Thrust factors

[kenboyd]

In theory, what effect would a one inch decrease in diameter have on thrust, in conjunction with the RPM increase that would be produced, by a propeller moving through the water, if no other factors were changed?
(This is an example to make understanding what I'm asking more understandable. These numbers are for simplicity and clarity only). For instance, if I had a 10 inch prop producing 100% thrust at a given RPM and NO other factors changed (horsepower, pitch, blade shape, style, or number, drag, viscosity, velocity, etc.), and I reduced the diameter 1 inch, the thrust would be 90% at the same RPM, then 80% for 2 inches, 70% for 3 inches....
Now, the question again in its final form. Using the above example: What would the thrust be with the increase in RPM? Would the thrust be less than 10% decrease or more than 10%, for each 1 inch diameter decrease?
I'm not interested in the effects hull design have on the equation. I'm just talking about a prop moving through the water. Blade shape, design, and number, exact figures or Reynolds numbers are irrelevant in this question. I'm aware that these factors all have an effect in reality, but I am not interested in these effects for this theoretical question.
Ken:)

[Mark75H]

Diameter is not as closely related to thrust as pitch is. Your question is beyond science and into the realm of art. No way to know without trying the prop on a boat, the variables are way too many and it will change dramatically from boat to boat, prop to prop and set up heights and angles.

[kenboyd]

Diameter is not as closely related to thrust as pitch is. Your question is beyond science and into the realm of art. No way to know without trying the prop on a boat, the variables are way too many and it will change dramatically from boat to boat, prop to prop and set up heights and angles.

Mark,
I don't think you understand the question. I'm not asking about the relationship between thrust and pitch, or pitch and diameter, or how props react when placed on one or another boat, or differences between props, or setups, or any other variables that might exist in that realm of reality. I am aware of how those things work and relate to each other.
Maybe I shouldn't have said the prop moving through the water. How about a prop in a tank or an enclosed controlled environment, not connected to a boat, but rotating in a fluid of a controlled viscosity and producing a given amount of thrust at a particular RPM, with a set amount of torque. Then reducing only the diameter which will increase the RPM, (I'm not talking about cutting down a prop. Let's say the prop is an exact replica except smaller, no other factors are changed), and then measure and compare the difference in thrust. Of course the resulting figures will be different for different types of props, but the relationships between the thrust differences would be similar. I'm interested in the direction and degree of those differences. I'm sure these factors have been explored by someone.
Ken:)

[Mark75H]

Mark,
I don't think you understand the question. .... I'm interested in the direction and degree of those differences. I'm sure these factors have been explored by someone.
Ken:)

I understood exactly what you meant. The only one I can imagine would be Paul Kamen aka "Fishmeal"

[david bryan]

i have only ben around boat racing for 50 years so dont know much about thrust but i do know about rpms and that is a propeler officiates increases with the rpm

[kenboyd]

Thanks guys.:)
Ken

[JohnsonM50]

