# Slip factors

Discussion in 'Hydrodynamics and Aerodynamics' started by Chris Pap, Jan 14, 2015.

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### Chris PapJunior Member

Hello,

I would like to ask if anyone knows the factors that affect propeller slip.
Hence, what can be done to avoid excessive slip.

Thank you,
Chris Pap.

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### daiquiriEngineering and Design

It would be necessary to understand first what do you intend by slip.

If you define it as the ratio of the theoretical distance traveled by the prop (imagining water as a solid medium) divided by the actual distance traveled, then the formula for the slip is
s = Lth / Lreal = (n P) / V = (P/D) / J
where:
n = revs per second,
P = pitch,
D = diameter,
V = advance speed,
J = advance ratio = V / (n D),
and you can immediately see from the formula what can be done to reduce the slip.

If you define slip as the quantity
s = (Vs - V) / V = (Vs/V) -1
where:
Vs = flow speed in the prop wake,
V = advance speed,
then it essentially depends on the disc loading - which is the thrust per unit disc area, T/A.

That's because Vs/V = sqrt(1 + Ct), where Ct is the thrust coefficient:
Ct = T / ( 0.5 rho V^2 A ).
So - the higher the disc loading, the higher the propeller slip. Hence, in order to minimize slip you have to minimize the disc loading of the prop.

It can be done in different ways, depending on which variables can be modified and which ones are fixed. Since you are the student here, I'll let you explore various paths which can lead you towards that goal.

Cheers

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### Chris PapJunior Member

More or less I define slip with the first way.
Firstly I find the theoritical speed by multiplying the pitch with RPM and a coefficient depending on the units. After that slip is defined by:
s=(theoritical speed-actual speed)/theoritical speed.

So my query lies by what and in which way slip is affected. It may be the weather, the current, the draft , the fouling , the pitch(for CPP), or the propeller material. But which other factors are affecting it? And if I have those data by a existing ship is there any way to analyze which percent of the slip is caused by each one of the abovementioned factors?

To be honest the the second way is very interesting. It would be a good idea to analyze slip by that equation too. But is there any way to calculate flow speed in the prop wake(Vs)? Or in order to connect the two ways of defining slip is there any way to calculate Vs by including the factors I mentioned before?

Thanks very much for your interest..
Chris Papandreou

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### DCockeySenior Member

Slip based on propeller pitch is essentially an alternative measure of the angle of attack of the propeller blades to the water.

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### gonzoSenior Member

You also need to calculate the speed of the water at the stern. There is a reverse of the flow, which affects the calculations.

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### Ben GJunior Member

Reducing disc loading means make the propellor bigger or reduce the power input, (assuming it's pitched correctly)

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### TeddyDiverGollywobbler

Size, as diameter and blade area. Hence, get a bigger propeller

You are welcome,
BR Teddy

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### sandhammaren05Senior Member

Camber and cup, especially near and at the blade tip, reduce slip. 'Slip' yields the pitch of the helical wake, which is less than the blade pitch. The blade pitch increases from leading to trailing edge on high speed props (flat blades are for ships and fishing motors). The low pressure side is highly cambered, the high pressure side should also be cambered. Slip can appear a a factor in the mechanical efficiency. For high speed craft (raceboats )mechaanical efficiency is .6 to .7, slip is about 5% at mid-chord, and a properly designed surface-piercing prop ha about zero slip at the leading edge.

Title and table of contents of a book ms I've just finished, am looking for a publisher-

The Physics of Surface-Piercing Propellers and High Performance Boats
Speed through drag reduction

Joseph L. McCauley
Physics Dept.
Univ. of Houston
Houston, Tx. 77204

1.Hydrodynamics

1.1 Basic equations of fluid flow
1.2 ‘Dry water’
1.3 The Reynolds number and scaling
1.4 Vorticity, the boundary layer, and the wake

2. Propeller efficiency

2.1 Propeller approximated as an ‘actuating disc’
2.2 A propeller blade creates a helical wake
2.3 Scaling of thrust, torque, power, and efficiency
2.4 Mechanical efficiency of propellers

3. Optimizing raceboat performance

3.1 Optimizing acceleration and top speed
3.2 Scaling of propeller diameter with power and RPM

4. Form drag, waves, and skin friction

4.1 Dynamics of vorticity and vortices
4.2 Boundary layer separation and eddy creation
4.3 Scaling of speed with power and weight

5. Theory of lift and drag on a hydrofoil

5.1 How an airplane flies, how a propeller propels
5.2 Theory of lift and drag on a hydrofoil
5.3 Propeller blades as rotating hydrofoils

6. High performance boat set up

6.1 Fast boat bottoms and set-up
6.2 Planing speed
6.3 Camber, center of pressure and stability of a tunnel
6.4 Rake and prop riding
6.5 Lubrication, tractor and overdrive gearcases, world speed records, and emissions

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### johneckSenior Member

The blade pitch doesn't really change from LE to TE, the sections are cambered and the camber leads to the appearance of a "cupped" pressure face. Pitch is often radially variable and thus the "slip" of the propeller is really a very imprecise way of measuring propeller performance. So your goal should be to maximize some aspect of propeller performance, not reduce slip. The equations provided by Daiquiri provide a way to look at efficiency. There are of course, many constraints and limitations either geometric or physical that need to be factored in.
Generally, if efficiency is the goal, then going bigger in diameter and lower RPM is the direction to head.

