# Define what a Surface-Piercing propeller is

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

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### baeckmoHydrodynamics

Since the pitch line of ALL radii on a propeller nominally have to cover the same axial distance for one turn of rotation, you may check ANY radius.

In reality on SC/SV propellers, the mean chord pitch is often reduced towards the tip. In inducers, on the other hand, mean pitch is kept constant along the radius. The notation of "progressive pitch" is used to describe the local angle along the chord of a cambered profile; a flat face propeller can not have "progressive pitch".

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### gonzoSenior Member

We are talking about surfacing propellers. Supercavitating propellers are submerged. Ventilated propellers do not cavitate.

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### baeckmoHydrodynamics

The rotating foil, constituing the propeller blade, does not know what gaseous substance there is in the gas cavity. So, when it comes to the lifting performance of the blade (and a blade in a row of blades in particular), it reacts practically the same when the cavity is full of h20 vapour, as it does when filled with ambient air.

The typical drop in performance at a certain angle of attack (=low advance ratio) is found in both operations. This is why I mention both varieties as "SC/SPP" in this discussion. There is a slight difference in inflow factors due to the difference in absolute cavity pressure, but that is not of significant relevance for the understanding of SC/SPP working principles, which is the subject of the thread at this level.

PS Attached please find a few diagrams, showing typical trends in performance for traditional fully wetted, supercavitating and superventilating ("surface") propellers. Also shown is typical thrust and drag requirements for planing hulls. Hope this will "light a candle" for you, Sandhammaren........DS

#### Attached Files:

• ###### On the operating principles for Supercavitating.doc
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### yipsterdesigner

now this i belive is an obvious example of a wellknown surface prop
having blade thickness straight at the end ventilating but other types are there
read that slow turning SP's may be used economicaly even for acean liners
asked arneson once over propwalk and learned vertical forges may be higher
and got some adresses to talk further on my idea's of a possible SD dual prop
never did go deepr into it as project was cancelled but certainly still curious
keep it going guy's, nice thread and yes, ofcourse the math too

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### sandhammaren05Senior Member

[Baeckmo is very close, as I expected. I'd say 13.8" for the case that I stated, so camber is absolutely necessary. In my actual experiment (1983 35 hp Johnson on a 350 lb 15' pad V-bottom shown in some photos I posted earlier) the leading edge pitch is 13" (Rundquist gauge), which violates intuition because the speed (via GPS) is 41.5 mph@6050 RPM. It means: naively estimated 'slip' is slightly negative (naively estimated 'efficiency' is slightly over 100%). This is ok, we've assumed that the trailing vortex sheet (induced drag) doesn't change the inflow, but it might. Without 'the right' cup, with too little or too much, the speed drops sharply to 37.5 mph. Leading into my second stated problem, which Baeckmo has been kind enough to respond to, the sharp transition described there occurs on this rig at exactly 37.5 mph. The transition occurs on other planning hulls where the 'slip' is quite normal.]

[Regarding negative slip, I'm talking here about apparent slip, not real slip. However, the boat planes high, and I would guess that the (turbulent) wake from the pad doesn't extend deep enough to create much of a wake fraction for the prop to run in. Or?]

Last edited: Jan 18, 2010
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### gonzoSenior Member

A supercavitating propeller works at pressures much below the 14.5PSI of the atmosphere. They are completely different designs. Ventilated propellers do not cavitate. The shape of the blades shows the great difference of operational parameters.

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### baeckmoHydrodynamics

You still don't get it, or you don't want to?? One last effort to try to get you onboard: We can differentiate between gas cavities and vapour cavities; the vapour beeing the gas phase of the surrounding working fluid. From hydromechanical point, the only difference is from the cavity closure. The vapour cavity is closing with a reentrant jet, causing implosion through rapid condensation, while the gas cavity is closing with the gas content beeing dispersed downstream in the flow. For the propeller performance, it does not matter which substance we have is gas phase in the cavity, as demonstrated by the test data from the two-blade propeller!!

With a full cavity over the whole chord, the cavity is closing behind the blade, and the closing procedure is not generally a dimensioning factor. If you still don't accept the explanations given here, please study any relevant textbook or research report on cavitation; I will not deal with your nonsense any further here!

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### gonzoSenior Member

This is one of your usual response with bizzarro world physics. Then you say you will not deal with someone's nonsense and immediatley after keep on posting. You are inventing words too. A cavitating propeller works with a vacuum created at the forward face of the blades. A ventilated propeller lowers the vacumm with atmospheric pressure, there are no explosions. They are two completely different modes of operation.

9. ### FrostyPrevious Member

If the root of the blade is on the surface what implodes? There is no vacuum, no cavititation, but airiation.

Cavitation is what a conventional prop does, rips a hole in the water.

A surface prop is more like a rotating sculling device.

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### sandhammaren05Senior Member

You can't 'lower a vacuum', you can only lower the pressure. Best to get your definitions straight, otherwise there's only confusion. Baeckmo's physics is correct, ignoring viscosity to zeroth order.

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### baeckmoHydrodynamics

As long as we are talking about cavities that are LONGER than the chord of the blade; ie supercavitating or superventilating by definition, the hydrodynamic difference between a "vapour cavitation cavity" and an "ambient air cavity" is negligable for engineering purposes.

In both cases, strictly speaking the cavities stay open into infinity; so there is no cavity collapse (=implosion) in the vicinity of the blade, or on it. All the lift is produced by the pressure side of the blade. Looking at the flow over a propeller blade in a test rig, the "hole in the water" created by pure vapour cavitation is very similar to the hole from a surface propeller, aside from the splashing on the surface.

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### gonzoSenior Member

Baeckmo:More bizzarro physics. The lift is produced by the difference in pressure between two sides of a foil.
Sandhammaren 05:Vaccum can be lowered or raised. It is negative pressure and just a convenient way of indicating that.

13. ### FrostyPrevious Member

Baeckmo said
The vapour cavity is closing with a reentrant jet, causing implosion through rapid condensation,

And then said,
the cavities stay open into infinity; so there is no cavity collapse (=implosion) in the vicinity of the blade,

So if I did'nt understand before I certainly don't understand now

Some one said I should shut my mouth and made deductions, Why --for asking for a clarification--- Baeckmo.

I see Gonzo has been hit too.

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### gonzoSenior Member

Baeckmo in this and other posts invents words. Then constructs sentences with them.

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### yipsterdesigner

easy gonzo, me thinks baeckmo is the scientist here and lets keep it frendly plz
arnesons story http://www.arneson-industries.com/page.php?type=products&id=drives
jimboat on props http://www.boatbuilding.com/article.php/designofpropellers1
http://www.boatbuilding.com/article.php/designofpropellers2
Paul Kaman's article http://www.well.com/~pk/SPAprofboat.html

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