Is a boat pushed or pulled by the prop.......that is the question

[Make-a-Wake]

Have long thought about this:

*Is the screw action of the prop biting into the still water in front of it pulling the boat forward?

*Is it creating faster moving water behind it as a forward thrusting action thru the slower moving water outside the cone?

*Is it both?

Seems like there could be arguments/beliefs in all three.

For those who believe its the thrust cone of faster water behind the prop, consider this:

If you are boring a screw into a wall for instance, the threads "bite" into the wall in a pulling fashion. If you are using a hex head screw and a ratchet wrench you dont need to apply any pressure to "push" it in once it is started as you would a phillips head to keep it in the grooves, just the energy of a turning motion. In this case the screw is not being pushed at all. It is boring into the matter in front of it pulling it, same as a prop would bore into the water in front of it. So in this case it is a pulling action.

A boat prop on the other hand is obviously also pushing a much faster moving cone of water behind it......but only because the water is being pulled in from the front by the prop................or not.................hmmm?

[Make-a-Wake]

I found this bit of info as well.....................now it gets into pressures on each side of the prop.

There is obviously some major physics involved here..................seems like it would be almost impossible to know due to all the variables

"As a propeller rotates, it pushes water back, as is done by your hand when swimming. At the same time, water must rush in behind the blade to fill the space left by the moving blade. This results in a pressure differential between the two sides of the blade, a positive pressure or pushing effect on the backside, and a negative pressure, or pulling effect on the front side of the blade."

[NativePaul]

At our size I think surface drives props are purely pushing, fast submerged props will be doing both but biased greatly towards pushing, and submerged displacement props both but maybe a little biased towards pushing. At full size surface drives are mainly pushing, Submerged displacement do both and I suspect like aircraft props slightly biased towards pulling. Impellors are probably pretty equal as there should be little to no camber and no cavitation.

[Darin Jordan]

Which side of the prop do you put cup into? That should answer your question...

[Make-a-Wake]

Well, the "cup" can also be construed as a larger more agressive bite into the oncoming water..................

Sounds like "pushing" is the response so far.

[Peter A]

Wow, has someone got nothing to do?

[Make-a-Wake]

Wow, has someone got nothing to do?

I have plenty going on, but its obvious that you are bored out of your skull to get on a thread and take the time to post something that has nothing to do with the topic, ya know, kinda like a Troll.....................maybe try knitting

[sundog]

I think the answer to your original question is yes. Prop screws into water creating an increase of pressure behind it, and it moves forward into the lower pressure in front of it, pushing the boat forward (if if were in front of the boat it would be called pulling). What do I win?

[Doby]

Push.

[SweetAccord]

I would maybe say it's doing both, but one is doing way more than the other. Being specific, when a prop turns, this results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect, on the underside and a negative pressure, or pulling effect, on the top side. This action, of course, occurs on all the blades around the full circle of rotation. So the propeller is both pushing and being pulled through the water.

These pressures cause water to be drawn into the propeller from in front and accelerated out the back, just as a household fan pulls air in from behind it and blows it out toward you. The marine propeller draws or pulls water in from its front end through an imaginary cylinder a little larger than the propeller diameter.

The front end of the propeller is the end that faces the boat. As the propeller spins, water accelerates through it, creating a jet stream of higher-velocity water behind the propeller. This exiting water jet is smaller in diameter than the actual diameter of the propeller. This water jet action of pulling water in and pushing it out at a higher velocity adds momentum to the water. This change in momentum or acceleration of the water results in a force which we can call thrust.

[sundog]

Well put. I forfeit my prize.

[flraptor07]

That SweetAccord guy has all the answers, very fine explanation!!