In theory, what effect would a one inch decrease in diameter have on thrust, in conjunction with the RPM increase that would be produced, by a propeller moving through the water, if no other factors were changed?
(This is an example to make understanding what I'm asking more understandable. These numbers are for simplicity and clarity only). For instance, if I had a 10 inch prop producing 100% thrust at a given RPM and NO other factors changed (horsepower, pitch, blade shape, style, or number, drag, viscosity, velocity, etc.), and I reduced the diameter 1 inch, the thrust would be 90% at the same RPM, then 80% for 2 inches, 70% for 3 inches....
Now, the question again in its final form. Using the above example: What would the thrust be with the increase in RPM? Would the thrust be less than 10% decrease or more than 10%, for each 1 inch diameter decrease?
I'm not interested in the effects hull design have on the equation. I'm just talking about a prop moving through the water. Blade shape, design, and number, exact figures or Reynolds numbers are irrelevant in this question. I'm aware that these factors all have an effect in reality, but I am not interested in these effects for this theoretical question.
Ken:)
Without knowing much science on the matter I have a prop I wanted more Rs out of so I carefully & methodically removed about 10% of the dia. Then with calipers as a measure I evened them & re-edged them [3 blader]. Last but not least I checked for balance. When done it gained about 200 Rs on the exact set up as B4 & about 2 mph. So, in percenages that will be 'about' 10% off = 4-5% RPM and speed. This started out as a 7 1/4 cleaver that is now 6 1/2 and my fastest prop for the boat I run it on.
What I leave to :confused: is by needing to trim back the leading edge some to re-shape, has the pitch increaased? Otherwise Id think Sam has it right that its an art. I got lucky, no way to be sure but to do it at your props risk.:cool:
What to disclaim: at the same time I thinned the prop a little more again using calipers to slide in & mark along the blades & connect the dots A pattern of thickness appears from the lines drawn to see where work is needed. [this was definitely a few late nights at the bench] :rolleyes:

[kenboyd]

JohnsonM50
That answer is in the direction that I was looking for. Thanks Mike
Ken:D

[JohnsonM50]

JohnsonM50
That answer is in the direction that I was looking for. Thanks Mike
Ken:D
Glad to help. I find by figuring potential speed [B4 slip/resist loss] & subtracting real speed that cleavers are very efficient. Of course 2 blade props wont be as efficient at all, the few round eared 3 bladers I have dont do as well. This doesnt mean cleavers are faster across the board, just that 2 of them are working for me. The fastest is a low rake cleaver, it airs the flat bottom out just right. The favorite is a high rake cleaver, its nearly as fast, has better traction & acceleration. The boat is...
http://i35.photobucket.com/albums/d151/mes355/442.jpg

[Mark75H]

Your results may differ, (batteries not included, see dealer for details, void in states where prohibited or taxed)

[zul8tr]

In theory, what effect would a one inch decrease in diameter have on thrust, in conjunction with the RPM increase that would be produced, by a propeller moving through the water, if no other factors were changed?
(This is an example to make understanding what I'm asking more understandable. These numbers are for simplicity and clarity only). For instance, if I had a 10 inch prop producing 100% thrust at a given RPM and NO other factors changed (horsepower, pitch, blade shape, style, or number, drag, viscosity, velocity, etc.), and I reduced the diameter 1 inch, the thrust would be 90% at the same RPM, then 80% for 2 inches, 70% for 3 inches....
Now, the question again in its final form. Using the above example: What would the thrust be with the increase in RPM? Would the thrust be less than 10% decrease or more than 10%, for each 1 inch diameter decrease?
I'm not interested in the effects hull design have on the equation. I'm just talking about a prop moving through the water. Blade shape, design, and number, exact figures or Reynolds numbers are irrelevant in this question. I'm aware that these factors all have an effect in reality, but I am not interested in these effects for this theoretical question.
Ken:)

From a pure theoretical aspect the thrust provided by a propellor pushing a fluid (water or air) per the momentum theory is related to

Thrust = mass flow rate thru propellor x Velocity

More specifically the variables are: density of the fluid, the area of the propellor disk and the square of the change in velocity (acceleration) of the fluid thru the propellor or:

Thrust = Ct x ro x area x (Vj^2 -V^2)

Ct = thrust coefficient, via experiment
ro = density, esentially constant for water but very variable for air
Vj = velocity of water jet exiting the prop i.e the water we see going
backwards from the prop. Note low roostertails = more thrust

V = velocity of boat, opposite Vj

Since area = pi/4 x D^2

D = diameter

Then

Thrust = Ct x ro x pi/4 x D^2 x (Vj^2 -V^2)

Note V and Vj are related to propellor rpm = engine rpm/ gear ratio
Also note that the thrust is a max when the boat is not moving or pushing at max rpm and not going forward. Also as the boat speed increases the thrust decreases and at max speed the thrust is exactly equal to all the drag.