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### sandhammaren05Senior Member

The blade pitch changes from leading to trailing edge. If it doesn't then you do not have a high performance propeller. Maybe you have an old ship prop. Cup is only a very sharp camber over a tiny chord at the trailing edge and should not be confused with the camber of the blade, which is necessary for high performance. Pitch should not vary radially, that only thickens the wake, which means increased drag.Slip on a high performance prop should not be more than 5% when the pitch is measured at mid-chord, the prop's mechanical efficiency is 60-70%. Bigger diameter and low RPM might be ok for ships, but that will place you dead in the back of the pack in a boat race! We want max. shp coming off the start dock and out of a turn (about 5500 RPM) and then we turn the motor 8000 RPM to get top speed at the end of the stretch. The advice to turn as big a dia. as slow as possible follows from the (faulty and non-falsifiable!) treatment of a prop a an actuating disc.

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### johneckSenior Member

I would define the pitch based on a straight line from a point defining the leading edge to a point at the trailing edge. The angle of this line to the blade center axis is the pitch angle. I believe that you are referring to camber which would be the arc that the actual blade section shape takes relative to this line. The camber is used to increase the lift coefficient of the section without increasing the angle of attack of the blade section. The camber and pitch can be treated independently and optimized for any desired radial circulation distribution. Obviously, as you mentioned, there are many ways of deciding what the "optimum" is depending on what you are trying to accomplish. In some cases an efficiency of 60-70% is reasonable, in some cases it can be as high as 90% or much, much lower.

Pitch is often varied radially to properly match the blade section performance to the non-uniform inflow caused by the body in front of the propeller. There may also be other reasons to vary the pitch for acoustics, vibrations, cavitation...

The turbo machinery guys talk in term of inlet angle versus exit angle. This may be what you are thinking about. I have no doubt that for race boat props that operate at really high flow speeds and in and out of the water, that the standard way of describing aerodynamics or propeller hydrodynamics probably isn't worth much.

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### daiquiriEngineering and Design

I hope that you are just using a very uncommon terminology, otherwise this phrase is evidently not true.
The pitch cannot change from leading to trailing edge. If you take a generic cross-section (or airfoil) of a propeller blade, the pitch is then by definition a theoretical distance which this airfoil will cover during one revolution. In order to define this distance, we need to define a pitch angle of the airfoil - which is the angle of the helix drawn by the airfoil during one rotation. This angle is measured relative to a straight line which connects the leading and trailing edge of the airfoil, and this line is called chord line (simplified to "chord") or nose-tail line.
Hence, since the chord is a fixed line for a given airfoil at a given radial station, and the pitch is measured relative to the chord, it means that the pitch cannot change from leading to trailing edge.
Perhaps you wanted to use the term "camber" instead of the pitch? The camber does change along the chord.
Again not entirely true, and again probably due to imprecise nomenclature used. There is difference between geometric pitch and hydrodynamic pitch.
For a given advance ratio J, the inflow angles of the fluid relative to the blade can vary by 40-50° at various radial stations from the root to the tip. If the local geometrical pitch is not adjusted accordingly, the blades at different radial stations would operate at a wildly varying range of angles of attack, some sections possibly even at negative ones (producing drag instead of thrust).
Hence, the geometrical pitch of airfoils has to vary along the blade (so-called blade twist), to follow the variable radial inflow conditions. This condition then can, but not necessarily, lead to a constant hydrodynamic pitch. It will depend on the characteristics of the airfoil used along the blade and on the type of propeller optimization at the design stage.
Cheers

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### daiquiriEngineering and Design

Ok, you are a much faster writer than I am.

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### DCockeySenior Member

Discussion of pitch varying along a section seems to originate with marine propeller designs that have sections with the back side straight. The pitch is then defined using the straight back side, and can be measured at anywhere along the straight back side with no need to define a line from the leading edge to the trailing edge. For a design where the back side is not straight some say that the pitch varies along the section.

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### daiquiriEngineering and Design

Hi David,
I still remember all the discussions we have had in this forum about the importance of correct terminology... If I call apple what someone else calls orange, we get nowhere. The propeller terminology and definitions are pretty much standardized throughout technical literature, codes and regulations - both aeronautical and marine ones. I think that we should all try to stick to the uniform terminology, lest we create confusion like this one. That is IMO even more true for someone who decides to write a book about this particular field, because using a non-standard terminology in specialist books (which are very often trusted without questioning by the general audience) is the best way to spread confusion and create misinformation - even if author's explanations of physical facts are perfectly valid.
My two cents worth.
Cheers

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