How do propellers propell a boat.

Valid and important points for an expanded discussion of propulsors in general, but the question posed in the original post (as I interpreted it) was the mechanism by which propellers generated applicable forces. For example, as any naval architect knows, the hull-propulsor interaction that is described above is quantified by the thrust deduction (admittedly in a pretty fuzzy way).

Duct (nozzle) interactions have two effects - a) to alter the flow field into the propeller, and b) exploit the curvature of flow at high propeller loading to create a useful forward component of thrust. In the practical use of a unit thrust coefficient KT for the propulsor (and component KTP and KTN for propeller and nozzle respectively) there is no distinction between "thrust" and "drag", just a summation of forces (as integrated by a summation of pressures). I see no particular advantage to a momentum calculation in this respect, which is just a different side of the same coin but which has some other weaknesses (such as modeling cavitation breakdown).

Don MacPherson
HydroComp

 

How do propellers propel a boat?
With a thrust bearing, of course.
Otherwise, the propeller would just propel itself.

 

Can you please explain why we have to remember this.

 

Many people think that vacuums suck but that is not the case.What happens is where air is displaced such as a motor piston going down the cylinder the atmospheric pressure "pushes" into the cylinder it is not sucked in.
Vacuum cleaners among many other things do not suck but displace air from from one place to another and atmospheric pressure pushes in to replace the displaced air. Briefly.

 

"You can't push on a rope"

"No such thing as suction in our universe, but there is pressure. Or lack of it."------------entirely correct.

In engineering school, examining structures as trusses, test problems often used a rope or cable as a stressed member. One thing students would do is find a compressive load in the cable, which is quite impossible with the application of common sense. The professor would always delight in stating "You can't push on a rope", indicating that ropes and cables can support tension forces only.

In the fluids arena, there is the same, but opposite condition. In the very first class about fluid dynamics, we were told that fluids cannot support tension loads, hence the same common engineering statement, "You can't pull on a fluid". This is a very true statement, that is often misunderstood, even by established engineers.

I have seen engineers thinking that they can use a vacuum pump to "Pull" on a fluid. Getting additional water into a condenser which operates at high vacuum, the common term is "Vacuum Drag". These terms are pure nonsense - Fluids will not support tensile loads, the only thing that pushes fluids around is pressure, never vacuum. There are also some other forces that can be at work here, such as gravity.

 

Well explained in non technical terms.

 

Don, I think many of the people here have not had the advantage of a rigorous look into the energy equations. Of all the things Jens Holm, Alan Rowan, and John Woodward taught that sticks with me is that you write out the whole energy equation first, at all the boundaries, then cut out the stuff you don't need. Many things, like momentum theory and nozzle "thrust", are just incomplete and degenerate parts of the overall equations.

True "propulsor" theory needs to be able to explain even the small stuff, like why your soup gets hot in a blender.