Thrust is very sensitve to diameter and velocity change since they are related as the square. Thus if all else remains the same and the diameter was decreased by say 1" on a 10" prop that would result in a relative thrust change of (9/10)^2 = 0.81 or a 19% reduction. However this assumes that there is no change in Vj and V the velocity change thru the prop and for a diameter reduction there would be changes in the velocity thru the prop.

There are many variables and thus the analysis is quite complex especially when surface piercing propellors are considered.

Here is an interesting analysis I found for a jet ski. The same principles apply in that the water jet moving backward is an anology to the prop moving the water backward.

http://greenhulk.net/forums/showthread.php?p=430459&highlight=thrust+output

Here is a text by Dave Greer on propellors that has some usefull insight

http://books.google.com/books?id=8w09O4hooWUC&pg=PA60&lpg=PA60&dq=propeller+thrust+formula&source=bl&ots=AdlFZfpBKZ&sig=aB9ClrViADU2wftJviqT71sgZtU&hl=en&ei=glwDSuY6lqW2B6OzkJUH&sa=X&oi=book_result&ct=result&resnum=9#PPP1,M1

[kenboyd]

zul8tr,
EXCELLENT:D!! You hit the nail on the head. Very interesting and informative read.
The links are great! They get very deep into related subjects. It looks like I'll have hours of stimulating insights, from all the information you provided through these links. Thanks Peter:D

[capnzee]

After you finish working all the numbers you may have to test, if for no other reason but to find out you wasted a lot of time you could have spent on the water having fun. :rolleyes::rolleyes::rolleyes:

[zul8tr]

After you finish working all the numbers you may have to test, if for no other reason but to find out you wasted a lot of time you could have spent on the water having fun. :rolleyes::rolleyes::rolleyes:

No doubt testing is the last word but understanding the principles (that he asked for) is definately a plus. If Edison better understood the principles he probably would have expended much less than 99% perspiration and had more than 1% inspiration for his inventions - he never should have fired Tesla.

[Smokin' Joe]

Gerr's book is well written for laymen, I'm reading it. Unfortunately, we do not really understand what goes on with surfacing props, except that they leave and reenter the water and behave like a rotating hydrofoil. From a basic standpoint an airplane wing, a prop blade and a sail all generate high lift(thrust)/drag by the same nonintuitive method: circulation is generated around the foil by shedding vorticity from the trailing edge. A too large angle of attack increases eddies shed at the leading edge, which only amounts to drag. The theory of propellers today is actually not better than it was 30 years ago, amounts only to scaling by various constants (Reynolds nr., Froude nr., etc), assuming that drag is some power of the dimensionless speed, then fixing coefficients empirically. That's what Gerr's book is about.






From a pure theoretical aspect the thrust provided by a propellor pushing a fluid (water or air) per the momentum theory is related to

Thrust = mass flow rate thru propellor x Velocity

More specifically the variables are: density of the fluid, the area of the propellor disk and the square of the change in velocity (acceleration) of the fluid thru the propellor or:

Thrust = Ct x ro x area x (Vj^2 -V^2)

Ct = thrust coefficient, via experiment
ro = density, esentially constant for water but very variable for air
Vj = velocity of water jet exiting the prop i.e the water we see going
backwards from the prop. Note low roostertails = more thrust

V = velocity of boat, opposite Vj

Since area = pi/4 x D^2

D = diameter

Then

Thrust = Ct x ro x pi/4 x D^2 x (Vj^2 -V^2)

Note V and Vj are related to propellor rpm = engine rpm/ gear ratio
Also note that the thrust is a max when the boat is not moving or pushing at max rpm and not going forward. Also as the boat speed increases the thrust decreases and at max speed the thrust is exactly equal to all the drag.

