How do propellers propell a boat.

Like the fastest guy on water in the world that nobody can seem to get close to after 35+ years....
Dont you have the genius first, then the university course followed by more genius's?
Just imagine selling a university course on a subject that doesnt yet exist, nope we all have to wait for the genius first

 

Probably no one brave or silly enough to challenge it, powerabout, I do hope the bloke rests on his laurels, though I hear he has plans to go at it again.

 

There is a technical problem passing the technical test designed by people that have never been fast, currently its not possible to attempt the record.
Ken has built 2 boats to the rules but before they were finished they changed the rules both times and the rules dont apply to boats currently raced that fit that speed criteria.

 

Is that true for any application with a real commercial benefit of having a better efficiency propeller (=faster)? None of the racing, speed record or leisure boat propellers fall into this category. It is just not worth the effort to design a special purpose well optimised propeller for these using e.g. CFD or even more tradiotional methods.

The surface propellers used in racing and speed records have very few commercial applications and are thus not the focus of propeller development. Also they are more difficult to model.

 

How do propellers propel a boat

I would love you to point me to the theory that you think is mostly likely to be reasonably correct. There are plenty to choose from. I have made some Google suggestions to start.
Does the term "screw propeller " make any sense. Does a propeller have to "screw" to push a boat or just accelerate water from one place to another.

 

"Screw propeller" is somewhat descriptive of the geometry of most propellers, but can be very misleading in how a propeller functions.

"I would love you to point me to the theory that you think is mostly likely to be reasonably correct."

From the first page of this thread:

Propellers generate thrust due to the variation in the pressure distribution around the propeller, and that variation in pressure distribution results from water being moved by the propeller.

Tom Kane, I did a Google search per you suggestions and the results showed some "interesting" variations on propellers but I didn't see anything describing a theory. Perhaps if there are specific "theories" you'd like to hear comments about you could provide links directly to those theories. Also, how familiar are you with the basic principals of fluid mechanics and aerodynamic/hydrodynamic theory? Are you familiar with the basic propeller theory as discussed in many aerodynamic and hydrodynamic textbooks?

 

If you clicked "more information" you should have got an explanation of how a propeller works.I have been reading about propellers and making them and using them in water and air for over seventy years.
You have given a basic theory about how a propeller works.
Propellers generate thrust due to the variations in pressure distribution around the propeller,and that variation in pressure distribution results from water moved by the propeller.
Most propeller designs (screw propellers) create drag and turbulence because they screw through the water leaving a helical wake, constant pitch (over the whole blade) propellers produce a much less disturbed wake and air and water flows straight through the the whole disk.

www.youtube.com/watch?v=afPbswbeOkM

 

If you've got a better one, Tom, a big world market awaits.

 

I get a YouTube screen with "Video is not available" when I click on Tom's link.

Edit: I Googled "propeller blades with vortex generators to reduce blade tip losses" and "high rake angle high speed boat propeller with low energy losses" and found several short video clips by George Tenchov of very simple "propellers made from bent aluminum sheet, with holes near the tip of the blades. I did not find any description or claims similar to "constant pitch (over the whole blade) propellers produce a much less disturbed wake and air and water flows straight through the the whole disk."

 

Conventional propeller blades are constant pitch, with the angle of the blade sections changing along the blade so that the tangent of the section angle = pitch/(distance of the section from the center). Based on the videos the "constant pitch" propellers shown actually have constant section angle.

What is meant by "screw through the water"?

Is the claim that these "constant pitch"/constant section angle propellers do not have any swirl component in their wake?

 

A comprehensive list of propeller terms is found at
www.olds.com.au
When at OLDS HOME, click Propellers, then click terminology.

Of interest is the cross sectional definitions and the note that the Troost profile is the most commercially viable section and they call this a "hydrodynamic profile (airfoil)"

As this profile is symmetrical, this section would hardly qualify for an airfoil section where the top camber length from leading edge to trailing edge is the same distance over the top is the same as the bottom. This cross section if operating in a horizontal mode, would not produce lift. It would if it had an angle of attack.

I was always of the opinion that a propeller derived its thrust from two components of the higher produced pressure at the trailing FACE of the prop : 1) the acceleration of the mass of water from say zero to whatever the velocity that it changes to and 2) the pressure differential derived from the foil profile (Bernoulli).


I was under the impression that this second component was of lesser quantitative value than the change due to the accelerated mass.

One reason is that because water is incompressible, virtually, the foil cross section becomes self limiting due to cavitation issues.

Tom has decided that most propellers are "screw propellers". Not sure how he gets here but perhaps he is referring to Archimedes Screw which does not have any application in current propeller design.
The link on OLDS has an actual screw (hate to say the word) propeller to move sand, etc inside a pipe.

