Define what a Surface-Piercing propeller is

Ok Frosty, we should have settled on definitions first; I use the professional terminology that is internationally recognized among researchers in this field. And I certainly don’t mind you asking. Attached you find first a typical picture of a supercavitating propeller with its long, well defined cavities, trailing downstream a considerable distance before finally imploding. You may see the blade as a shadow within the cavity in the top position. (This pic from ”Principles of Naval Architecture” by Comstock et al).

Second pic is the shape of a typical supercavitating propeller with its cambered blade, sharp leading edge and truncated trailing edge. This prop, type no 3767, is one of the most thoroughly tested in a variety of conditions, both supercavitating and ventilating (the first pic is taken from a test with a very similar prop). Note that today the trend for foil shape is to move the maximum camber further aft on the chord.

Pic three shows the prop no 3768, which is a geosim of the 3767 (same shape as 3767, but bigger), operating in surface mode (”SPP”). The top frame shows the trailing cavities at design speed (design advance ratio), the lower at reduced advance ratio. You can see the cavities opening to the surface, but still holding their basic shape, while travelling downstream as ”dents in the water”. Pics 2 and 3 from Hadler and Hecker ”Performance of Partially Submerged Propellers”.

Pic four is another propeller in surface operation, pic unfortunatly not too clear (these rascals are difficult to picture). Anyway, propeller is at right, fluid moving from right to left. The surface is seen where the trailing blade cavities meet a ”bubbly” area. Still coherent cavities, open to surface and travelling downstream, finally dispersing as a ”bubble cloud” in the water. (From personal archive).

Finally a simple sketch hopefully showing the two situations (ok I’m no Michelangelo so don’t start another war for that please….). For clarity the SC blade is shown with its cavity not yet in spiraling motion. The rotating foil; ie. the propeller blade, cannot ”feel” what happens behind its trailing edge. This is the key to understanding the physical phenomena around propellers designed to operate with a gas-phase cavity. As soon as the cavity on the suction side of the blade is fully developed, ie. it covers the complete chord, the pressure on this side is fixed (vapour pressure on SC and atmospheric on the SPP), and can not add further thrust, even if the rotational speed or incidence is increased. Any additional thrust has to be generated by the pressure side of the blade.

 

Gonzo, a correct terminology is necessary for a constructive discussion. Vacuum is an absence of gas molecules. It is zero pressure, by definition. It cannot be lowered nor raised. A pressure can be lowered or raised, not the vacuum.
Next, a pressure can never be negative. It is always positive, or zero (when it is vacuum).

 

In English technical terminology, a vacuum can be absolute or partial. Partial vacuum can be lowered or raised.

 

Hmmm I remember that there can not be a perfect vaccum on the earths surface. You can not form a vacuum where there was not one in the begining.

So down here on Earth all vacuums must be partial.

 

Getting back to what a surface piercing propeller is, I think that is should be one that is designed to work on that mode. The key to their operation, is that ventilation prevents cavitation.

 

No, the phrase vacuum is precisely defined in physics: no matter. Outer space approximates a vacuum but consists of a very sparse gas. People often wrongly write 'vacuum' while discussing (or trying to discuss) cavitation, but a cavitation bubble is filled with water vapor. A cavitation bubble does not in any sense approximate a vacuum. One gets an approximation to a vacuum (a 'partial vacuum') in the lab by pumping the gas out of a container.

"If you can't define your terms precisely then you don't know what you're talking about."

Julian Palmore (mathematician)
George & Harry's Bar in New Haven, ca. 1966.

 

So you claim that nobody uses vacuum gauges?

 

A 'vacuum gauge' is simply a pressure gauge in some chosen units. In the lab, for practical purposes, one has a 'vacuum' when the pressure or density of a gas in the container is below some desired measurable limit, to so and so many digits. In a cavitation bubble one has water vapor at pressure close to vapor pressure, the density and pressure are not negligible (otherwise the bubble would collapse much faster). As I stated above, if you don't define your terms precisely then you don't know what you're talking about. You need to do some serious homework in physics, and get your terminology in line with physics, in order to be able to contribute anything other than amusement to this discussion.

 

Nice,

The deep voice of unstudy made baeckmo to explain the phenomenon of supercavitating and surface operation quite well. Otherwise, these guys would only use their professional jargon. Thanks gonzo, you've helped a lot.

A little study of physics could'nt harm you. Try Young & Freeman University Physics, it's not so difficult and help you to see things.


What a great thread!