Thrust is very sensitve to diameter and velocity change since they are related as the square. Thus if all else remains the same and the diameter was decreased by say 1" on a 10" prop that would result in a relative thrust change of (9/10)^2 = 0.81 or a 19% reduction. However this assumes that there is no change in Vj and V the velocity change thru the prop and for a diameter reduction there would be changes in the velocity thru the prop.

There are many variables and thus the analysis is quite complex especially when surface piercing propellors are considered.

Here is an interesting analysis I found for a jet ski. The same principles apply in that the water jet moving backward is an anology to the prop moving the water backward.

http://greenhulk.net/forums/showthread.php?p=430459&highlight=thrust+output

Here is a text by Dave Greer on propellors that has some usefull insight

http://books.google.com/books?id=8w09O4hooWUC&pg=PA60&lpg=PA60&dq=propeller+thrust+formula&source=bl&ots=AdlFZfpBKZ&sig=aB9ClrViADU2wftJviqT71sgZtU&hl=en&ei=glwDSuY6lqW2B6OzkJUH&sa=X&oi=book_result&ct=result&resnum=9#PPP1,M1

[zul8tr]

Gerr's book is well written for laymen, I'm reading it. Unfortunately, we do not really understand what goes on with surfacing props, except that they leave and reenter the water and behave like a rotating hydrofoil. From a basic standpoint an airplane wing, a prop blade and a sail all generate high lift(thrust)/drag by the same nonintuitive method: circulation is generated around the foil by shedding vorticity from the trailing edge. A too large angle of attack increases eddies shed at the leading edge, which only amounts to drag. The theory of propellers today is actually not better than it was 30 years ago, amounts only to scaling by various constants (Reynolds nr., Froude nr., etc), assuming that drag is some power of the dimensionless speed, then fixing coefficients empirically. That's what Gerr's book is about.


Agreed on what Gerr's book is about and I was only offering it to provide some basic info for prop theory and agreed that surfacing props are difficult to understand and predict performance. But I have found my rather simple relations have proved to be very useful in setting up my hydro.

As far as the lift/drag theory of wings, props and sails by nonintuitive I would add that for sails there is another theory out there. Since a sail is a thin cambered sheet when properly set in the wind there is no relative difference in cambered shape of the front and the back face like there is with a wing and prop blade. Therefore the usual wing theory of lift by pressure difference from the lower pressure on the greater curved side than the flatter does not totally apply to a sail. A more reasonable explaination results from the sail simply changes the direction of the air from the leading edge to trailing edge that passes around it and that deflection constitutes a force from Newton's F=ma. The angle that the air enters the sail is different than when it leaves at the trailing edge thus resulting in sail force. Therefore for sails the greater the deflection the greater the force (within limits of course). No doubt that shedding eddies has some contribution as well as other items. If interested here is a fairly good explaination of the theory:

Prelude
http://www.sailtheory.com/wrongtheory.html

Theory
http://www.sailtheory.com/sail.html

[Smokin' Joe]

Thanks for the communication. Regarding the link, the writer hasn't studied enough hydrodynamic theory. A flat plate at an angle to a fluid flow experiences both a lift and a drag, but without circulation about the plate you can't get an efficient lift/drag ratio (a boat bottom is indeed like a flat plate with an incompressible fluid on only the bottom side, neglecting the air flow over the top to first order). But an airplane wing can't lift off the ground like that. Another way to say it: if you neglect drag (vorticity creation), and so treat the fluid as ideal and then calculate the net force on the plate or sail then the net force vanishes vanishes (this is a standard theorem in hydrodynamics--if you throw a block of wood into a hypothetical idea flow then the river does not carry the block downstream). In a real, viscous fluid like air or water a sail can't work efficiently simply with wind blowing against it, the drag will match the thrust/lift. If you can get the circulation started around a mast with no sail, then the mast will experience a side force relative to the wind. I have to warn you that freshman and sophomore physics texts get this wrong, in reviewing those books in the past I railed against that mistake to no avail. But engineering texts do not get it wrong, nor do advanced physics texts on fluid mechanics (like Landau-Lif****z, e.g.).

