Define what a Surface-Piercing propeller is

[tom kane]

From Hickmans design to todays surface-piercing propellers is it possible to define a propeller as just being a surface-piercing propeller.

[Ad Hoc]

What does it matter?

Whether it be a prop or anything else that is "designed". If said "item that has been designed", does the job, then it has been designed in accordance with the requirements for its application. It may not always be the most efficient, or the cheapest or the "best way overall", but if it works, what is wrong with it? Why reinvent the wheel if the wheel works, but if used in a different context, so what?

Words are used to describe the application to give context to "things" so that the idea/notion can be conveyed. If a normal prop is used as a surfacing piercing prop, does it make it no longer a normal prop or does it suddenly make it a surface piercing prop because of its application?

What is considered avant-garde or weird today may be tomorrows norm.

"...What's in a name? that which we call a rose, by any other name would smell as sweet.." sums it up!

[tom kane]

I am inclined to agree with you.You would need to take a close look and handle a propeller to see what it is intended to do better than another propeller.Just a casual glance can be deceiving.

[Frosty]

A Surface prop and a normal prop is chalk and cheese, the similarity is that they propel a boat through water. How that is achieved is very different.

The surface prop is half submerged for a start off thats why they call it a "surface" prop. the root and the shaft itself is larger to take the constant pounding of thrust on the blade on and off.
swapping them would kinda propel the boat,in as much as move it.

[baeckmo]

Referring to AH I would say that a spp is a propeller that is intentionally designed to operate with mainly natural ventilation from a free surface. That excludes the 200.000 ton tanker propeller that is cutting the surface in light conditions, but includes Hickman's screws, even if they did not have the camber of todays spp and scp. The outboard prop with exhaust ventilation through the hub has much in common with the spp (and scp) designwise, but there is a difference due to the ventilating gas being introduced to the blade root isof to the outer radii.

Tom, do you have more info on the boat in your pic.s? Weight, power, speed aso. From one photo it seems to have a V-bottom, or is it only my monday vision that is diffuse?

[tom kane]

Prop is a Rola with a very aggressive cup on the trailing edge.First trials 27.5 knots with 200 RPM over- revving ride was excellent and high speed manouvering was much improved but at low speed all was not well especially in reverse when the prop walked the stern sharply to starboard.
a quick look at text..12000 lbs displacement.quarter-beam buttock angle 1.75"
Hull is a one off deeply modified.Definitely capable of generating sufficient lift to justify increased power..300 HP at a quick look at text.

[tom kane]

A surface-piercing prop,so we must look at what props in the early 1950`s
caused a boat to prop-ride,not only Hydro planes but deep v hulls. They were not called surface-piercing props then but heavily worked propellers.Not only cleaver props with heavy cupping.

[baeckmo]

........all was not well especially in reverse when the prop walked the stern sharply to starboard.
.

Even a normal prop would do the sidewalking thing with this installation, even if not as sharply. The spiralling backwash from the screw during backing will hit the vertical structure (holding the shaft) and generate a force to stbd. To counteract this, a vertical fin down from shaft line (check Arneson legs!) will help. (You will notice the same effect with an outboard having lost part of its lower fin.)

The reversing backwash from props with blades designed for cavitating or ventilating flow has more swirl than a std prop for the same astern force. Some propeller designs (Levi for instance) have an appendix at their trailing edges that both strengthens the blade and improves flow in backing.

[tom kane]

Reading from the text of this magazine article..surface area of the very tall narrow rudder was increased by 80% to give more steering authority.Most of this was at the top where the rudder would be fully immersed at low speeds but at high speeds much of it would be out of the water and would not add significant drag. In Aprill 1995....returned to the water for her final trials.She achieved 28.7 knots (measured by Radar) and the enlarged rudder was a roaring success-she was easier to control in reverse,and her handling was positive at all speeds....

[PetterM]

I am inclined to agree with you.You would need to take a close look and handle a propeller to see what it is intended to do better than another propeller.Just a casual glance can be deceiving.

Looks good, who made this drive?

