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Big_Stick
December 30th, 2008, 18:32
When Alexander Graham Bell was in the process of developing a flying machine, he quipped to his wife that there should be no difference between the thrust of a propeller in air or water. His wife replied that "you can row a boat in water with an oar but you can't row a flying machine in the air". Thus, Bell didn't pursue that angle. But I think he was right.

The question is, can anyone with a good familiarity with fluid dynamics state with any certainty whether it is true that for a given amount of thrust, the power required is the same whether a propeller be in water or air? Let us set aside all questions of density variation due to temperature and assume standard conditions obtain. To put it another way, is the power required per pound of thrust essentially the same regardless of the medium, assuming a properly-designed propeller?

Henry
December 31st, 2008, 09:57
That is a good Question!
i am guessing here but No
H

fliger747
December 31st, 2008, 10:00
Check out the concept of Reynolds Number, even a factor within a phase type (gas or liquid).

Of course you knew this!

Best wishes: T.

Big_Stick
December 31st, 2008, 13:56
Check out the concept of Reynolds Number, even a factor within a phase type (gas or liquid).

Of course you knew this!

Best wishes: T.

I don't want to reinvent the wheel, I'm just looking for a quick yes or no from anyone who's familiar enough with fluid dynamics and prop efficiency to know. I suspect they are similar for air and water since air is considered as an incompressible medium at subsonic speeds. The real differences ought to be mechanical; the prop in air will be pitched very differently and spin much faster but can achieve the same thrust per unit of energy. That's my guess, because drag decreases with density allowing the speed to increase proportionally. There are folks out there, somewhere, who could just look at the question and quickly answer yes or no. Could be the wrong place to ask, though. Used to be a couple of pretty sharp guys around here but based on the way things seem to be going, maybe they've flown the coop.

Big_Stick
December 31st, 2008, 14:08
This would tend to support Bell's initial hunch. Basically, my understanding of what is being said here is as I'd thought before; unless compressibility is a factor, water or air could be used to test designs and the result would be the same. The design of the propeller would be different for an airplane than for a boat because of the density but the essential behavior is the same with respect to fluid dynamics. The aircraft prop would have a much flatter pitch and spin much faster but could do so with the same amount of power per thrust unit because the air is much less dense than water. At least, that what this seems to suggest. A good engineer would know right off.

Example of the importance of the Reynolds number

If an airplane wing needs testing, one can make a scaled down model of the wing and test it in a wind tunnel using the same Reynolds number that the actual airplane is subjected to. If for example the scale model has linear dimensions one quarter of full size, the flow velocity would have to be increased four times to obtain similar flow behaviour.

Alternatively, tests could be conducted in a water tank instead of in air (provided the compressibility effects of air are not significant). As the kinematic viscosity of water is around 13 times less than that of air at 15 °C, in this case the scale model would need to be about one thirteenth the size in all dimensions to maintain the same Reynolds number, assuming the full-scale flow velocity was used.

The results of the laboratory model will be similar to those of the actual plane wing results. Thus there is no need to bring a full scale plane into the lab and actually test it. This is an example of "dynamic similarity".

Reynolds number is important in the calculation of a body's drag (http://www.answers.com/topic/drag-physics) characteristics. A notable example is that of the flow around a cylinder. Above roughly 3×106 Re the drag coefficient (http://www.answers.com/topic/drag-coefficient-1) drops considerably. This is important when calculating the optimal cruise speeds for low drag (and therefore long range) profiles for airplanes.

Big_Stick
December 31st, 2008, 14:23
And the final answer, from encarta, is that propellers are actually more efficient in air than in water...contrary to Mrs. Bell's pronouncement. The best designs for marine use operate at up to 77 percent efficiency and the best for aircraft can reach 86 percent efficiency, but in practice, aircraft props average about 80 percent efficiency and marine props 56 percent.

In the best of all worlds, then, they are quite close, giving a lot of credit to Alexander Graham Bell's "hunch". Not bad for a guy who only invented the telephone.

http://encarta.msn.com/encyclopedia_761561557/Propeller_(mechanics).html

fliger747
December 31st, 2008, 15:45
Actually air is compressable at subsonic speeds and forms shockwaves at the speed of propagation of the pressure waves (speed of sound). Though the speed of sound in water is higher than in air, pressure waves propigate even at very low velocities. The wave making resistance of a ship hull is related to hull length. For example a tug boat making say 12 knots might be pushing a huge wall of water in front of it, whereas a Destroyer will create very little disturbance ahead even at say 20 knots.

In the design of WWII Battleships, a tradeoff that had to be considered between increased hull length and reduced required SHP for a design speed, and the greater difficulty and weight expenditure for protection of the greater length. The Shorter SODAK were able to afforded thicker protection that the North Carolina's, but required much more HP to obtain the same speed. Clever design of the machinery spaces allowed this to be possible, even though it was a less hydrodynamically efficent design.

Some other issues between the two, mach compression and drag and in water, cavatation issues. The boundary effects between air and water affect suface ship propusdion and drag to a great extent also.

Indeed all the smart guys have left..... Say hi for us!

Cheers: t.

Henry
December 31st, 2008, 18:59
This is too intelligent for me
but i still say NO!
even the density of different liquids would make a change
NO?

Big_Stick
December 31st, 2008, 19:12
This is too intelligent for me
but i still say NO!
even the density of different liquids would make a change
NO?