Apparently, it would be necessary to spell Lif****z in Russian to get this name past the web control: Лифшиц





Agreed on what Gerr's book is about and I was only offering it to provide some basic info for prop theory and agreed that surfacing props are difficult to understand and predict performance. But I have found my rather simple relations have proved to be very useful in setting up my hydro.

As far as the lift/drag theory of wings, props and sails by nonintuitive I would add that for sails there is another theory out there. Since a sail is a thin cambered sheet when properly set in the wind there is no relative difference in cambered shape of the front and the back face like there is with a wing and prop blade. Therefore the usual wing theory of lift by pressure difference from the lower pressure on the greater curved side than the flatter does not totally apply to a sail. A more reasonable explaination results from the sail simply changes the direction of the air from the leading edge to trailing edge that passes around it and that deflection constitutes a force from Newton's F=ma. The angle that the air enters the sail is different than when it leaves at the trailing edge thus resulting in sail force. Therefore for sails the greater the deflection the greater the force (within limits of course). No doubt that shedding eddies has some contribution as well as other items. If interested here is a fairly good explaination of the theory:

Prelude
http://www.sailtheory.com/wrongtheory.html

Theory
http://www.sailtheory.com/sail.html

[Mark75H]

I have read that advanced research always presents angle of attack as the strongest force in lift.

Going thinner and increasing angle of attack increases lift, which is contrary to "airfoil" lift theory. A wing has lift if it has angle of attack, regardless of how thin it is.

[Smokin' Joe]

When the angle of attack of an airfoil is too large then big eddys form at the leading edge. this is called 'stall', the plane loses altitude. Ask yourself what too large an angle of attack would lead to with a prop.

Prop cup is just like dropping the flap on the trailing edge of an airplane wing, it increases the circulation about the foil by shedding more vorticity off the trailing edge (the latter is drag, and to keep lift/drag high the foil's wake, the vorticity shed in the form of helices from a prop blade) must remain thin. I played with leading edge camber in 1978 before any outboard prop specialist thought about it. Actually, some Fla. racers looked at my props and tried to copy, then in 1981 or so we got props from OMC and Mercury with some leading edge camber. I'm not about to say 'why' that's important, I never got enough together either empirically to write an article about it.




I have read that advanced research always presents angle of attack as the strongest force in lift.

Going thinner and increasing angle of attack increases lift, which is contrary to "airfoil" lift theory. A wing has lift if it has angle of attack, regardless of how thin it is.

[Mark75H]

Yes, "stall" comes in at the extreme of excessive angle and low speed, I was referring to the other end of the curve.


All wings and props meet a stall limit somewhere, even yours.

[Smokin' Joe]

I tried to recollect coherent thoughts while going to the grocery (I taught grad classes in hydrodynamics a few times). Here's a summary in a nutshell. You start with D'Alembert's Paradox, that in the flow of an 'ideal fluid' (viscosity=0) past an object there is no net force exerted on the object (the sum of all local forces over the entire body adds exactly to zero). There's no drag, no lift, no nothing. The object is not only not dragged downstream, it isn't even wet (first layer of fluid slides past without sticking). Next comes the fact that drag (the main fluid force, the one that's always present) is due to the (real, viscous) fluid having been decelerated in layers flowing near the object (think of a fixed object, flow runs past it, the first layer of fluid sticks to the object, this is how the object gets wet and experiences a net force). That fluid forms the wake, and mathematically is represented by vorticity. Vorticity and circulation are related but there's no circulation here. A plate at an angle to such a flow experiences a small side force but mostly drag, the lift/drag ratio is low. Now, circulation (related to vorticity) is approximately conserved if the viscosity is not too large (as in air, water). When a plane runs down the runway, the wake has the form of a vortex sheet (a velocity discontinuity, with speed higher on top side of sheet). But the sheet is unstable and eventually rolls up into a vortex/eddy (with finite circulation about it). Since circulation is nearly conserved, a vortex/eddy of opposite sign has formed about the wing. When the trailing vortex splits off then the wing lifts because an eddy in a flow experiences a side force. Prandtl explained this all in a short paper ca. 1916.

