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Old 12-30-2012, 06:24 PM   #81
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Marin your'e thinking GB as I think the 2.57-1 ratio is much more common.

I remember when I was 19 I flew from Anchorage to the Bethel area on an F27 Fairchild. My first turbine flight and I was probably almost afraid it climbed so steep.

I had an ultralight that climbed at 1500fpm at 40 mph. It felt like about 45 degrees. I had to look between my legs to see the ground looking ahead. That bird used a high aspect ratio 4 blade prop. Interestingly it had a 2.57-1 gear ratio. Just number of teeth I guess.

I could probably use a lower gear (3-1 or more) and swing a considerably larger prop. Willard choose an 18" dia prop. Very deep ratios I think unload much quicker (that is w less rpm drop) than smaller and faster props. That's why OBs and especially small OBs have very small props. They don't know what boat the engine will be required to push so they must have a combination that won't allow the engine to over rev or over load too much. With a small engine in a heavy disp trawler w a deep gear ratio backing down while entering a slip basically will need almost full rpm to slow her down. With a 4-1 gear one would need to run at a higher rpm (all other things being equal) to load the engine the same as w a higher geared boat w a smaller faster prop. Summing up ... deep gearing is not very flexible. And basically you'll only get more efficiency at full load/speed.
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Old 12-31-2012, 01:15 AM   #82
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Marin your'e thinking GB as I think the 2.57-1 ratio is much more common.
I don't know if Borg Warner made custom gear ratios for specific manufacturers or not. I know of other boats than GBs that use the same ratio transmissions as our boat. So I suspect the transmissions in our boat are off-the-shelf units offered by BW at the time.

Single engine GB36s used the 2.1:1 ratio transmission.
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Old 12-31-2012, 12:21 PM   #83
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Mark, a little tail has never hurt anyone.

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Old 12-31-2012, 01:45 PM   #84
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Mark, a little tail has never hurt anyone.

Where's the skier??
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Old 12-31-2012, 02:53 PM   #85
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The hull-speed Coot's four-blade propeller seems well matched with the 2400 RPM 80-horsepower JD engine (don't recall the gear-reduction ratio). ...
It's a 2:78:1 ratio ZF transmission
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Old 01-01-2013, 01:58 AM   #86
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I know on helos...the reason for more/shorter blades varies but at some point all aircraft propellers/rotors have to watch that the speed of the tips don't go supersonic as it totally messes up the aerodynamics.
I don't know if they are getting supersonic but I've ran airboats that have had electronic ignitions so they could be easily and precisely governed because of prop tip speed issues. I do know that if they went higher than spec'ed the noise changed significantly. Made a sound like it was "beating" the air, if that makes sense.
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Old 01-01-2013, 12:09 PM   #87
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I don't know if they are getting supersonic but I've ran airboats that have had electronic ignitions so they could be easily and precisely governed because of prop tip speed issues. I do know that if they went higher than spec'ed the noise changed significantly. Made a sound like it was "beating" the air, if that makes sense.
That's what I was taught in aerodynamics...

The whop whop of a Huey 2 blade is them approaching supersonic..that's why the faster and more maneuverable Cobras went to shorter 4 blades.

.here's another tidbit...

NACA research during the mid-1940s did show that supersonic prop-driven aircraft were feasible, but the key was developing propellers that could operate at or near the speed of sound. NACA began a high-speed propeller development program that culminated in the supersonic propeller, but this kind of prop is substantially different from those used on the fastest propeller-driven aircraft of the day, like the P-51 Mustang. Beyond speeds of Mach 0.9, the prop blades must become significantly shorter and thinner and the blade angle (the angle of attack of the blade) must be decreased compared to blades used on previous aircraft. An example is the test propeller mounted to the nose of an XF-88 research aircraft.

The reason a supersonic prop looks so different from standard propellers is because of shock waves that form on the propeller blades as the aircraft approaches Mach 1. Even before the plane reaches the speed of sound, portions of the blades do reach or exceed Mach 1 creating "bubbles" of supersonic flow. As we discussed in a previous question on supercritical airfoils, these supersonic regions are accompanied by shock waves. These shock waves not only greatly reduce the efficiency of the propeller, but create tremendous forces that can tear the prop apart. Making the blades shorter and thinner and reducing the blade angle effectively makes the blades supercritical meaning these regions of supersonic flow form at higher speeds than is possible with standard props. By delaying the onset of shock waves and reducing their strength once they do form, the supersonic prop can operate at higher speeds.
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Old 01-01-2013, 12:18 PM   #88
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This is one of the cool things about this site. Loads of experience from widely differing backgrounds.