If we adopt the same terminology as the OLDS website it will help

I disagree with Tom on his statement that a constant pitch prop does not produce a swirl in the prop cross section area but the water just flows straight through. There is just too much pressure changes across the prop face, drag from the low pressure side, vortices across the edge of the prop, and the velocity component due to the pitch angle to say that the water flows straight through the whole disc.
How about some data on this Tom,

 

Propellers work by accelerating mass in the net direction of travel of the vehicle motion. This is whether its an aircraft propeller, boat propeller, hydrojet, helicopter rotor or paddle wheel. There are many individual differences but that is basically what happens. Doesn't matter whether its negative pressure (suction) or positive pressure, its still moving mass and transferring the energy expended to the vehicle, minus all the drag and other losses, of course.

Lost energy mostly becomes heat. If the vehicle is restrained from moving, all the energy expended becomes heat. To be more accurate, all the expended energy in the propeller becomes heat.

Mr Kane obviously is trolling for some specific answer or answers that he would like to refute. Not a totally honest way to start a conversation. Getting into details sends the conversation on tangents, causes confusion and headaches.

Edit: If there were true zero slip then we have a real screw with no accelerated mass, but in reality I've not known of zero slip. Perhaps Mr Kane has seen zero slip in a propeller. If so, it would be enlightening to see how that works. In that case all the energy would be lost in friction and none on mass acceleration. This is not meant to be anything negative to Tom Kane, I just don't know where he is heading.

 

I think the easiest way to understand a propeller is to think of each blade as wing or foil moving through a fluid. That wing is attached at one end to a rotating shaft, so it is a wing going around and around. the foil section, twist and cord distribution is developed on this exact "model", and the same concepts apply: a high aspect ratio wing is more efficient, as is a high aspect ratio prop, a thin airfoil has less drag, as does a thin propeller blade, etc.

The term "screw" propeller is obsolete, but it does describe the motion of the blade tips through the water so it continues to be used.

The term "slip" is a complete misnomer and was applied from when before it was understood how lift (or thrust in the case of a propeller) was generated. You can not get a force out of a fluid unless you have motion, so the "slip" is not a loss, but the very motion that actually creates the thrust. It would be the same incorrect idea if you describe the angle of attack on an airplane in level flight as "slip", because the airplane is not climbing at that angle.

When people use these terms to describe the complex interaction of foil and fluid to generate lift or thrust, it indicates to me that they have a poor or incomplete understanding of the how fluids generate a force. Please use the correct terminology or there is nothing meaningful that can be discussed, only a frustrating argument by poorly informed people results.

 

Petros, if the only pressure against the rear face of the propeller was generated by the higher velocity of the fluid over the top camber as compared to the bottom, foil lift, then a straight parallel, constant angle vane/blade would not provide thrust but it does.

Tom28571 explains it correctly, the change in momentum, the acceleration of the water mass as providing the pressure against the back of the blade. After looking at the cross sections referenced on the OLDS website above, there is also a lift or additional pressure created by the shape of the foil.

An airplane propeller, because it operates in a compressible fluid, generates its main thrust from the foil shape.

An airplane does not need an attitude of attack to fly. The foil shape will provide
the lift without an attitude of attack which is merely more drag from air impingement on the front bottom and more drag acting on the leeward side of the top of the wing against the direction of the airplane in a horizontal direction

An example would be when an airplane is about to land and the nose is down, ie negative attitude of attack, when the flap angle is increased, the plane will gain altitude .

Ie even with a negative attitude of attack, you can get lift out of a foil and a zero attitude of attack offers the least amount of drag

 

As far as there is no cavitation and flow velocities are not close to speed of sound there is no difference between a propeller (or a wing) in air vs. in water. You just need to take the Reynolds number and density into account. Air can be taken as incompressible as well.

An airfoil or even a curved flat plate can produce lift with zero angle of attack, but that is more of an artifact of the definition of angle of attack than anything else. The drag is not always minumum at zero AoA. It can be lower at some other angle and for many profiles it is virtually constant in some AoA region, often called a low drag bucket. There is not much difference between an aeroplane having a symmetric wing attached so that it provides the needed AoA or a asymmetric one with zero AoA. The asymmetric one can be designed to have a bit lower drag and maybe even a bit higher maximum lift, but otherwise they are equal.

You can't really separate flow velocities, directions and pressures. It is like chicken vs. egg. You can't have one without another. So it is equally wrong or right to say lift (and thus thrust) is due to velocities or pressure created by the propeller.

What you can say is that the interaction between propeller blade surface and fluid can only happen via pressure (along surface normal) and friction (along surface tangent). This interaction changes the velocity AND pressure field in the fluid thus changing the interaction untill an equillibrium is reached.

One clearly wrong theory is the one saying that lift is due to longer path on the suction side of the profile. The error in this is that there is no reason that fluid molecules need to travel the different paths at the sime amount of time. This theory also would not allow a flat plate or even a symmetric profile to produce lift.