It's the circulation about the wing/foil that produces the high lift/drag ratio necessary for flight (or for a prop or sail). Otherwise, you have a small lift/drag ratio, as with a flat plate in a flow with no circulation about it.

Theoretical physics can't be avoided if you want to understand props.... .






Yes, "stall" comes in at the extreme of excessive angle and low speed, I was referring to the other end of the curve.


All wings and props meet a stall limit somewhere, even yours.

[Smokin' Joe]

You can think of the sail as a cambered sheet with wind moving fast over the convex side, and with 'dead air' (a big trapped eddy at some Reynolds nr.) inside the concave side. So, taking the boundary layer into account, the net profile looks more like an airfoil of finite thickness, thickest near the middle of the sail. Edgar Rose would surely enjoy this discussion, given that physics has entered .... .


Agreed on what Gerr's book is about and I was only offering it to provide some basic info for prop theory and agreed that surfacing props are difficult to understand and predict performance. But I have found my rather simple relations have proved to be very useful in setting up my hydro.

As far as the lift/drag theory of wings, props and sails by nonintuitive I would add that for sails there is another theory out there. Since a sail is a thin cambered sheet when properly set in the wind there is no relative difference in cambered shape of the front and the back face like there is with a wing and prop blade. Therefore the usual wing theory of lift by pressure difference from the lower pressure on the greater curved side than the flatter does not totally apply to a sail. A more reasonable explaination results from the sail simply changes the direction of the air from the leading edge to trailing edge that passes around it and that deflection constitutes a force from Newton's F=ma. The angle that the air enters the sail is different than when it leaves at the trailing edge thus resulting in sail force. Therefore for sails the greater the deflection the greater the force (within limits of course). No doubt that shedding eddies has some contribution as well as other items. If interested here is a fairly good explaination of the theory:

Prelude
http://www.sailtheory.com/wrongtheory.html

Theory
http://www.sailtheory.com/sail.html

[kenboyd]

Yes, "stall" comes in at the extreme of excessive angle and low speed, I was referring to the other end of the curve.


All wings and props meet a stall limit somewhere, even yours.

I would like to add that angle of attack, is the main factor in producing stall. A stall can occur at any speed, if the angle of attack is great enough to cause the fluid medium to separate from the foil.:)

[Smokin' Joe]

Correct. A good picture book of flow patterns is S. Goldstein's "Modern Developents in Fluid Dynamics", vol. 1, boundary layer separation is shown there. There's a real picture book of flow patterns but I can't recall either the author or title.




I would like to add that angle of attack, is the main factor in producing stall. A stall can occur at any speed, if the angle of attack is great enough to cause the fluid medium to separate from the foil.:)

[Smokin' Joe]

When testing race boats, motors, props, I kept written records of everything. Otherwise I couldn't remember from one time to the next what I'd done and what was the result. I made some good props, and some good boat bottom modifications. I would not have found it fun to buzz around on the lake without improving performance. Unfortunately, propeller theory is nearly non-existent, nearly everything we know comes from trial and error. I would still like to understand how a surfacing prop works (I mean REALLY works).




After you finish working all the numbers you may have to test, if for no other reason but to find out you wasted a lot of time you could have spent on the water having fun. :rolleyes::rolleyes::rolleyes:

[nonslip tip]

:) Robert Kress has done some good papers on surface or super cavitating propellers. Also to add to all that has been said,look up patent #6352408, and yes it does work. Airplne tips , props all work the same. Alway so props with these tips have been run through the Navy's David Taylor model basin and a folding prop for sailboats has been truogh the test tank in Berlin.:)

[Mark75H]

Wouldn't the added weight on the tip limit this kind of prop to relatively low rpm use?