[tom kane]

As the info is from an old Magazine article I felt was very worthy of keeping I guess there is no harm in puting it on this forum. The Evolution Company Camden Maine. The Evolution Marine Shafting System.From the thrust bearing back to the propeller,the shaft ran in an oil bath inside back to the propeller,the shaft ran inside a sealed tube,and turned in bearings normaly lubricated by oil bath.but would continue to run safely if the seal failed...The system did away with normal slow leak system of other systems.
WoodenBoat 126 September/October 1995

[Submarine Tom]

Tom,

I'm surprised at your aversion to the starboard prop walk in reverse (Post #6).

This can be turned (no pun intended) into a real asset if handled correctly.

Cheers, Tom

[tom kane]

Hi Submarine Tom,I was not giving may opinion but reading from a magazine article about trials of the surface drive in that WoodenBoat Magazine article as stated many times.My surface drive gave me lenght of boat turning while manouvering at low speed. I do not think that many boaties realize the limitations of rudders especially at speed where a rudder can only give at best 10 degrees each way from straight ahead before the stall position is reached stall and drag sets in.

[sandhammaren05]

Prop riding is an extreme case, V-bottoms with surfacing props usually don't prop ride.





A surface-piercing prop,so we must look at what props in the early 1950`s
caused a boat to prop-ride,not only Hydro planes but deep v hulls. They were not called surface-piercing props then but heavily worked propellers.Not only cleaver props with heavy cupping.

[gonzo]

Surface piercing propellers, as the name implies, pierce the surface. Only the lower blades are supposed to be in the water. That is why sometimes exhaust gas is used to start ventilation so the boat can get on a plane.

[sandhammaren05]

Surfacing props on outboards are fully submerged when the boat is accelerated from idle. Exhaust gas is not needed for planning.




Surface piercing propellers, as the name implies, pierce the surface. Only the lower blades are supposed to be in the water. That is why sometimes exhaust gas is used to start ventilation so the boat can get on a plane.

[gonzo]

Do you have any examples of outboards with surface piercing propellers?

[sandhammaren05]

All V-bottoms and tunnels raced since ca. 1965 are surface piercing! With a short shaft outboard the cavitation plate is even with the bottom of the boat with about a 17 5/8" transom height. On V-bottoms (55-85 hp) we ran closed course races at 21.5"-22" transom height. In general, on a fast pad V-bottom about 1/4-1/3 of the top of the gearcase will line up with the pad. On a tunnel there is no disturbance of the water coming into the prop, so that in that case the bottom of the gearcase is even with or higher than the deepest edges of the catamaran sponsons.

I now, for fun, run a 350 lb 'bass boat' with a 35 Johnson shortshaft (tiller handle) at a transom height 19"-19.5", the prop is a surface piercing (41.5 mph 2-way average). It isn't a cleaver, and need not be. Our fastest surfacing props on 18-20' pad V-bottoms with 235 hp outbooards ca. 1980 were not cleavers, but were high rake round eared props.

Typically, 'bass boats' are running surface piercing props.



Do you have any examples of outboards with surface piercing propellers?

[gonzo]

The wrongly called cavitation plate is there to prevent ventilation and the prop to be surface piercing. The photos show typical submerged propeller setup and operation.

[sandhammaren05]

You're wrong, it's surface piercing.



The wrongly called cavitation plate is there to prevent ventilation and the prop to be surface piercing. The photos show typical submerged propeller setup and operation.

[gonzo]

What makes you think it is a surface piercing propeller, and how do you define it?

[sandhammaren05]

It's surfacing because one blade is always above the water surface. That the out of the water blade runs through some spray thrown up from the gearcase bullet provides no thrust. I.e., at most 2 of 3 blades are providing thrust at any given time.

If you don't like my example, then you can be satisfy yourself with any outboard tunnel, where the entire gearcase runs above the water's surface.

I think that researchers who have mainly read a bit about surfacing props, and have not run high speed outboards set up for peak performance, have trouble getting the right picture. Or?

Breaking the surface is only where it starts. Without the right 'cup' surfacing props don't work.



What makes you think it is a surface piercing propeller, and how do you define it?

[gonzo]

Have you taken any photos of the propeller operating how you claim?

[sandhammaren05]

No, one can't steer a boat a photograph at the same time. but with a 35 hp motor with tiller handle, one can look back at the cavitation plate above the water.