Not so complex...I was just asking a very simple yes/no question regarding the efficiency of a propeller in water versus air. Which is more efficient, or are they about the same? Intuition, like Mrs. Bell says, would make ya think that water is going to be the more efficient medium but unless the encarta article is wrong (and encarta is not Wikipedia), props are actually somewhat more efficient in air. That's all I was after. A discussion as to WHY is a whole 'nother thing but I think it'd center on the fact that the flatter pitch and higher speed of the propeller, in air, which has less drag than water, is the reason an aircraft prop can be designed more efficiently. Advance ratio is the key.

Click on the link the article and scroll through, it explains it pretty well in lay terms.

fliger747
January 1st, 2009, 10:25
As a lad I remember standing on the vibrating fantail of a WWII vintage Fletcher class deatroyer making 35 knots. They really squat and dig their heels in at speed, the churning mountain of white water, disturbed by 50,000 hp towered well above my head.

Therin lies one of the main differences between potential propulsive efficency beetween surface ships and airplane props. The presence of an escape hatch for what would otherwise be a cleaner helix represents wasted energy, hence propulsive efficency. Another item, most marine propellers have historically been fixed pitch, cast from bronze as a single piece. There have been various adjustable pitch models which have made appearance in recent times. Sometimes worth the inherent problems in a marine propulsive environment, sometimes not.

deep diving Submarine propellers probably make the closest comparison to an aviation application, but the mediums are very different. Really apples and fruitcake comparison... Orville et al. can be applauded for paying attention to the inherent differences between the liquid and gas phases and ariving at a more optimum solution. Early ideas about airscrews certainly derived from marine propulsion, which orginated from the ideas of the Archemedies Screw.

Happy new year!

T.

PRB
January 1st, 2009, 14:16
Interesting article!

While airplane propeller blades are little rotating wings, is the same true of ship propeller blades? Or are ship propeller blades closer in design to a typical ceiling fan blade? Does a ship’s propeller just “move water” to produce thrust, as opposed to an airplane propeller, which generates “lift”?

fliger747
January 1st, 2009, 19:03
Ship propellers tend to rely more on a great deal of curveature to provide camber rather than thickness such as might be found in an airscrew. As such this tends to emphsize rearward mass flow to develop thrust. Though water is not compressable per se, Bournouilli's Theorum does apply and a minor amount of lift may apply due the the necessity to accelerate the flow at the top of the propeller (water wing). Generally ship propellers tend to be thin as possible to reduce the blade form drag and incorporate cupping at the aft end to enhance the direction of the mass flow.

Unlike airscrews, marine applications do not has the "supersonic tip" issue and have plenty of medium to bite on. However, especially in multi screw applications vibration is a serious issue at high speed/power outputs. Enough so as to break or render inoperative sensitive optical and electronic equipment! For submarines, noise is a critical imperitive. All these require very different design parameters for optimal performance for the primary task at hand. For instance propellers with more blades may create less vibration (The Iowa's use 4 blade props inboard and 5 blade outboard) and modern sub props tend to incorporate many blades for noise reasons, at a penalty in efficency.

Big_Stick
January 1st, 2009, 19:33
Ship propellers tend to rely more on a great deal of curveature to provide camber rather than thickness such as might be found in an airscrew. As such this tends to emphsize rearward mass flow to develop thrust. Though water is not compressable per se, Bournouilli's Theorum does apply and a minor amount of lift may apply due the the necessity to accelerate the flow at the top of the propeller (water wing). Generally ship propellers tend to be thin as possible to reduce the blade form drag and incorporate cupping at the aft end to enhance the direction of the mass flow.

Unlike airscrews, marine applications do not has the "supersonic tip" issue and have plenty of medium to bite on. However, especially in multi screw applications vibration is a serious issue at high speed/power outputs. Enough so as to break or render inoperative sensitive optical and electronic equipment! For submarines, noise is a critical imperitive. All these require very different design parameters for optimal performance for the primary task at hand. For instance propellers with more blades may create less vibration (The Iowa's use 4 blade props inboard and 5 blade outboard) and modern sub props tend to incorporate many blades for noise reasons, at a penalty in efficency.
That makes sense...article did not say why efficiency was less than it could be but noise and and vibration are good reasons to sacrifice ultimate efficiency. Now I have to go back and watch "The Hunt for Red October".:costumes:

PRB
January 1st, 2009, 20:49
Yep, North Carolina experienced severe vibration due to resonance of the 4 four-bladed propellers, leading to the mixed 5/4 blade configuration in South Dakota and Iowa class ships. What I don’t get is why other ships didn't suffer from this resonance vibration, like all the pre-North Carolina class battlewagons?

... Now I have to go back and watch "The Hunt for Red October".:costumes:

Captn., we’re cavitating!!! They can hear us!!! :d

fliger747
January 2nd, 2009, 07:54
Why, they dinn'a have the power! The North Carolina's were the first US fast (28+ knots) battleships. Also the various skeg designs in those, the SODAK's and the Iowa's had some effect. SODAK's skegs were on teh outside, Iowa's on the inside pair. One of the ideas behind the skegs was to provide a tunnel for better water flow. Another benifit was increased structural strength and hopefully some protection against torpedo damage in this sensitive area. Model basin tests indicated a 5% reduction in resistance using the skeg designs at 28 knots.

Sometimes just a certain speed range was the issue. The Juneau and her ilk had some nasty vibrations of that type.