[Smokin' Joe]

I'm always bothered by the term 'supercavitating' and the assumption that a cleaver with a thick trailing edge is necessary. In Mod VP preparing for Havasu in 1981 we found that choppers and other round eared props with normal trailing edge were better. I doubt that a cavity of any size can form over the low pressure side of an efficient (i.e., one that does the job) suction side of a blade. A cavity would represent an effectively very thick blade and more drag. Maybe there are just some air bubbles, underwater photos that 'stop' the blades would be necessary. One thing is fairly obvious: a normal submerged prop has a triple helical wake. A surfacing 3-blade prop with one blade entering as the other leaves has less than 1/3 of that wake, so less drag.
Clear that a prop with 4 or more blades will create more drag, ceteris paribus.

I haven't understood the bent upward tip on a wing, haven't done my homework. Can imagine that the purpose is to make the trailing vortex sheet (wake) smaller, drag reduction. Not clear why that would apply to a helical vortex sheet ... .



Wouldn't the added weight on the tip limit this kind of prop to relatively low rpm use?

[Smokin' Joe]

Can you give me the references, or email them in pdf form? Am reading
Gerr's nice book and haven't read prop papers since ca. 1980. Do you have the paper by Railton & Kamen on surfacing props? I'm landlocked in an alpine village til Aug. Where's a photo of the bent-tip boat prop?

Thanks,
jmccauley@uh.edu



:) Robert Kress has done some good papers on surface or super cavitating propellers. Also to add to all that has been said,look up patent #6352408, and yes it does work. Airplne tips , props all work the same. Alway so props with these tips have been run through the Navy's David Taylor model basin and a folding prop for sailboats has been truogh the test tank in Berlin.:)

[Mark75H]

Any idea why racers are having success with 4, 5 and 6 blade props?

[Smokin' Joe]

Mystery to me. Are they as fast as 3 blade? Hydrodynamics is nonintuitive because nonlinear.





Any idea why racers are having success with 4, 5 and 6 blade props?

[Mark75H]

Mystery to me. Are they as fast as 3 blade? Hydrodynamics is nonintuitive because nonlinear.

Faster. In the classes where 4 and 5 blades are allowed, you aren't competitive without them.

[Smokin' Joe]

Interesting. Closed course, then. Straightaway records?




Faster. In the classes where 4 and 5 blades are allowed, you aren't competitive without them.

[Mark75H]

Both extra letters

[Smokin' Joe]

Very interesting. How many blades for straightaway records?


Both extra letters

[Mark75H]

Generally 4, but some are having success with 5 and 6, too.

I suspect it is blade area that is beneficial and counteracting other factors where a greater number of blades detracts from performance.

Many factors to balance in this dance.

[nonslip tip]

:)Sorry it took so long to get back,but had to finish a bunch of props so people can bang up them up this weekend and i can do it again next week:D.Anyway to answer some questions,no it does'nt add weight by having tips because you have to reduce dia. when you tip because if the increased loading of the prop,other wise if you tip at dia. you will overload the engine because of the increase in efficiency of the prop.How tipped props work is by tipping you eliminate or greatly decrease tip vortices,for as water comes off the tip it creates vortices which draw water off of the suction side and the pressure face,so if you can stop this you create more suction on the suction side which in turn creates more pressure on the pressure side which creates more efficiency. The benefits are more thrust straight back,improved handling,decreased fuel,more overall boat lift, not transom or bow but overall more lift all of which leads to go speed. So in a nutshell you decrease your slip.I do have Krees's papers and the Railton&Kamen papers i will dig them out(Doug Railton used to spend a great deal of talking to my dad Bob Kilian and myself at Pitchometer Propeller years before any papers were written)trkilian@yahoo.com 510-357-3480

[nonslip tip]

:D I do have pictures of all types of props we have tipped,I will get them scanned and post next week,also if you want i can mail you copies.Tim R. Kilian

[Mark75H]

Tim I still would like to see results showing operation at higher rpm

[nonslip tip]

How high an rpm or speed are we talking? We have them on boats with 6000 rpm engines running up to 35% over in the vee drive and on boats that exceed 125 mph.