I've watched tunnels run where, with the motor turning 7000-7500 RPM but geared 2:1, I could see the entire gearcase hub and propeller blades above the water. When the motor's jacked up to a surfacing condition, you can feel it: there's a transition from slow to fast where, suddenly (like an airfoil lifting off), the cup becomes effective, the boat lifts and gains speed considerably.
I set the record in EP Class (V-bottom/stock 50 c.i. outboard) with the waterline nearly splitting the gearcase hub (70.560 mph). Another few millimeters and the motor no longer pumped, because the water intake is located about halfway between the gearcase hub and the cavitation plate. The motor pumped adequately so long a there was enough spray from the gearcase hub. This is all well known by outboarders.

I think you should go to a boat race, see for yourself. The technology is old. Arneson didn't invent anything that wasn't known before, so far as I can see.




Have you taken any photos of the propeller operating how you claim?

[sandhammaren05]

Here's a shot where you can nearly see the nose of the gearcase

http://images.google.at/imgres?imgurl=http://farm4.static.flickr.com/3430/3870197474_188a0ba1c5.jpg&imgrefurl=http://flickr.com/photos/37021845%40N00/3870197474/&usg=__MVt19eTJ7IcTyMKfxt4YaqleBcg=&h=321&w=500&sz=116&hl=de&start=10&tbnid=5G0X98vbtjqPaM:&tbnh=83&tbnw=130&prev=/images%3Fq%3Dcees%2Bvan%2Bder%2Bvelden%26gbv%3D2%26hl%3Dde%26sa%3DG

[gonzo]

It is a very subjetive personal opinion. Outboard manufacturers have spent large amounts of time and money to keep the propellers from ventilating. I trust their data more.

[gonzo]

The photo you posted show a fully submerged lower unit.

[Frosty]

The rear of a surface prop blade is concave the normal prop rear is convex.

And yes outboards have ran surface props for 30 years, we used to call them choppers and were big round eared props. No air was needed to get them to break free because they were so close to the surface.

They were not used for skiing just racing and flywheel weight was reduced for out of corner acceleration. The rooster tail was enormous.

[sandhammaren05]

After winning 2 National Championships in 1977, the chief engineer in charge of props at a major outboard co. :P was smart enough to ask me to do research for them. So you listen to them, and they listen to me. Actually, they're very smart. They know that racing is the real testing ground, so they're always interested in what racers are doing with props.

Photos: 2008 and 853 are me, the other
boats were run by friends/competitors.

Working definition:
A surfacing prop is any propeller, of any design, with the motor set high enough on the transom that, with say a 3 0r 4 blade prop, at most 2 blades provide thrust via circulation about the blade at any given time. With a 3 blade prop it's only 1 blade at time that provides thrust. The photos show examples of outboards running surface piercing propellers. Surface piercing setups are typical for outboard closed course racing since at leat the midince the mid 1960s, and for straightaway records, at least since the 1950s



No, one can't steer a boat a photograph at the same time. but with a 35 hp motor with tiller handle, one can look back at the cavitation plate above the water.

I've watched tunnels run where, with the motor turning 7000-7500 RPM but geared 2:1, I could see the entire gearcase hub and propeller blades above the water. When the motor's jacked up to a surfacing condition, you can feel it: there's a transition from slow to fast where, suddenly (like an airfoil lifting off), the cup becomes effective, the boat lifts and gains speed considerably.
I set the record in EP Class (V-bottom/stock 50 c.i. outboard) with the waterline nearly splitting the gearcase hub (70.560 mph). Another few millimeters and the motor no longer pumped, because the water intake is located about halfway between the gearcase hub and the cavitation plate. The motor pumped adequately so long a there was enough spray from the gearcase hub. This is all well known by outboarders.

I think you should go to a boat race, see for yourself. The technology is old. Arneson didn't invent anything that wasn't known before, so far as I can see.

[gonzo]

Which major outboard manufacturer?

[sandhammaren05]

Here's an exercise for you. The manufacturers couldn't answer it correctly in 1980, I doubt they can answer it now. We have a high speed, high-planning hull (very little wet surface or 'bow wave' from the bottom near the transom), the outboard is jacked high enough that the prop has at best 1-2 blades in the water at one time during a revolution. the gear ratio is 14:27, and the diameter has been chosen optimally for that gear ratio. The top speed should be 41 mph at 6100 RPM. What is the optimal pitch at the leading edge, and (hydrodynamically) why? Pitch is measured via a Rundquist pitch gauge, i.e., over approximately a 3/4" chord.

