Defining propeller pitch

[TANSL]

there are several ways of measuring the pitch

Wise words.
Any distance can be measured in various ways, but that does not change the value of the distance or what it represents.

 

[DogCavalry]

Very interesting @jehardiman . My only marine source, as opposed to air souces, is Gerr's book. At the time of writing, props typically had constant pitch. But then that book is more than 30 years old, and state of the art is always a moving target. So pitch is eased off at both root and tip to achieve an eliptical load on each individual blade? Fascinating stuff.

 

[Barry]

Actually, most standard marine propellers have decreasing pitch as you get closer to the hub, that is why pitch at 0.7 r is typically defined as the pitch of the wheel. This has to do with real world hub length and blade strength issues. 0.7 r is effectively the center of the working pressure face. It is rare to have a wheel with the same pitch from root to tip, especially if you have a full on wake adapted wheel.

Jehardiman, has entered .7 as the radius to use to determine the pitch of a propeller and the link that I provided used 2/3rds and others about the same. So one might make the conclusion that if at .7ish radius, the calculation of forward
travel of a revolution in a no-slip situation could be the number that gets stamped on the propeller.

We know that the pitch is variable across the blade. (root to tip-for clarification) The way that it was explained to me too many years ago was that the pitch decreases in numerical value as it approaches the hub, in order to keep the water accelerated to the same
output speed across the blade face /momentary disc cross section/out put accelerated cylinder. Ie at the outer tip, for every revolution, minimum pitch location, the tip travels further in the water than at the hub (2 pi r) , maximum pitch location in order for the blade at the hub to accelerate the water closer to this speed, the pitch must be lower.
My understanding is that this reduces/minimizes turbulence drag at the high pressure blade side. Unfortunately, I have not been unable to find any "official" papers that support my comments.

If the pitch did not decrease towards the hub, this area of the blade could offer little acceleration of the water disc/cylinder and depending on inlet speed , ie boat speed plus the water speed due to the upstream water speed caused by the
pressure differential.

I am using speed of the water instead of velocity as there is always a flow component not axial to the propeller. Certainly the decreased pitch physical shape will increase the strength of the blade, but so could increased cupping. Further
displacement hulls will create upstream parameters to the propeller.

 

[jehardiman]

We know that the pitch is variable across the blade.

Perhaps you meant along the blade, i.e. root to tip. "Across", at least to me, implies a chordwise change, which I don't think you intended.

The way that it was explained to me too many years ago was that the pitch decreases in numerical value as it approaches the hub, in order to keep the water accelerated to the same
output speed across the blade face /momentary disc cross section/out put accelerated cylinder. Ie at the outer tip, for every revolution, minimum pitch location, the tip travels further in the water than at the hub (2 pi r) , maximum pitch location in order for the blade at the hub to accelerate the water closer to this speed, the pitch must be lower.
My understanding is that this reduces/minimizes turbulence drag at the high pressure blade side. Unfortunately, I have not been unable to find any "official" papers that support my comments.

If the pitch did not decrease towards the hub, this area of the blade could offer little acceleration of the water disc/cylinder and depending on inlet speed , ie boat speed plus the water speed due to the upstream water speed caused by the
pressure differential.

I am using speed of the water instead of velocity as there is always a flow component not axial to the propeller. Certainly the decreased pitch physical shape will increase the strength of the blade, but so could increased cupping. Further
displacement hulls will create upstream parameters to the propeller.

Ummmm, there is a lot more going in the inner portion of the wheel than can be summed up in a few lines. And yes, there needs to be "new research" and papers to address it. However, in 'large' wheels over 2-3 meters D, there is a real material issue that it requires the blade section be thickened and pitched down for strength. It has been this way since Ericsson's first screw propeller patent, the HMS RATTLER trails, and Brunel's GREAT BRITAIN in the 1830's and '40's.

Anyway, for a complete discussion in the differences of pitch distribution from various lifting line and lifting surface theories prior to RANS, I would suggest C A Johnsson's "An Examination of Some Theoretical Propeller Design Methods"; Goteburg, 1962 (it is on the net and you can see why 0.7 r is selected to define 'pitch' in figures no. 8 and 9). This is after the works on pitch wake adaptation by Eckhardt & Morgan (1955), Van Manen (also 1955), and Troost (1956); but before Kerwin (1964) and the whole lifting line/lifting surface war was waged.

 

[DogCavalry]

This forum is bloody awesome!

 

[hoytedow]

Right(Write) on!

 

[Barry]

Perhaps you meant along the blade, i.e. root to tip. "Across", at least to me, implies a chordwise change, which I don't think you intended.

I should have included root to tip. Though many if not all blades will change the pitch from the leading edge for a given radius to the trailing edge. But I believe the term for this is "progressive pitch"

Ummmm, there is a lot more going in the inner portion of the wheel than can be summed up in a few lines

Agreed, it is much more complicated than a few lines as illustrated on just a few of the books that you quoted, plus the works of Kutta, Prandtl et al.

 

[sandhammaren05]

I suppose in it's most basic form it is the distance travelled forwards in one complete revolution of the propeller, assuming that there is no 'slip' at all.