[Smokin' Joe]

Sorry too, I had to try (with all books and papers in Houston) to remember some of what I knew 30 years ago. The purpose of a bent-upward tip on an airfoil is to try to constrain the flow over the foil to 2D (2 dimensions). Here's what's going on.

Circluation about a foil (wing, blade, sail) is like a vortex/eddy about the foil (Ludwig Prandtl, ca. 1916). A vortex cannot end in the fluid (conservation of circulation), it can only end at a boundary or interface (or close on itself like a smoke ring). One end of the vortex ends on the body of the plane (hub of the prop). so the vortex extends way beyond the wing/blade and must end someplace. With the plane, the other end of the tip vortex ends on the ground where it started. A tip vortex coming off a prop blade ends on the surface of the water, where it started. Vortices are unstable against viscosity, so they diffuse away. You can best see them by playing with your paddle while sitting in the boat. The tip vortex is one 3D effect that only adds to drag, but cannot be eliminated (due to circulation conservation in the fluid). Here's a related 3D effect.

An infinitely long wing would have strictly 2D flow over it. A real wing has a velocity component transverse to the wing (radial to the prop blade) and only stretches the vortex over the wing/blade, adding drag but no lift/thrust.
Adding the bent-upward tip forces the radial component of fluid velocity (let's use prop language) to be zero along the bent-upward tip (the fluid velocity vanishes at any solid boundary). This constrains the flow over the suction side of the blade to be 'more like 2D flow', reducing the drag. Of course, the bent-upward tip adds drag in the form of extending the vortex sheet off the trailing edge, just as would adding prop diameter. So a bent tip should work on a prop.

Yesterday (during a break in a jazz festival near the Inn River in NE Austria) I saw a 5-6m dia. variable pitch ship prop, a rusty relic on display. The prop had 'bent-upward leading edges' (welded on) along a chord short of the max. radius. This is the same idea as the airfoil tip (try to make the flow over the suction side more 2D), so the notion's been around for ships for some time.












:)Sorry it took so long to get back,but had to finish a bunch of props so people can bang up them up this weekend and i can do it again next week:D.Anyway to answer some questions,no it does'nt add weight by having tips because you have to reduce dia. when you tip because if the increased loading of the prop,other wise if you tip at dia. you will overload the engine because of the increase in efficiency of the prop.How tipped props work is by tipping you eliminate or greatly decrease tip vortices,for as water comes off the tip it creates vortices which draw water off of the suction side and the pressure face,so if you can stop this you create more suction on the suction side which in turn creates more pressure on the pressure side which creates more efficiency. The benefits are more thrust straight back,improved handling,decreased fuel,more overall boat lift, not transom or bow but overall more lift all of which leads to go speed. So in a nutshell you decrease your slip.I do have Krees's papers and the Railton&Kamen papers i will dig them out(Doug Railton used to spend a great deal of talking to my dad Bob Kilian and myself at Pitchometer Propeller years before any papers were written)trkilian@yahoo.com 510-357-3480

[nonslip tip]

:D My Grandfather and my father started doing tipped props in the late 40's and early 50's.Their are patents on various tipped props going back to the 30's, so the idea is not new,making them work has always been the trick,especially at speed beyond displacement and semi-displacement speeds. I still have some of the patterns left from when they were making them.

[Smokin' Joe]

Tim,

Posted is interesting for the community, I can then download, thanks!

Joe


:D I do have pictures of all types of props we have tipped,I will get them scanned and post next week,also if you want i can mail you copies.Tim R. Kilian