 

Wouldn't it be much easier to calculate with pitch directly? If you want zero AoA, that means zero slip. Thus P=Va/n = 18.3/52.7 = 0.347 = 13.7". If you want 1 deg AoA, the pitch is not constant anymore.

What would be a typical difference between the leading edge pitch (or angle) to the nominal pitch of an outboard racing propeller. Eg. I had a 12x22 Ron Hill cleaver from mid 80's. It would be nice to calculate the AoA I had. Is it much different from the current ones?

What is the best method for getting the target AoA? RPM will change at the same speed depending on how deep the propeller is and also motor trim has an effect.

 

This region was not covered in the curves of post #63. How big is the drop typically? Just trying to figure out what happened 20+ years ago. I first had a 10x18 cleaver, that took full rpm at start and then got a grip at ~15 kn. The 12x22 never did that, it was much like a normal unventilated aluminum propeller at starts. Both where so high, that they were certainly ventilated once the transom was dry.

The engine was Yamaha 40 hp (3 cylinder), with 24:13 and rpm limiter at 6000 rpm. The boat was a 3.8 m ~15 degree V. Total weight including driver about 250 kg. The top speeds were 44-45 kn with 10x18 at 6000 rpm and 48 kn with 12x22 at 5300-5700 rpm depending on the height of the propeller (the propeller shaft ~ at the water surface).

Was the 12x22 just big enough not to ever go under Ja ~0.4*P/D (=60 % splip)? I think we usually had about 5500 rpm at ~15 kn with the 10x18 just before gripping. That would be Ja~0.34*P/D. Then I think rpm was something like 2500 and almost linear to top speed with very good acceleration.

I recently sold the propeller and it is now slightly modified and doing ~55 kn with an old 75 hp Stinger and a 5 m boat that is more than 100 kg heavier.

 

Joakim, the example cited was to answer Joe's quest specifically on leading edge pitch. But in general a cavitating/ventilating foil (no matter if it sits on a rotating propeller or a hydrofoil vessel) must be operating with a leading edge incidence close to zero. Note that I was referring to the pressure side, not the mean chord line!

The lift coefficient is mainly a function of (incidence + camber + camber position); the zero-lift incidence is consequently negative in many cases, and the design lift and lift/drag ratio is found with a small positive incidence. With that in mind, the (leading edge)-to-(trailing edge) pitch can be quite misleading. This is my reason for checking LE pitch, also for pumps as in wjets or industrial applications, where cavitation can be critical.

I would say that the foil shape has gone from the classical wedge with nearly circular arc pressure side (your Ron Hill cleaver??) to a concentrated load far aft of the chord. In one of the classic studies of supercavitating propellers, Tulin showed that this was theoretically correct. More than a decade earlier, Pozdunin in Russia had shown this effect empirically. In fact, according to theory, all lift should in a supercavitating flow should be produced at the trailing edge for minimum losses!

That result was regarded bizarre for a long time, but when you look at the pressure distribution on a cupped foil and compare its cavitating performance to a conventional circular arc, the results verify Tulin's findings.

Ouch, you are quick, I'm still with your #85 note; gimme some time man.......!

 

The naive method is best here, as I stated elsewhere above in the real experiment the speed is 41.5mph@6050RPM, and the leading edge pitch is 13".

Generally, an OMC 12.5x23, e.g., measured 23" at the leading edge, and an OMC 10x15 measures 15" pitch at the leading edge. I'd have to measure the Hill prop to see what 12x22 means (sounds like a prop for my favorite motor, the loop charged 3 cyl. 75). 'Nominal' could mean leading edge pitch. I've reworked props that Ron sent to the GT-Pro racers in Minn., he also uses something like a Rundquist gauge and measures the pitch along the blade from leading to trailing edge (as do I) at each radius.

 

Sorry about that! Here are a few pictures of the propeller. I don't have it anymore, thus I can't check the geometry. But as you can see, it clearly has cup and I think the new owner added more cup and got better top speed.

http://kotisivut.fonet.fi/~jmajande/potkuri/

The 10x18 propeller was made by Radice (1982??) and it is more like the traditional shape. I still have it on the same boat with a Mercury 25 (1984, 1:2,25 gear), but it performs very poorly (33 knots, equal to badly under-pitched standard aluminum) and I haven't had the inspiration to do something about it. Actually the boat hasn't been on water for 5 or so years.

 

Has the sharp camber in cup I would agree with. Would need to put it on a pitch gauge. I'm slow right now, will have to respond to other details later ... .