[Peter A]

I have plenty going on, but its obvious that you are bored out of your skull to get on a thread and take the time to post something that has nothing to do with the topic, ya know, kinda like a Troll.....................maybe try knitting

I'm sorry you think I was "trolling". You try to compare a prop with a screw into wood was just too random for me. They are not even close in any aspect. I struggle with the concept that you thought that a boat gets 'pulled' through the water by the propellor. It would if the prop were at the front of the boat, but as our props run off the rear of the boat, the boat is pushed!
You didn't have to be snarky about it either.
Apologies to everyone for my foobar!

[Make-a-Wake]

Props are also called screws btw, for a reason im sure, and i was making an analogy that i thought would make it simple.

Also, if a chain is hooked to the rear axle of a rear wheel drive car and run under it out front and hooked to a tow vehicle................it is being pulled...............by the rear mounted driveline............which propels the vehicle when under its own power. Things can be pulled from anywhere.

[Fluid]

One reason that surfacing propellers are "faster" is because they create a low pressure 'vacuum' area in front of the prop, this is well documented. The result of the pressure differential is strong forward movement.



.

[tlandauer]

Interesting thoughts, I don't know if it is any different, but on certain aircrafts where you have props behind the trailing edge of the wing, they are referred as "pusher props"
But a scholarly look into this subject is far more involved.
XB-35.jpgConvair_B-36_Peacemaker.jpg
And then, you have the counter rotating props as with the famous Russian Tu-95:Russian Tupolev Tu-95 Bear Strategic Bomber.jpg

I am definitely liking this thread!

[Make-a-Wake]

I think the answer to your original question is yes. What do I win?

A trip to Eureka Springs to hang out with the cool hippies, all expenses paid.........................by you

[Make-a-Wake]

One reason that surfacing propellers are "faster" is because they create a low pressure 'vacuum' area in front of the prop, this is well documented. The result of the pressure differential is strong forward movement.




.

That sounds logical................. I knew pressure differences would come into play at some point

[tlandauer]

I guess to me it is a separate aspect. The prop is pushing the water regardless if it is mounted in the back or the front of the boat. However, the force that is exerted on the components are different. If the prop is in the front, there would never be a thread about THRUST Bearings between the collet and motor or at the strut. The prop would be pulling the rotor of the motor. ( provided the motor is mounted pointing forward) NO?
I go back to pushing a car, if you are behind, you are pushing, if you are infront pulling with rope, you are pulling. But in both cases you legs are PUSHING against the pavement to propel yourself forward.
Then, what do I know, I have a concert to play tonight.

[Doug Smock]

Remember that thing called thrust?

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system. The force applied on a surface in a direction perpendicular or normal to the surface is called thrust.

BTW Some people refer to props as "wheels" also.

[keithbradley]

Props are also called screws btw, for a reason im sure, and i was making an analogy that i thought would make it simple.

Also, if a chain is hooked to the rear axle of a rear wheel drive car and run under it out front and hooked to a tow vehicle................it is being pulled...............by the rear mounted driveline............which propels the vehicle when under its own power. Things can be pulled from anywhere.

I don't mean to rain on your parade, but the "out front" placement of the tow truck means:

1) Your argument is moot, because the truck is in FRONT of the vehicle relative to the direction it is travelling. Your scenario doesn't dispute Peter's statement in any way.

2) The tow truck driver is clearly an idiot. Don't let him touch your car.

[Make-a-Wake]

Thanks Keith, good to know that the driver in my made up scenario is incompetent, I'll be sure to hire a different fake driver next time to avoid a possible insurance claim which could raise my rates and may cause me financial trauma.

[tlandauer]

Thanks Keith, good to know that the driver in my made up scenario is incompetent, I'll be sure to hire a different fake driver next time to avoid a possible insurance claim which could raise my rates and may cause me financial trauma.

Hmmm.... M-a-W, you and I have a lot in common.

[iop65]

Remember that thing called thrust?

Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system. The force applied on a surface in a direction perpendicular or normal to the surface is called thrust.

