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Marine propeller theory

by mdekooker
Tags: marine, propeller, theory
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mdekooker
#1
Mar9-08, 12:12 PM
P: 1
Good day

I hope someone can help. I am looking for marine propeller theory. Things like how do the diameter of a prop determine the speed of a vessel ? What effect does the blade angle or shape for that matter have on the functionality of the prop. If someone can help with the theory behind as well as the calcs I will be grateful.

Thanks
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ray b
#2
Mar9-08, 05:31 PM
P: 428
aka the screw propeller http://en.wikipedia.org/wiki/Propeller
as it screws thru the water
limits are HP RPM and cavitation [air bubbles] http://en.wikipedia.org/wiki/Cavitation
props are sized by dia and blade angle
sandhammaren
#3
May21-09, 06:38 AM
P: 5
Only rough rules based on scaling/empirical constants are known. See Dave Gerr's Propeller Handbook. Racing props require extensive trial and error.

Physics Prof who raced outboards successfully

Dr.D
#4
May21-09, 12:41 PM
P: 619
Marine propeller theory

A boat in the water is not a simple system such that the propeller alone determines the speed of the boat. The propeller provides a force driving the boat forward, but drag tends to hold the boat back so the actual motion is governed by Newtons 2nd law. Drag is a complicated subject and depends greatly on the type of hull and the regime in which the boat is operating (down in the water, planing, etc.).

For a suitably large propeller on a displace3ment hull with adequate power, the propeller essentially operates as a screw with a small amount of slip advancing through the water at relatively low speeds. At higher speeds, the slip increases, so this simple model does not hold up over a very wide operating range.
sandhammaren
#5
May21-09, 01:14 PM
P: 5
Look, I don't merely know hydrodynamics, I raced successfully for 13 years (5 national championships, 2x U.S.-1, 3 speed records). A propeller is not at all like a screw, it operates via circulation (providing high lift/drag) like an airfoil, hydrofoil, or sail. Understanding the drag is relatively easy compared with understanding the thrust generated by the blade (analog of 'lift' on a hydrofoil) generated, especially by a surfacing propeller. Surfacing propellers run at relatively high efficiency and push light boats at relatively high speeds.



Quote Quote by Dr.D View Post
A boat in the water is not a simple system such that the propeller alone determines the speed of the boat. The propeller provides a force driving the boat forward, but drag tends to hold the boat back so the actual motion is governed by Newtons 2nd law. Drag is a complicated subject and depends greatly on the type of hull and the regime in which the boat is operating (down in the water, planing, etc.).

For a suitably large propeller on a displace3ment hull with adequate power, the propeller essentially operates as a screw with a small amount of slip advancing through the water at relatively low speeds. At higher speeds, the slip increases, so this simple model does not hold up over a very wide operating range.
Dr.D
#6
May21-09, 02:44 PM
P: 619
In spite of all that sandhammaren knows, large vessels such as navy ships treat the motion of the ship at low speeds much like the advance of a screw with a small amount of slip. I have heard the plan of approach to a harbor given in terms of 105 revolutions of the screw on this heading, then 227 revolutions on that heading, etc. That is not national championship racing, only handling large ships.
sandhammaren
#7
May21-09, 03:21 PM
P: 5
Beware people who've read something and imagine they understand it. No propeller, period, works like a screw, and naval architects know that. Efficiency, high thrust/drag is achieved only via circulation about the blade. Make yourself familiar with the literature and phenomena before claiming to be able to explain something.
Dr.D
#8
May21-09, 03:41 PM
P: 619
On the contrary, I know that theory at least as well as you do, but I also know a lot more about explaining things. The screw explanation is useful without getting into a lot of mathematical details that a beginner probably does not have. I did a doctoral dissertation in hydrodynamics, so you really should back off; you are way out of line.

At the same time, I said nothing that was not true. I have been in the ward room of a destroyer and heard the junior officers describe their plan of approach to the harbor in exactly the terms I gave. The next morning I saw them execute it exactly that way.

Being a racing champ does not confer all knowledge. It evidently does not bring much civility, either.
sandhammaren
#9
May21-09, 03:49 PM
P: 5
Based on your statements, I seriously doubt that you understand hydrodynamics very well, unfortunately. Your screwy explanation dates to ca. 1900. Juynior officers in a war room surely do not understand hydrodynamics. The use of 'screw' for propeller is a historically fixed term that is, from the standpoint of fluid mechanics, not accurate. A propeller leaves a helical wake, the pitch is the pitch of the blade which is greater than the pitch of the wake, but the propeller doers not go through water like a screw through wood: circulation about the blade is necessary and is generated by the vorticity shed (as wake) by the trailing edge. Vorticity is generated when the leading edge of the blade moves through the fluid.

Did anyone read any claim by me to 'all knowledge'? That looks awfully much like a jump to a conclusion without support.


