OT Efficiency of air propeller versus water propeller?

[John Doe]

Are you familiar with the experiment. A water float with an air
propeller above the water, connected through a shaft, to a water
propeller under the water. Put a fan on the air propeller, and the
float moves towards the fan.

Can that be considered a demonstration of the efficiency of an air
propeller versus a water propeller? The air propeller is less
efficient than the water propeller, that's why the float moves
towards the air propeller?

Thanks.

 

[John Doe]

Correction

 

[Phil Allison]

"John Doe"

Are you familiar with the experiment. A water float with an air
propeller above the water, connected through a shaft, to a water
propeller under the water. Put a fan on the air propeller, and the
float moves towards the fan.

** Nope. Was it on U Tube?

Can that be considered a demonstration of the efficiency of an air
propeller versus a water propeller?

** Course not.

The air propeller is less
efficient than the water propeller, that's why the float moves
towards the air propeller?

** Both types of propeller can be highly efficient - but this requires
careful optimisation to the particular application.

For example:

With high speed racing boats, by far the most efficient propulsion is with a
prop that has only one half submerged. This is commonly known as "surface
drive" and creates a spectacular plume of water behind the boat.

The water entering the prop must be undisturbed by the hull or drive
haft - so it is used with hulls that ride right on the surface or twin
hull designs that have a central air gap for the prop shaft.

It is efficient because it eliminates cavitation due to lack of smooth water
flow into the prop and is further aided by surface tension on the unbroken
water as each blade chops down.

Special props are designed for this kind of use.


.... Phil

 

[Phil Allison]

"Tim Wescott"

Define efficiency. Power efficiency? Force vs. size efficiency? Force
vs. angular velocity? Number of replies per meaningless post?

** With propellers, it is usually take to be the ratio of torque to thrust
under given conditions.

There is quite an engineering science around it.

Cos it matters a bit.


.... Phil

 

[John Doe]

Regular troll

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NNTP-Posting-Date: Wed, 14 Dec 2011 00:18:55 -0600
From: Tim Wescott <tim seemywebsite.com>
Subject: Re: OT Efficiency of air propeller versus water propeller?
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Too many variables. Make the "water propeller" one inch in diameter, and
the "air propeller" six feet in diameter -- do the results end up the
same?

Define efficiency. Power efficiency? Force vs. size efficiency? Force
vs. angular velocity? Number of replies per meaningless post?

Etc.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com