# Exposed or Enclosed Propeller Shaft?

Discussion in 'Hydrodynamics and Aerodynamics' started by AlreadyWet, May 7, 2014.

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I’m not an engineer...of any kind…or studying to be one. However, in my personal research I have read that a spinning shaft in a fluid environment (water for this case) actual increases in diameter because of the surface tension of the water thus making it less efficient. However, the information I read assumes the angle of attack for the shaft is perpendicular to the fluid.

My question is specifically, which would be more hydro dynamically efficient:

1) An exposed shaft of .875” in diameter, 36” long, with an attack angle of 6*, a rotational velocity of 10,000 RPM’s and a linear velocity of 176 ft./sec.

2) Or… the same shaft enclosed in an elliptical hydrodynamic strut?

This seems to have been a debate for many years and any feedback would be appreciated!

AW

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### Mr EfficiencySenior Member

Seeing 10,000 rpm is orders of magnitude greater than shaft rotation speeds in practical use, this exercise seems somewhat academic.

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### RurudyneSenior Member

You want to go a scootch over 104kts? Or is 176ft/sec too high?

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Sorry mate, but don't understand your comment. There are thousands of examples of this already in use.

My question was simply to determine which example would provide the least hydrodynamic drag on the vessel. IE. a boat traveling 120 MPH or 104 Knots!

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### Mr EfficiencySenior Member

Well, I think your comment about the shaft growing in diameter at high speeds was a struggle to follow, so we're even.

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### waikikinSenior Member

AW,
open shaft at that speed

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waikikin,

Thank much for the answer! It seems it was pretty obvious to you, but kind of defies conventional wisdom...obviously why they call it conventional.
Is it because of the rotational shaft speed? Just seems a contoured elliptical shaft housing would have a leading edge with less drag not to mention how the water would flow around it.

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### gonzoSenior Member

Even at lower rotation speed a shaft enclosed in a round pipe will have less drag. The drag increases with the square of the increase of velocity.

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### waikikinSenior Member

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### Mr EfficiencySenior Member

So we have two answers, one says the open shaft, other says go the enclosed shaft. I have NFI, especially since I don't understand the question so good !

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Picture worth 1000 words...

First pic says it all and gives the perspective of application in exposed configuration...except of course boat is usually in the water!

Second and third pics are one persons interpretation of what might have less drag.

In either case, the exposed, spinning shaft or enclosed shaft housing is moving through the water at 120 MPH average with a 6-7 degree angle.

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### gonzoSenior Member

I have no idea what the first picture says. The measurable fact is that an enclosed shaft has less drag.

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### baeckmoHydrodynamics

Actually, at those speeds the complete shaft and bracket are supercavitating (or ventilating) and none of the "standard answers" apply.

A strut section for minimum cavitating drag has a shape that is close to parabolic, with the blunt end aft. This means that the shaft should operate in the open cavity/wake behind a parabolic fairing. The problem with such a fairing is to make it stiff enough, since it is exposed to very strong forces, including vibration in resonance.

It may even be possible to get very good results with a supercavitating s.c. cavitator in front of the shaft (instead of the parabolic fairing), but again, the structural strength will be an issue.

The same applies to the bracket, a parabolic, or semielliptic shape gives the best combination of mechanical strength and drag. The usually short length makes it easier to design than the shaft fairing though.

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Current conventional...

Better pics of current, conventional configuration used by literally thousands of performance boat racers around the world for the last 50+ years. (1st pic is for circle racing and 2nd pic is drag boat) You can count those who have experimented with an enclosed shaft on this type of boat in the last 50 years on one hand.

Is it that the shaft is in super-cavitation to a point where it actually produces less drag?

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### baeckmoHydrodynamics

With the flat planing surface, there is a lot of free air following the bottom. My guess is that there is enough air to ventilate both shaft and propeller. This massive ventilation will create a cavity in the wake, very similar in shape to a vapor supercavity, but without the cavity implosion.

If you look at the shaft section in a cut along the streamline, it will be a slender ellipse. A truncated (half) ellipse comes close to the ideal nose shape for low cavitation drag. The front half will be wetted and the flow will break away from the shaft surface slightly in front of the mid-section. To be quite precise, the flow will get a slight bias in the shaft-wise direction, but that is not changing the main picture of the cavity.

The peripheral speed of the shaft is 11.6 m/s, which is about 21 % of the forward speed. Since the friction is depending on speed squared, the influence of the shaft rotation is less than 5 % (ie 0,21^2).

There is some improvement possible to the front of the bearing housing as far as I can see from the pics (but perhaps the bracket and bearing tubing is not finished on the picture?)

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