# Define what a Surface-Piercing propeller is

Discussion in 'Propulsion' started by tom kane, Jul 12, 2009.

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### sandhammaren05Senior Member

Sorry to take so long in getting around to this, and thanks for "On the Operating Principles ...". With J_A ≈.4P/D that is not the transition I'm describing. In that gtransition the parameters I stated have J_A>1. Also, I doubt that there can be a cavity after transition, the form drag would be enormous (we gain considerably in racing by running blades so thin that cracks often develop). So I guess it's time to stick my neck out and post the guess that I sent you before you posted your solution. Here's my uneducated guess:

I think the sudden transition is like liftoff of a wing. Extra circulation is created about the blade by the cup, the shedding of the trailing vortex is the transition. That is, by the asymmetry of the surfacing prop the blade under water is a bit like a hydrofoil running upside down. Naturally, a trailing vortex was shed when the prop made the boat move from rest in the first place, but if you remove the cup then the transition under consideration will disappear. That's why I asked if your answer holds when there's no cup. I'm not convinced that supercavitation occurs, I would guess that the water is simply aerated and always wets the blade that's under water. A supercavitating prop could not be used to push an outboard to top performance, too much form drag from the cavity.

Note added later: One could also speculate that the transition is due to collapse of a cavity. In any case, airlift acting on the bow plays a big role in speeding up the transition.

Well, as I said I'm speculating. When in 1978 I built racing props trying to match the LE to the inflow I wasn't thinking about cavities. Any sharp edge generates an eddy, so there's eddy drag generated by a sharp prop blade moving relative to the inflow. I was naively trying to reduce the drag by getting rid of that eddy and then used camber to make up for the lost angle of attack. Every racer knows about 'cup', which is an extreme for of camber at the trailing edge but when racers discovered leading edge 'cup' they simply bent the blades without a pitch gauge and had no idea what they were doing. They gained acceleration and lost top speed. I used exactly the same prop to run le Mans starts and run closed course races that I used for my straightaway record run, because the leading edge camber increased both the acceleration and top speed.

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### ChrisN67Senior Member

Surface piercing props VS None Piercing

The major differences are that a surface piercing (super ventilating propeller) create the majority of thrust by positive displacement of water.

A submerged propeller is a foil; most of its thrust is from "pulling" the boat forward.

Another interesting characteristic of surface piercing propellers is that balance is not very important. They operate in an environment of dissimilar dynamic load (half prop in water and half in air) Thus it is not to important that they are well balanced.

Surface props have blade pulse vibration that is characteristic of a propeller operating in different media (air and water)

Last and most importantly...they are ridiculously expensive....this is why the attached picture is of my boat parked outside my house but my Rolla props are under my bed, next to the nightstand with the machine gun.

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### sandhammaren05Senior Member

propulsion by propeller

No, your physics/hydrodynamics is wrong. The propeller pushes in both cases, and the angle of attack is equally important in both cases. In both cases the lift/thrust is created by circulation about the foil/blade.

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props suck and blow
ask anyone on a vessel where they need bollard pull to do their job
When you are moving you cant produce the same bollard pull as there is current inflow to the prop hence reducing the suction side performance

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### ChrisN67Senior Member

?

My statement was regarding the principal difference between a surface piercing propeller (ventilating) and a foil propeller that is submerged. The DIFFERENCE...that means aspects of the two types of propellers that are not the same or "dissimilar"

I generally do not controvert people unless I am 100% sure. I guess we are different.

What do you think generates more propulsion force on a submerged non- ventilating propeller....the vacuum or pressure side?

If you say the vacuum side then you are correct and in agreement with the world today.

If you say the pressure side you are hanging out with conspiracy theorists and noone can prove you wrong.

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### tom kaneSenior Member

Well I do not think any one really understands how a propeller works.
Any propeller pushes a boat forward by displacing a volume of water rearward away from the back of the boat. Like a paddle boat. How that is achieved does not matter so long as the process of moving the water is as efficient as possible. A screw propeller does not screw through the water and thus move the boat forward. What counts is the volume and velocity of the expelled water. The displaced water is replaced by the atmospheric pressure pushing down on the water. Vacuums do not suck (or propellers)
Atmospheric pressure pushes in to replace displaced water air. Not my idea but proven scientific experiment.

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### ChrisN67Senior Member

Ok...

" A proven experiment" ? Not sure what to make of this. I have never of an experiment that has been proven -I understand that experiments are used to validate hypothesis that can lead to some accurate rule or guidance on a subject matter.

I am not being snobby, but if you want to come into a forum and act as a teacher settling a dispute with the assumption of an elevated intellect worthy of resolving debaters, at least show a fundamental grasp of basic principals you claim to have mastered by your self appointed subject matter expertise. There are no experiments to be proven, experiments in of themselves are just experiments (tools), that can lead to conclusions regarding the accuracy of a hypothesis. A hypothesis which has been validated (by experiments and/or observations) may become a rule or law barring any evidence to dispute it.

