Exposed or Enclosed Propeller Shaft?

AlreadyWet · May 13, 2014

baeckmo said: ↑

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.

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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This makes sense and is how I pictured it.

Many of these boats have recently started running a tapered safety collar in front of the bearing housing which, at least in static form, negates the blunt end of the strut tube.

Thank you again for your reply!

baeckmo · May 13, 2014

AlreadyWet said: ↑

This makes sense and is how I pictured it.

Many of these boats have recently started running a tapered safety collar in front of the bearing housing which, at least in static form, negates the blunt end of the strut tube.

Thank you again for your reply!
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Having a rotating collar is a bad idea, since the power loss ("pumping effect")increases by diameter^5. Better fix it statically to the bearing housing as a nose cone.

gonzo · May 13, 2014

That is correct.

bpw · May 14, 2014

gonzo said: ↑

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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What is your experience with hi speed craft? Just curious where your info is coming from.

Rastapop · May 16, 2014

AlreadyWet said: ↑

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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The enclosure will widen the profile and add form drag, and the increased wetted surface area will add viscous drag (which at high speed will be a large percentage of total drag).

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Exposed or Enclosed Propeller Shaft?

AlreadyWet Junior Member

baeckmo Hydrodynamics

gonzo Senior Member

bpw Senior Member

Rastapop Naval Architect