Send me your answer and I'll have a good idea whether you might be able to teach me something, or vice-versa.





Which major outboard manufacturer?

[gonzo]

There is no such thing as a "pitch at the leading edge" You are trying to change the subject and not answer a straightforward question: What outboard manufacturer did you work for? Also, we have to believe that propellers are surface piercing because no one can steer and take photos of a prop. Have you ever heard of camera mounts? Too much BS.

[sandhammaren05]

It's clear that you haven't the slightest idea what you're writing. I have no further time to waste.



There is no such thing as a "pitch at the leading edge" You are trying to change the subject and not answer a straightforward question: What outboard manufacturer did you work for? Also, we have to believe that propellers are surface piercing because no one can steer and take photos of a prop. Have you ever heard of camera mounts? Too much BS.

[gonzo]

You haven't answered the question. What outboard manufacturer did you work for? It is interesting how upset people that make wild claims get when asked for evidence.

[baeckmo]

Sorry Gonzo, but Sandhammaren is completely on track here, and you are way off. The "leading edge pitch" is used to define the local angle of attack on a cambered or cupped blade, particularly when we talk about ventilated or supercavitating propellers. For these, the inflow factors normally used for fully wetted props, are not applicable. At the design operating point, the angle of attack to the leading edge on a SC or SPP prop is more important than the average pitch in order to control cavity size and thus performance.

If you check the pic on the outboard on the transom, you will see that it is adjusted vertically, so that the flow leaving the step is meeting the rig far below the antiventilation plate. This is not an uncommon view in racing circles.

So back off Gonzo, you don't know what you are talking about here!

[gonzo]

Pitch is a nominal measure. That is industry standard. I asked about his claims that it is not possible to take a photo of the propeller and that he worked for an outboard company. Show me a photo

[Frosty]

Merc Mod VP V6 fuel injection were used, no warranty-- 101 octane had be got from the airport. The solid hubless props would shatter the gearboxes so starting in gear was necessary. The exhaust would go into the blades and almost Max RPM was used to take of until they would bite. The foot controled throttle was necessary also as jabbing throttle op to full and off again was needed to get them up.

The gearboxes were so high they could not pick up water for the engine, various adaptors were made but a separate pick up scoop on the transom did the trick. 2x3 inch holes were drilled in the leg approx 6 inches down, there were also mods on the exhaust trumpet, i forget if that was on the mod VP.

26x30 I think they were, rabbit eared 3 bladers. If I remember correct cleavers were for rough water. All a long time ago.

[sandhammaren05]

Good to hear from you. I discovered the importance of camber by accident in Louis Baumann's prop shop in Houston in 1978. His old pitch gauge measured over about a 3" or more chord, I wanted a 22" down from 23" but the stainless was still too thick so I bent mainly the leading edge. Was an eye opener for me how it accelerated and ran on top end. That was before I'd done any theory homework in Newman or Landau-Lifshitz to help me to understand what I'd done. Paul Allison independently discovered 'cup' in a worse way: he pulled his trailer up his driveway with the prop dragging. He took out the damage and lost speed. That was 1955, river racing in Knoxville with a 25 Johnson on a homemade boat. Three of the photos shown by me are Allison boats, the other is a Laser, all made 1975-1981. Thanks again for adding to the discussion.



QUOTE=baeckmo;333386]Sorry Gonzo, but Sandhammaren is completely on track here, and you are way off. The "leading edge pitch" is used to define the local angle of attack on a cambered or cupped blade, particularly when we talk about ventilated or supercavitating propellers. For these, the inflow factors normally used for fully wetted props, are not applicable. At the design operating point, the angle of attack to the leading edge on a SC or SPP prop is more important than the average pitch in order to control cavity size and thus performance.

If you check the pic on the outboard on the transom, you will see that it is adjusted vertically, so that the flow leaving the step is meeting the rig far below the antiventilation plate. This is not an uncommon view in racing circles.

So back off Gonzo, you don't know what you are talking about here![/QUOTE]

[Frosty]

Altering pitch is difficult infact altering leading edge pitch and trailing is all you can do, as you will know no more than 1 inch is enough.

An old cylinder block with all its nooks and crannies and cylinder holes are great for tweaking props.