Quote Quote by Dr.D View Post
On the contrary, I know that theory at least as well as you do, but I also know a lot more about explaining things. The screw explanation is useful without getting into a lot of mathematical details that a beginner probably does not have. I did a doctoral dissertation in hydrodynamics, so you really should back off; you are way out of line.

At the same time, I said nothing that was not true. I have been in the ward room of a destroyer and heard the junior officers describe their plan of approach to the harbor in exactly the terms I gave. The next morning I saw them execute it exactly that way.

Being a racing champ does not confer all knowledge. It evidently does not bring much civility, either.
zul8tr
#10
Jul11-09, 05:26 PM
P: 6
Quote Quote by sandhammaren View Post
Based on your statements, I seriously doubt that you understand hydrodynamics very well, unfortunately. Your screwy explanation dates to ca. 1900. Juynior officers in a war room surely do not understand hydrodynamics. The use of 'screw' for propeller is a historically fixed term that is, from the standpoint of fluid mechanics, not accurate. A propeller leaves a helical wake, the pitch is the pitch of the blade which is greater than the pitch of the wake, but the propeller doers not go through water like a screw through wood: circulation about the blade is necessary and is generated by the vorticity shed (as wake) by the trailing edge. Vorticity is generated when the leading edge of the blade moves through the fluid.

Did anyone read any claim by me to 'all knowledge'? That looks awfully much like a jump to a conclusion without support.
--------------------------------------
I raced outboard hydros as well (20ss and 25ss and had many wins in Region 5) and know the detailed theory you speak of being a fluids engineer, however to keep it simple and workable in the field I rely on the helical screw analogy as noted in Greer's book to calculate theoretical max speed with no slip. This has worked quite well for the surfacing props when compared to actual measured speeds (avg over both ways over known measured distance and time usually about 400ft I also record a water pitot speedometer reading). With this procedure I have been in the slip range of less than 5%. Since performance measuring changes are best done using a relative change process and the screw anology provides a convienent easy proxy for the theoretical max speed I see nothing wrong with it if used properly. Since RPM and gear ratio is easy and accurate to obtain that leaves pitch as the only parameter that requires some detailed measurement to get it accurate. When the prop is receiving torque in the water it is thrusting with the overall pitch and area of all blades. Thus a good measurement of pitch is desirable and at the least a pitch measurement procedure that is always consistent for all testing done. Note that since rpm and gear ratio are accuratly measured and pitch is a calculated value than it has a direct relation on the amount of slip that will be determined from testing. So slip is biased with the value used for pitch. But if pitch is determined by a consistent method this does not matter when relative results are desired.

As you know the surface props we use are variable pitch from lead edge to trail edge at any given radius. It would be nice to get a pitch contour map of each blade with laser optics and get the average geometric pitch of the entire prop. But that requires a lot of cost and work. I keep the consistency by measuring the pitch (actualy angle of blade slope at 90 degrees to prop shaft centerline) at about the 70% radius along that arc from the lead edge to the trail edge at 1/2" chord intervals (and neglect cup). Then take the average of those values to provide a pitch proxy at the selected radius. Pitch can then be calculated from the helix spiral formula below. Note average pitch calculated from the average of the slope angles is pretty close to the average pitch calculated from the average of the pitches at each increment. Pick one as long as the same procedure is used all the time for consistency.

Pitch @ any point on blade = 2 x Pi x R x Tan (slope angle),

Pi = 3.14.............
R = radius,
slope angle = angle between blade slope and 90 degrees to prop shaft centerline

Then it's to the water!
awermedia
#11
Jul20-09, 06:54 AM
P: 2
first u calculate ur vessels speed.some programs calculate hull speed like hullspeed and then there r some tables which shows u propeller dimensions
zul8tr
#12
Jul20-09, 07:53 AM
P: 6
Quote Quote by awermedia View Post
first u calculate ur vessels speed.some programs calculate hull speed like hullspeed and then there r some tables which shows u propeller dimensions
Your response is not on target to my tread.
stewartcs
#13
Jul24-09, 03:18 PM
Sci Advisor
stewartcs's Avatar
P: 2,284
Quote Quote by sandhammaren View Post
Beware people who've read something and imagine they understand it. No propeller, period, works like a screw, and naval architects know that. Efficiency, high thrust/drag is achieved only via circulation about the blade. Make yourself familiar with the literature and phenomena before claiming to be able to explain something.
First off, lose the attitude.

Second, nothing that Dr. D said was untrue. The term "screw" is used by Naval personnel when referring to the propeller...not to a "fastener" that holds two pieces of wood together.

Based on the OP, one must consider the total effect on the vessel (including the drag forces) not merely the propeller design. That, as it seems to me, is the only thing Dr. D was pointing out.

CS


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