BTW Some people refer to props as "wheels" also.

actually it"s Bernoulli's law : P x V=cst

the change in pressure due to the difference in speed the water has on the 2 sides of the blade makes the prop wants to move to one side and so creates thrust , same reason as why an airplane goes up due to the shape off it's wings or a helipcopter wants to go up when turning the rotor

in fact Newton's laws have nothing to do with the questions why the boats wants to go forward or an helipcopter wants to go upwards
Newton's laws explain why a boat has torque roll or make it necessary for an helicopter to have a tailrotor( or an other solution like contra rotating props)

[Darin Jordan]

This prop is "pulling" the boat... :)

IMG_0419.JPG

IMG_0420.JPG


Maybe it's been mentioned above... I don't know, I haven't ready this whole thread (this kind of minutiae makes my head hurt... ) but I think any "push" or "pull" argument is really relative to where the "propelling" device is located in relation to the object being moved.

How the prop actually moves ITSELF through the water can be open for debate, or detailed analysis, or cage fighting, but clearly the prop is "Pulling" the hull if out front, and "Pushing" if out back... Unless you hit reverse, which you CAN do if you own a Pro Boat RTR...

OK... enough of this... time to go finish prepping race boats... where I could give a rip if the prop is pushing or pulling, and instead be more concerned if I remembered to tighten the prop nut...

[Make-a-Wake]

I guess i may have said it wrong initially:

Should have said, is the prop "boring/biting" into the water in front of it, drawing water in towards it......................"pulling" itself into the undisturbed water ahead of it.......................I wasnt actually referring to where it was on the boat.

Like Darin just said, how the prop moves itself thru the water..................which is how the boat is propelled by being attatched to the hull 'somewhere'.

It was just a physics question.

[T.S.Davis]

It's all thrust.

There isn't enough blade to create useful negative pressure on the front side of the prop like a wing so it's not that. The screw analogy doesn't really fit because the water isn't a solid. If the screw going in actually moved the material that the wall is made of then they would be the same. Haha Maybe it would if the screw were turning at say 25k rpm. Then the screw becomes a propeller instead of a screw. "Propeller" as in propulsion.


2 cents. Opinions will vary.

[T.S.Davis]

I would think too that if the propeller was only pulling itself through the liquid then the thrust cone would not affect the ride attitude. We all know that's real.

[fox88gt]

I'm copying a link to website I found that contains information derived from a book that Mercury Marine published a number of years ago. The book is called "Everything You Need To Know About Propellers." Mercury probably designs and builds more propellers than any other company in the world.

http://www.killcaremarina.com.au/ind...ropellers.html

Here's an exerpt from the text in the link:
The "Push/Pull" Concept

To understand this concept, let us freeze a propeller just at the point where one of the blades is projecting directly out of the page (Figure 3-1). This is a right-hand rotation propeller, whose projecting blade is rotating from top to bottom and is moving from left to right. As the blade in this discussion rotates or moves downward, it pushes water down and back as is done by your hand when swimming. At the same time, water must rush in behind the blade to fill the space left by the downward moving blade. This results in a pressure differential between the two sides of the blade: a positive pressure, or pushing effect, on the underside and a negative pressure, or pulling effect, on the top side. This action, of course, occurs on all the blades around the fall circle of rotation as the engine rotates the propeller. So the propeller is both pushing and being pulled through the water.

Thrust/Momentum

These pressures cause water to be drawn into the propeller from in front and accelerated out the back, just as a household fan pulls air in from behind it and blows it out toward you (Figure 3-2).

The marine propeller draws or pulls water in from its front end through an imaginary cylinder a little larger than the propeller diameter (Figure 3-3). The front end of the propeller is the end that faces the boat. As the propeller spins, water accelerates through it, creating a jet stream of higher-velocity water behind the propeller. This exiting water jet is smaller in diameter than the actual diameter of the propeller.

This water jet action of pulling water in and pushing it out at a higher velocity adds momentum to the water. This change in momentum or acceleration of the water results in a force which we can call thrust.

[T.S.Davis]

Reading that makes it sound like neither exist independently. Thrust results from the sum of the two. Interesting.........