I always altered my own props after driving miles to take a brand new prop for cupping to the manufacturer and he just hammered the hell out of it,---I though right well I can do that.

[sandhammaren05]

We had to start the 49.9 c.i. Evinrude 75 loopers in gear too. It was 1981 at Havasu that Mod-VP boats were allowed to run a non-stock undergearcase-hub water pickup. The Mod-VVP tunnel speeds went from about 85mph to about 100mph because of the drag reduction because they could run with the gearcase hub above the water in typical tunnel style. You couldn't raise the motor that high on a V bottom and still turn, or have any control at all. I ran boat #2008, a Laser/Evinrude 235 that year, finished 13/50 and enjoyed the fine handling of the boat (I turned inside of everyone) although the speed was but 85 mph. It was that year that I noticed that Hendricks at OMC had picked up on my adding camber to leading edge of props, but we still ran a 28" pitch Mercury Chopper, not his prop. There was another round0eared Mercury prop that wasn't bad, but I've forgotten the name. The cleavers did not provide enough bow lift on the heavy boats.



Merc Mod VP V6 fuel injection were used, no warranty-- 101 octane had be got from the airport. The solid hubless props would shatter the gearboxes so starting in gear was necessary. The exhaust would go into the blades and almost Max RPM was used to take of until they would bite. The foot controled throttle was necessary also as jabbing throttle op to full and off again was needed to get them up.

The gearboxes were so high they could not pick up water for the engine, various adaptors were made but a separate pick up scoop on the transom did the trick. 2x3 inch holes were drilled in the leg approx 6 inches down, there were also mods on the exhaust trumpet, i forget if that was on the mod VP.

26x30 I think they were, rabbit eared 3 bladers. If I remember correct cleavers were for rough water. All a long time ago.

[sandhammaren05]

A first rate photo of outboards running surfacing props can be found on pg. E14 of the following well-known reference:

Robert F. Kress, High Speed Propeller Design, The Society of Naval Architects and Marine Engineers, paper E, presented at the Spring mMeeting, Lake Buena Vista, Fla., Apr. 2-4, 1973.

I have the paper on hand in case anyone wants a copy, belongs to my stack from ca. 1980.:P

[PetterM]

Could I have copy?

[baeckmo]

Place a note on the reference in Rudi's thread on surface prop litterature in the "Surface drive section". And yep, I would like a copy as well.

[sandhammaren05]

Give me your email address and I'll fax a copy. I'll also list other lit on Rudi's thread that I have if it's not already there.

jmccauley@uh.edu



Place a note on the reference in Rudi's thread on surface prop litterature in the "Surface drive section". And yep, I would like a copy as well.

[sandhammaren05]

Will scan and send to the email address on your website.


Could I have copy?

[sandhammaren05]

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


Give me your email address and I'll fax a copy. I'll also list other lit on Rudi's thread that I have if it's not already there.

jmccauley@uh.edu

[Rik]

Here's an exercise for you. The manufacturers couldn't answer it correctly in 1980, I doubt they can answer it now. We have a high speed, high-planning hull (very little wet surface or 'bow wave' from the bottom near the transom), the outboard is jacked high enough that the prop has at best 1-2 blades in the water at one time during a revolution. the gear ratio is 14:27, and the diameter has been chosen optimally for that gear ratio. The top speed should be 41 mph at 6100 RPM. What is the optimal pitch at the leading edge, and (hydrodynamically) why? Pitch is measured via a Rundquist pitch gauge, i.e., over approximately a 3/4" chord.

Send me your answer and I'll have a good idea whether you might be able to teach me something, or vice-versa.

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.

1. HP of your engine?

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 ass 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?

[baeckmo]

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.

[PetterM]

Will scan and send to the email address on your website.

Thanks

[sandhammaren05]

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).




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.

1. HP of your engine?

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 ass 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?

[sandhammaren05]

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.

[Frosty]

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.

[sandhammaren05]

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.



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.

[baeckmo]

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?

[sandhammaren05]

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!



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?

[baeckmo]

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.

[Rik]

It's all math, well at least in this world :D 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.

[tom kane]

How about trying your maths on the thread A strange propeller design. As maths is not an exact science please be careful.

[Rik]

How about trying your maths on the thread A strange propeller design. As maths is not an exact science please be careful.

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.

