# Define what a Surface-Piercing propeller is

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

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### sandhammaren05Senior Member

I mean I'll scan and email, not fax, sorry.

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### RikSenior Member

Not looking to teach, rather curious to the answer.

So let's see if we speak the same language.

14:27 is the hard way of saying you had a 1.92:1 reduction ratio in the outboard.

6100 rpm/1.92 = 3,162.963 rpm

Speed = 41 mph (actual)

With these known constantans we would then need to know a few more constantans to answer your pitch question.

2. Displacement weight of said boat?

3. Efficiency of said boats power train?

4. Type of propeller, how much cup?

5. Number of blades on the propeller?

6. Do you actually trust the Rundquist pitch gauge? I've had one for years and they measure an average not precise as the measuring head cannot account for a variable pitch design especially on a high camber propeller. Taking a tangent measurement is not exactly accurate.

A more accurate measuring system would use a stylus like the Hale MRI incorporates. http://www.halepropeller.com/MRIphoto.html

7. Do you allow for progressive pitch, average pitch or straight pitch designs and the amount of cup in your design?

WAG. (wild *** guess) being high X dim, low hp, most likely a 3 blade propeller, small displacement boat I would say the leading edge pitch would be 16”

How far off am I?

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### baeckmoHydrodynamics

For the supercavitating profiles I have been using, a good starting point for design is with the leading edge (~25 % of chord) pressure side having zero to one degree angle of attack. So:

Prop rps= 6100*14/27/60=52.7 rps
Va=41 mph=41*1609/3600 = 18.3 m/s (no wake included)
Check on arbitrary diameter, say 0.2 m

Ja = Va/(np*D) = 1.74;

Inlet flow angle "betazero" = arctg(Ja/pi) = 28.92 degr.

Add 1 degr a of a =29.92 deg. = "betaone"

Resulting P/D = pi * tg (betaone) = 1.808

On the diameter 0.2m, that is a basic pitch for the leading edge pressure side of 0.362 m, or 14.2 in. Note that this is NOT by any means a relevant figure for the rest of the blade. It is the pitch necessary to minimize the cavity for base-vented operation for best efficiency. Also note that there are other leading edge shapes, "asking for" different approaches. For instance, the sc five-term profiles even work with a slight negative angle of attack to the geometric leading edge.

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### PetterMSenior Member

Thanks

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### sandhammaren05Senior Member

I very much like your response! Part of my aim was to find out if anyone knows what I knew ca. 1980. Baeckmo/Bäckmo has shown me by his response that it's apparently pretty well known now. Read his response carefully and you'll get the right picture. Still, I haven't written anything about what I discovered while reworking my own racing cleavers but might still (I'm thinking of writing a book on the history of OPC racing with a chapter on prop theory included) so I'll try to answer your questions best I can without stating my entire viewpoint. I.e., I'll leave the answer for you to arrive at. It's only arithmetic guided qualitatively by basic ideas from foil and wake theory (ala ch 5 in Newman's Marine Hydrodynamics or pts 46-48 of Fluid Mechanics by Landau & Lifshitz).

14:27 is 1.93

Actual speed is 41 mph

Aside from RPM nothing else is needed to make the calculation. You are inherently estimating the leading edge 'slip' in providing the answer. That's the whole point of Baeckmo's response to Gonzo.

Any surfacing prop must be appropriately cupped, otherwise the whole enterprise is in vain. You can't profitably raise the motor on the transom to surfacing conditions without cupping the prop. Too much cup and you accelerate fast but lose top speed, too little cup and you accelerate too slowly and lose top speed. On my 35hp Johnson/15' Allison, it's a very, very thin line between 37.5 mph and 41.5 mph. With adequate power to lift the pad bottom out of the water (that 350 lb boat is underpowered) it's less critical.

I use only a Rundquist pitch gauge, because it measures a chord length (3/4") that I can change with a hammer and trailer hitch ball (or anvill).
Measuring 'pointwise' is a useless activity: how do you want to use the information that the pitch over a 1/10" (or less) chord is so and so much? Totally useless! You might measure the pitch digitally to within .001", but what use is that to anyone with an 8" diameter (or more) prop? Measuring over 1/32" might well be of use to a hobbyist with a model boat and tiny prop.

I don't understand your statement about not being able to use the Rundquist gauge to measure camber, that's exactly what I use it for. The chord is the best approximation you can get to the tangent to a hypothetical helix, and a 3/4" chord (for 10-15" diameter props) is adequate.

All outboard props are variable pitch with the pitch increasing from leading to trailing edge. I worked recently on Yamaha racing cleavers where the pitch increases with radius along the leading edge, that's a bad design (I have the performance results with raceboats).

There's no displacement to mention, these are planing boats with only a small fraction of wet surface. Even my 35 hp powered 15' pad V-bottom 'fishing' boat rides with most of the surface dry.

I don't know how to quantify 'amount of cup', except that it should be sudden at the trailing edge, as small a chord as possible with a ball peen hammer, and extending not necessarily to the prop hub (where it runs into a big hub vortex boundary layer) but fading to zero very near to the maximum diameter to help to reduce the size of the top vortex.

So far, we haven't tried 'tip vortex reducers' like the bent tip of a jet wing, although I have a photo of an old prop from a ship, now used as a monument on the Inn River south of Passau, where the tip is indeed bent 'downward' on the high pressure side. That sort of modification is too hard to perform in a garage workshop with a 10-12" diameter prop, given the hardness of the metal.

16" pitch is way too high, would kill both acceleration and top speed. Tell me how you arrived at that number. 16" pitch would require about 6700 RPM and 46 mph (the so-called GT-Pro Class in Minnesota, where the tried to run the Yamaha cleavers).

