they will compensate each other´s momentum and the ship will keep a straight(er) course.

On non-seagoing multi-propped vessels, the propellors still turn in opposite directions, but this time they turn inward when moving the vessel forward. I'm not sure, but I'm guessing this has to do with the different hydrodynamic conditions of shallower and/or sweet water.


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Re: Which way do the props turn?

Postby StuartL » Sun Feb 26, 2012 3:47 am

Robert F. wrote:As a rule, propellors on multi-propellor[ u]seagoing[/u] vessels turnoutward when moving the ship forward. Having the propellors turn outward prevents the hydrodynamic forces created by them from reinforcing each other, which might cause damage to the ship and/or the props (cavitation for example).


Do you know why outwards rather than inwards? The only difference I can think of is the compression version expansion effect of the hollow between the props and the hull...
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