[Rik]

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

It is the pivital constant in your answer.

[baeckmo]

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".

[gonzo]

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

[baeckmo]

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

[yipster]

http://www.boatdesign.net/forums/attachments/propulsion/39442d1263316147-define-what-surface-piercing-propeller-twin.jpg
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

[sandhammaren05]

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

My response is sandwiched below in [ ...]

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.


[Forget mean pitch]


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



[The trick is to understand that there is only one unknown, to zeroth order.]



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.



['Cup' changes trailing edge pitch, yes. It's like the flap on an airplane wing.]



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.



[Forget a stylus, won't work usefully on a 10-20" (or more) diameter prop. The Rundquist gauge tells you exactly what you need to know on 10-15" dia. props.]



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.



[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?]

[gonzo]

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.

[baeckmo]

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.

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!

[gonzo]

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.

[Frosty]

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.

[sandhammaren05]

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.

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.

[baeckmo]

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.

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.

[gonzo]

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.

[Frosty]

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.

[gonzo]

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

[yipster]

easy gonzo, me thinks baeckmo is the scientist here and lets keep it frendly plz
Surface piercing propeller
From Wikipedia, the free encyclopedia

The surface piercing or ventilated propeller is a propeller that is designed to intentionally cleave the water and entrain atmospheric air to fill the void, which means that the resulting gas layer surrounding the propeller blade consists of air instead of water vapour. Less energy is thus used, and the surface piercing propeller generally enjoys lower drag than the supercavitating propeller. The surface piercing propeller also has wedge shaped blades, and propellers may be designed that can operate in both supercavitating and surface piercing mode.


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

[baeckmo]

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.

[daiquiri]

Vaccum can be lowered or raised. It is negative pressure and just a convenient way of indicating that.

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).

[gonzo]

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

[Frosty]

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.

[gonzo]

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.

[sandhammaren05]

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

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.

[gonzo]

So you claim that nobody uses vacuum gauges?

[sandhammaren05]

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.

[terhohalme]

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!

[Joakim]

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.

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.

[Joakim]

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.

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.

[baeckmo]

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.......!

[sandhammaren05]

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.

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.

[Joakim]

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

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.

[sandhammaren05]

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 ... .

[sandhammaren05]

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.

You have a pitch gauge? You'd need one to do anything about it. I bought my Rundquist for about $200 in 1958. Now they 'discount' for $1500. Lack of competition.

[baeckmo]

Checked your pic, Joakim. Like Joe says, without measurements there is only guesswork. But personally I find this tip shape a bit weird. Look at the outer 10 to 20 mm's of the blade. The cupped part is a very huge part of the total chord; in the tip region there is "only cup". Without the front part of the foil, there is not much thrust generated there, ie loss of power. It may produce a lot of lift though, but that lift could be produced by other means.

I feel modern SP props have more of a constant ratio cup/chord all along the radius. When the blade emerges out of the wet stuff, it is still carrying a substantial volume of water as a film over the pressure side. In the absense of a surrounding volume of water, this film is thrown off tangentially into the air instead of beeing pushed aft for thrust.

To counteract this tendency, either the cup must "go around the ear" as in the chopper or the "classical" Newton-Rader prop (or the Russian SK series for that matter), or the TE must be cut forward, so there is an inward force acting in the tip region.

BTW, do you have access to a lathe? I use mine, with a 360 degree scale on the chuck for blade shape control. A micrometer (don't get the English word for it right now) on the doll plus a length scale to the doll and that's it, plot with spline function in any CAD application. If you happen to have a stroboscope, first check the prop rotating under strobe light; you will immediately spot the out-of-shape blade. It is not unusual to find substantial pitch differences coming with new props right out of the box!

[Frosty]

Below is some technical drawings of levi surface drives with diamond back Seafury 3 blade cleavers 24x24

Top picture left is the original levi rudder. 44 cat 2x250HP

Second pic is that of my latest modification. Removal of 1 rudder each side increase in speed and dramatic improvement in 20KT steering. Was this because of more air to the prop or less wetted surface of the cut rudder.



3rd pic is my intended mod shortly, following on the success of above (more may be better). More air may have been the reason. Some think that the prop wash can be re directed into thrust , my opinion is that this wash is spent and has no benefit.