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### sandhammaren05Senior Member

What's happening when a surfacing prop 'catches'?

Here's another good question. My experience is with outboards on pad V-bottoms and tunnels, 35-235 hp, 40-90 mph. All run surfacing props.

Here's what happens after you plane off, and I'll use 'good ol' boy' language since the problem's not been quantified. The boat's on a plane, the motor's screaming, and all of a sudden the prop 'catches': the bow lifts, the transom lifts later, and the speed goes way up. The transition was sudden, what's going on?

I think I understand it qualitatively but can't quantify anything. We can have a contest if you like, if someone requests it then I can send my answer beforehand to Baeckmo, who's well informed, and everyone including Baeckmo (before he sees my answer) can post a guess. Again, this is not written up anywhere but I'm thinking of including a chapter on prop theory in a history of OPC racing. Hint: only a qualitative understanding (ala Prandtl 1916) of how a foil works is required.

7. ### FrostyPrevious Member

I Know what you mean, mine does that and at 14 tons its a bit of a mystery, but I have a lot of prop slip, I think it needs more air to the props.

My bow will come up at 14/15 and the stern shortly after, revs start to decrease and away we go, I have no Idea why.

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### sandhammaren05Senior Member

Thanks for your response, it adds 'weight' to the discussion, so to speak! I think I understand it but let's see if anyone comes up with a good idea. I could give a hint what I'm thinking, but the hint is nearly the whole idea.

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### baeckmoHydrodynamics

Oh, its quite simple, the supercavitating and ventilating propeller has a nonsteady thrust performance, while a fully wetted rotor has a more or less steadily falling character. If you plot kt versus Ja for supercavitating operation, kt starts at a low value, slowly increasing until Ja ~0.4 times P/D. There the flow quite suddenly changes from a full cavity on the blade suction side, to baseventilated flow. The phenomenon resembles what is happening when you spill your coffe along the cup's side; increase the inclination and suddenly the flow breaks away.

This characteristic has been well known since Pozdunin presented the supercavitating propeller in the mid-forties and has to be catered for in the design of pumps, inducers and propellers. Unfortunately, few of the racing enthusiasts take the effort to learn the science behind the heating and beating, thus the "black magic" attitude. Supercavitating/superventilating flows, ie multiphase flows in turbomachines behave completely different compared to singlephase flow.

I'll send a typical propeller diagram for a sc prop as soon as my scanner has been replaced, ok?

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### sandhammaren05Senior Member

First, let's agree on the terminology. By kt assume you mean speed in knots, and J(a) is the advance coefficient, or? For readers unfamiliar with the terms, Dave Gerr's 'Propeller Handbook' is highly readable even if you're a layman.

Then P and D are pitch and diameter-? I'm not sure the coffee cup analogy is the best one but will think about it. Also, will your explanation hold if there's no cup at all, prop is merely smoothly (or not at all) cambered? This is important in the discussion.

I agreed to stick out my neck and send you my naive speculation, so I'll do it. Other readers should not be frightened by the engineering terms above, feel free to post your ideas!

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### baeckmoHydrodynamics

Sorry guys, when rereading my previous note it unintentionally sounds a bit patronizing, please have patience..... Correct remark on terminology, kt in the prop world is "thrust coefficient" (T/(density)/rps^2/D^4), where T=thrust in N, density ~1000 kg/m3, rps = revs per second and D = prop dia in m.

In fact, some flat face propellers have an extremely sharp transition and my reasoning goes for all types. In supercavitating flow, where cavities are longer than the blade chord, the specific gas in the cavity is not changing the process other than the cavity closure, so in our context here, the characteristics of a supercavitating propeller is similar to a superventilated one, the difference is the cavity pressure. In both cases, the cavity pressure is the sum of individual gas pressures; for cavitating flow, the vapor pressure is dominant, for ventilated flow, atmospheric pressure.

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### RikSenior Member

It's all math, well at least in this world but you stated the rpm, reduction ratio and speed.

Assuming a slippage variable, will determine the pitch of the propeller be it even a mean average. You have now stated an unknown in your response. "All outboard props are variable pitch with the pitch increasing from leading to trailing edge".... You assumed the readers knew the characteristics of the propeller design you were using.

You stated 6,100 rpm = 41 mph.... The two unknowns are pitch and slippage.

You also stated with the same reduction ratio that 16" would require 6,700 rpm and would achieve = 46 mph so your slippage value is 12.61%

The amount of cup can effectively act as pitch so if I was to use the scenario of top speed achieved of 41 mph (not accelerated speed) I can WAG it a few different ways but guessing that this has to be a small boat, small power and achieves a (relatively speaking) low speed then the slippage will not be very low (<10%). SO I used a 15% slippage factor for your application.

As for the measurement device, if the stylus is to wide (meaning its contact produces an area of a gap on it contact surface) then it is not accurately measuring the surface. A small circular stylus can (as many are doing) reverse engineer the propeller blades.

Baeckmo said 14.2" and I understand that this is the "leading edge pitch" as the boat would have a theoretical speed of only 42.53 mph without slippage if this were the total pitch for the propeller.

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### tom kaneSenior Member

Last edited: Jan 14, 2010
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### RikSenior Member

Sorry, I did not claim I design propellers, which you need a great deal of math or Maths to design accurately. I merely commented about the question as to what the pitch could be in an effort not to say I am correct, but rather to see what the correct answer is. This is why I gave a WAG not a definitive, argued answer. I understood one had to give an answer to the question in order to get the true answer. Just reading the rules.

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### RikSenior Member

Baeckmo, I have to ask (I follow your math) why did you chosse or how did you come to the conclusion of the diameter?