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.



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