[Joakim]

I think also, that it is important, that the water film is not thrown in the air. The Ron Hill in the pictures was much better at this than the 10x18 Radice, which produced a big rooster tail. When you look at boats really performing well, there is no visible rooster tail:

http://pellinge.fi/fresegustafson.com/Snygga%20bilder.html

Look also at the propeller section:

http://pellinge.fi/fresegustafson.com/prop.htm

Is there much of a difference between the cup of these compared to the Ron Hill?

I don't have a pitch gauge, but a friend of mine has a lathe and many measuring devices for it. If I find some spare time, I try to do some measurements of the Radice, but it will take some time.

What kind of efficiency (not slip) would you expect for the Ron Hill in the conditions I specified? It would be nice to compare to the predictions of my Savitsky program.

[daiquiri]

Below is some technical drawings...
Frosty... :D :D :D

[sandhammaren05]

I think also, that it is important, that the water film is not thrown in the air. The Ron Hill in the pictures was much better at this than the 10x18 Radice, which produced a big rooster tail. When you look at boats really performing well, there is no visible rooster tail:

http://pellinge.fi/fresegustafson.com/Snygga%20bilder.html

Look also at the propeller section:

http://pellinge.fi/fresegustafson.com/prop.htm

Is there much of a difference between the cup of these compared to the Ron Hill?

I don't have a pitch gauge, but a friend of mine has a lathe and many measuring devices for it. If I find some spare time, I try to do some measurements of the Radice, but it will take some time.

What kind of efficiency (not slip) would you expect for the Ron Hill in the conditions I specified? It would be nice to compare to the predictions of my Savitsky program.

I can't comment/compare cup without measuring the props, 'eyeballing' is misleading. The cleaver on top, blade shape near tip, looks like the ones I made for myself. I've posted photos of some of my raceboats, will add a few here. You don't see much rooster tail, that's only wasted power. As for predicting efficiency of a Hill or any other prop, I'd have to measure it on my pitch gauge and know the boat/motor, some details. I never used Savitsky, I always found the existing theory to be way too crude for top performance. Theory was always a qualitative guide for me: try to minimize the tip vortex, and the leading edge vortex as well. So I don't like blades that have a tip like a 'knife'. I ran a Mercury chopper on 2008, ran 12x23 OMC cleavers on 853 (I had four cleavers that I'd reworked for myself in Louis Baumann's prop shop). Leading edge was than 23" pitch, obviously! Evinrude 75 turned about 6700RPM, geared at 15/28, speed about 67mph (I could get about 1mph more by reworking a 12x25, in the end only the pitch distribution along the blade at constant radius was different with the leading edge pitch set about 1" higher than on my 12x23s). The Evinrude 235 ran a 28" chopper, turned about 6500-6700RPM (best I recall) at 85 mph. Geared 2:1.

[yipster]

http://www.boatdesign.net/forums/attachments/propulsion/39789d1264042620-define-what-surface-piercing-propeller-.gif
:D nevertheless interesting experiments

[sandhammaren05]

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.

The blade shape is basically good, I easily could improve it with a file. The cup? Would have to measure it. Also, is the pitch along the leading edge constant as radius varies, or (like some mis-engineered 10x16-17 Yamaha cleavers) does it increase with radius along the leading edge?

[sandhammaren05]

Below is some technical drawings of levi surface drives with diamond back Seafury 3 blade cleavers 24x24

Top picture left is the original levi rudder. 44 cat 2x250HP

Second pic is that of my latest modification. Removal of 1 rudder each side increase in speed and dramatic improvement in 20KT steering. Was this because of more air to the prop or less wetted surface of the cut rudder.



3rd pic is my intended mod shortly, following on the success of above (more may be better). More air may have been the reason. Some think that the prop wash can be re directed into thrust , my opinion is that this wash is spent and has no benefit.

Your engineering drawing is impressive, Popeye. In the local Austrian Gymnasium they unnecessarily force our son to use CAD.

[sandhammaren05]

Checked your pic, Joakim. Like Joe says, without measurements there is only guesswork. But personally I find this tip shape a bit weird. Look at the outer 10 to 20 mm's of the blade. The cupped part is a very huge part of the total chord; in the tip region there is "only cup". Without the front part of the foil, there is not much thrust generated there, ie loss of power. It may produce a lot of lift though, but that lift could be produced by other means.