In a submerged, non ventilating, non cavitating propeller, the MAJORITY of propulsion is derived from the low pressure (vacuum) generated by the foil in front of the propeller.

This effect generates more propulsive force than the pressure side of the propeller.

Ask any competent sailor and he will tell you the same thing...his vessel is pulled forward by the low pressure area created by sale (which is a form of aerodynamic foil).

This phenomena is not applicable to surface piercing propellers where the force is primarily generated by the reaction of the displaced water at high speed, they are truly "paddling" (as you say) the water. The equal and opposite reaction to the displaced water is the majority of the propulsive force.

In my life I am not concerned with my ignorance, I am most concerned with avoiding circumstances where have inappropriate confidence in the validity of my positions. I do not think this is the case.

In a submerged, non ventilating, non cavitating propeller, the MAJORITY of propulsion is derived from the low pressure generated by the foil in front of the propeller.

Hint: If you think this is false then you either have no access to reference materials or you insist on being ignorant.

....I am not trying to be ugly, but I am shocked people are willing to spew their opinion as gospel without spending so much as 30 seconds to validate their premise.

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### sandhammaren05Senior Member

I don't need to ask, people ask me because I teach about props. A propeller is a foil surface, period. Study the subject, if you can, instead of writing nonsense. I also build surface piercing racing props for outboards.

The Physics prof.

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### ChrisN67Senior Member

This is a discussion in which you chose to put in your 2 cents. I do not care what you do for a living or who asks for your opinion. For all I know you live in a trailer park surrounded by cretans where just spelling your name makes you an expert; I am soliciting your opinion regarding my understanding of the subject matter.

So, is the following statement true of false:

In a submerged, non ventilating, non cavitating propeller, the MAJORITY of propulsion is derived from the low pressure generated by the foil in front of the propeller.

Please answer the question or then what the heck are you doing on a "forum" if you only came here to sing your own song ad nausium and do not care to entertain a "discussion".

What is it that you think is incorrect about my position in this matter? Or are you just going to parrot another quip about "foils" and your unvalidated "credentials" ?

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so you agree its a foil so one side has low pressure and the other has high pressure

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### ChrisN67Senior Member

I did not start this discussion to be abrasive....but I have read , and was taught by people who own major prop shops that most of the propulsive force of a submerged propeller is generated by the low pressure side of the propeller.

It seems I am having a heck of a time trying to extract a response to that premise.

There are people with vast experience in boating that manage to get it right through trial and error experience and historical precedence as opposed to proper knowledge of engineering fundamentals.

In this case it seems that a contradictory opinion is viewed with hostility. So the discussion is overlooked and it becomes a personal issue to prove that age and time tinkering with boats should trump learning something new and legitimizing accurate information - Especially if that information may controvert long established understandings attained through practical experience in absence of formal education and fundamental knowledge.

One of my favorite lines is from Flight of the Phoenix '65 @ 51:30 "That is precisely what is wrong, he has remembered everything and learned nothing".

I am here to learn and share acquired knowledge, not peddle attitude and insecurity. I never argue with a "professor", especially one who challenges what I understand to be basic science.

I consider the matter closed.

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### tom kaneSenior Member

"A proven observable experiment" by Galileo and Torricelli circa 1644 showed that a vacuum does not suck but that atmospheric pressure PUSHES in to replace displaced air. Google it.. (This not part of experiment,) thus causing aeration etc., in propellers.

In a submerged non ventilating non cavitating propeller ALL the thrust is from the reaction of the displaced water pushed rearward (reaction). Low pressure on the front of the blades is an effect caused by the water air displaced giving drag rather than forward thrust. So we have need for S/P propellers that can clear away this drag.

An absolute vacuum is 19.5 psi. not much hope of power from that. We could go on to quantum mechanics and the behavior of sub atomic particles.

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### sandhammaren05Senior Member

Absolutely, whether submerged or surface piercing. In either case the pressure difference is created by circulation about the foil.

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### sandhammaren05Senior Member

I didn't see any science in your discussion, only uninformed assertions. F for the course so far.

The prof.

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### ChrisN67Senior Member

There are two distinct and separate effects that occur on each side of the blade. On the forward side, the foil creates a low pressure system resulting in a pressure imbalance between the water in front of the blade and behind the blade. There is also the dynamic effect of the blade acting as a screw and ejecting the water.

Mercury Marine http://www.mercurymarine.com/propellers/about/how-propellers-work/ calls it the "push/pull" effect.

All I am trying to impart is that the "pull" of the low pressure side of the blade has more effect on propulsive thrust than the "push" side.