Every point on every blade of one side (take the high pressure side, e.g.), traces out the same helicoidal surface with pitch pp as the propeller advances at speed U over the period of one revolution. That pitch pp is not the pitch of the propeller, it's what we call the path pitch because it reflects exactly the path followed by that blade surface. The fact that there are two blade surfaces gives the helicoidal wake its thickness, reflecting both induced and form drag. A blade with constant progressive pitch measured over the blade would have a helicoidal shape, meaning that there is no camber. Camber added along every radial arc on the helicoid takes the blade off the helicoid except at the leading and trailing edges. The pitch is therefore the pitch of the helicoid and is measured from leading to trailing edge at constant radius. There is a formula defining progressive pitch in terms of camber as a function of radius and arc length.

 

[DogCavalry]

Another excellent thread from @gonzo , like the electric motor thread.

 

[gonzo]

Camber added along every radial arc on the helicoid takes the blade off the helicoid except at the leading and trailing edges.

That is incorrect any point of the blade will generate a helix if the propeller is advancing at a constant velocity.

 

[johneck]

It is very interesting to read all the different viewpoints on this. It is what makes propeller design such an interesting topic in my opinion. It simply depends on the problem one is trying to solve and what the constraints and optimization goals are. Sandhammaren is operating in a world where there is no non-uniform wake and the pressure surface is the only wetted part, so that is all that matters. Others in in a world where optimum efficiency is the goal and wake adaption and elliptical load distribution maybe is the most important. There are also other aspects of the design goal that can play a part in the design optimization, such as cavitation inception and radiated acoustics. Then there are those that just want to buy the cheapest prop that will push the boat. And it all works!

 

[gonzo]

I agree. The main point of this thread is to define the parameters to be able to have meaningful discussions.

 

[sandhammaren05]

It is very interesting to read all the different viewpoints on this. It is what makes propeller design such an interesting topic in my opinion. It simply depends on the problem one is trying to solve and what the constraints and optimization goals are. Sandhammaren is operating in a world where there is no non-uniform wake and the pressure surface is the only wetted part, so that is all that matters. Others in in a world where optimum efficiency is the goal and wake adaption and elliptical load distribution maybe is the most important. There are also other aspects of the design goal that can play a part in the design optimization, such as cavitation inception and radiated acoustics. Then there are those that just want to buy the cheapest prop that will push the boat. And it all works!

Interesting. Except: (1) with through hub exhaust there is no exhaust deposit on the low pressure blade side, and (2) the thinner the blade the higher the speed, and it's always the low pressure side that we thin. Both of these argue against ''the (high-) pressure side is the only wetted part...". The evidence from practice is that both blade sides are wet after re-entry.

Racers and high performance enthusiasts want the cheapest prop and the fastest prop and the two rarely coincide, if ever. CNC props from Mercury and Herring can cost upward of $20000. No one with an Arnesen surface drive will look for the cheapest prop.

No one addressed in this forum the challenge that I set elsewhere in the forum: In prop design, I have derived a formula showing how camber determines progressive pitch and vv. Of course, if the reader has a confused notion of pitch then he/she/it will only be confused by my challenge. Two coarse results from the formula, along a radial arc on a blade, are: (i) pitch p(r) at radius r = progressive pitch at the camber extremum along an arc at radius r, and (ii) the integral over progressive pitch divided by the arc length at radius r is the pitch p(r) at radius r. So we have pitch p(r) at radius r. We will get p(r)=p independent of r iff. a blade from a casting was CNC-machined using a CAD program where constant pitch p independent of r was written into the program, as in the case (our case) where the blade shape is cut from a helicoid of pitch p, and camber is then added-in the CAD program that is fed into a CNC machine.

None of this has anything to do with where I operate, it's hydrodynamics and mathematics, independent of location, orientation, and time.

The usefulness and advantages of our formula are many. E.g., we can use it to test models of camber and compare the results with those from measurements of racing props. An example is that Tulin-Burkhardt and Johnson 3- and 5- term camber where both were defined for 2D hydrofoil sections. When we adapt those equations to adding camber along the radial arc of a helicoid, thus taking the blade below the helicoid, then the progressive pitch is far too small at the trailing edge once the leading edge condition required for surface piercing has been satisfied. Etc.

 

[gonzo]

You keep on confusing camber with pitch. There are two completely different parameters. If you refer that the tangent to the camber changes, well, otherwise it would be flat. Unless you define what you are talking about, no one can understand. Camber and pitch are well defined, and universally accepted, as two separate parameters. I can't make any sense of your description. Further, it is irrelevant to the discussion of pitch and camber whether it is designed with CADD or cut with CNC. Also, please don't hijack this thread with your nonsense challenge. You decided to close your thread. Reopen it if you want to continues it; but not here.

 

[powerabout]

I can measure a cambered face race prop and get a large range in pitches just depends where on the blade you choose to measure it.
Pitch is only simple when you cut a prop out of the jam tin lid and have flat blades.
Thats also zero rake which can be another variable when added
Did we mention cup?