I feel modern SP props have more of a constant ratio cup/chord all along the radius. When the blade emerges out of the wet stuff, it is still carrying a substantial volume of water as a film over the pressure side. In the absense of a surrounding volume of water, this film is thrown off tangentially into the air instead of beeing pushed aft for thrust.

To counteract this tendency, either the cup must "go around the ear" as in the chopper or the "classical" Newton-Rader prop (or the Russian SK series for that matter), or the TE must be cut forward, so there is an inward force acting in the tip region.

BTW, do you have access to a lathe? I use mine, with a 360 degree scale on the chuck for blade shape control. A micrometer (don't get the English word for it right now) on the doll plus a length scale to the doll and that's it, plot with spline function in any CAD application. If you happen to have a stroboscope, first check the prop rotating under strobe light; you will immediately spot the out-of-shape blade. It is not unusual to find substantial pitch differences coming with new props right out of the box!

Consider Baeckmo's statement: "...To counteract this tendency, either the cup must "go around the ear" ...". I agree partly from experience.

With my small sportboat (35 hp, 350 lb Allison pad Vee) the cup must full at max radius, or else the 'sharp transition' to higher speed that I stated in my second 'problem' on this site doesn't occur (instead, there's only a bigger roostertail). If the cup goes around the tip (beyond max radius) then the transition to bow lift occurs very fast but then top speed is reduced by about 10%. I didn't find this to be so with my larger hp raceboats, where the cup could feather near to zero at the tip.

[sandhammaren05]

[QUOTE=sandhammaren05;336312]Consider Baeckmo's statement: "...To counteract this tendency, either the cup must "go around the ear" ...". I agree partly from experience. And, yes, the cup at the tip does produce bow lift.

With my small sportboat (35 hp, 350 lb Allison pad Vee) the cup must full at max radius, or else the 'sharp transition' to higher speed that I stated in my second 'problem' on this site doesn't occur (instead, there's only a bigger roostertail). If the cup goes around the tip (beyond max radius) then the transition to bow lift occurs very fast but then top speed is reduced by about 10%. I didn't find this to be so with my larger hp raceboats, where the cup could feather near to zero at the tip, and here's why: with greater hp the required bow lift is otherwise dynamically generated. The Allisons had airlift built into the bow, so at a high enough speed air did the trick. The pad felt greater water pressure and also lifted. The Laser (#2008) did not have much bow lift, but had small lift strakes rear along the chines. These tunneled enough air to lift the transom, and also made the boat turn very tightly on a race course.

[hoytedow]

:D :D :D Frosty, I tried to give you rep points for your "technical drawings", but I must "spread repu...blah..." first.

[Frosty]

My drawings seem to have become a source of amusement. They took me quite a while to do even sprinkling talcum on the touch pad to get the round curves.

I thought they were rather good. Should I have another go and see if I can improve them?

[hoytedow]

No. They are perfect.

[sandhammaren05]

Checked your pic, Joakim. Like Joe says, without measurements there is only guesswork. But personally I find this tip shape a bit weird. Look at the outer 10 to 20 mm's of the blade. The cupped part is a very huge part of the total chord; in the tip region there is "only cup". Without the front part of the foil, there is not much thrust generated there, ie loss of power. It may produce a lot of lift though, but that lift could be produced by other means.

I feel modern SP props have more of a constant ratio cup/chord all along the radius. When the blade emerges out of the wet stuff, it is still carrying a substantial volume of water as a film over the pressure side. In the absense of a surrounding volume of water, this film is thrown off tangentially into the air instead of beeing pushed aft for thrust.

To counteract this tendency, either the cup must "go around the ear" as in the chopper or the "classical" Newton-Rader prop (or the Russian SK series for that matter), or the TE must be cut forward, so there is an inward force acting in the tip region.

BTW, do you have access to a lathe? I use mine, with a 360 degree scale on the chuck for blade shape control. A micrometer (don't get the English word for it right now) on the doll plus a length scale to the doll and that's it, plot with spline function in any CAD application. If you happen to have a stroboscope, first check the prop rotating under strobe light; you will immediately spot the out-of-shape blade. It is not unusual to find substantial pitch differences coming with new props right out of the box!

Is there a German word for 'cup'? Scandinavian?

[sandhammaren05]

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?

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.