Ii just wish some were more open-minded, and would acknowledge that everything they did isn't necessarily the only way things can be done (and while not widely practiced sometimes neither is wrong).
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Old 01-01-2013, 12:21 PM   #89
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My back ground is with offshore powerboats As the speeds have increased with some boats over 200mph and prop blade pitch upwards of 40'' The cast blades have become unreliable, slinging blade tips and complete blades off at high speed . A few small shops now make forged props with cnc finishing. Of course this has nothing to do with out 10kt trawlers Thankfully we do not have to spend $15000 for a set of props
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Old 01-02-2013, 12:56 PM   #90
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[QUOTE=Marin;122648]That may be true for water propellers, I don't know. It is not true for air propellers.

I should add that the 2-bladed prop used on float-equipped Beavers is even wider and more square at the tip than the one pictured. And possibly longer-- I can't tell that from the photo.

Dont know about planes but your post reminded me of a conversation i had with a mercrusier engineer in thier prop department. I asked almost the same question in regards to props, 4/ 3 blade and blade width. His answer was basically the higher the operating rpm the narrower the blades for best efficiency. Function of total prop load, rpm, design speed.
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Old 01-02-2013, 01:39 PM   #91
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His answer was basically the higher the operating rpm the narrower the blades for best efficiency. Function of total prop load, rpm, design speed.
Air propeller length is governed by the tip speed. Assuming conventional propeller design, the tip speed cannot exceed the speed of sound. If it does it introduces the potential for structural problems.

The slower the propeller turns the longer it can be as witness helicopter rotors. For prop planes, given the blade rpm limit the only variable left with regards to developing thrust is blade width and blade design. This is why planes with a lot of power, like the WWII Corsair, had big fat blades on them. The blades still had to turn at about the same speed as the blades on a Cessna, but they could be made way broad to take advantage of the tremendous horsepower available to turn them so they could develop a massive amount of thrust.

The deHavilland Otter was designed for an 800 hp engine that never materialized. So they compromised and used a 600 hp engine but put a gearbox on it to produce a higher horsepower at the output shaft so it could swing a fatter propeller at the same rpm than it could with only 600 hp. This was the only way enough thrust could be developed to get the Otter off the ground and climbing with a full load.

How all this-- tip speed and blade width-- relates to marine propellers I don't know since the medium a marine prop is running in is not compressible where the medium an air propeller is running in is. So it's probably apples and oranges.
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Old 01-02-2013, 02:54 PM   #92
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Air propeller length is governed by the tip speed. Assuming conventional propeller design, the tip speed cannot exceed the speed of sound. If it does it introduces the potential for structural problems.

The slower the propeller turns the longer it can be as witness helicopter rotors. For prop planes, given the blade rpm limit the only variable left with regards to developing thrust is blade width and blade design. This is why planes with a lot of power, like the WWII Corsair, had big fat blades on them. The blades still had to turn at about the same speed as the blades on a Cessna, but they could be made way broad to take advantage of the tremendous horsepower available to turn them so they could develop a massive amount of thrust.

The deHavilland Otter was designed for an 800 hp engine that never materialized. So they compromised and used a 600 hp engine but put a gearbox on it to produce a higher horsepower at the output shaft so it could swing a fatter propeller at the same rpm than it could with only 600 hp. This was the only way enough thrust could be developed to get the Otter off the ground and climbing with a full load.

How all this-- tip speed and blade width-- relates to marine propellers I don't know since the medium a marine prop is running in is not compressible where the medium an air propeller is running in is. So it's probably apples and oranges.
I think the same laws of physics apply to both so explanation is valid. The mathematical calculations if one takes into consideration of all the variables will be the same.
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Old 01-02-2013, 04:52 PM   #93
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I think the same laws of physics apply to both so explanation is valid. The mathematical calculations if one takes into consideration of all the variables will be the same.
I hope you aren't suggesting that aerodynamics and hydrodynamics use all of the same physical principles....if so...not even remotely close....

here's a website clip....

Many complex flow problems are common to aerodynamics and hydrodynamics as they relate to fundamental phenomena such as turbulence, separation, and wake vortices. The same digital models (Euler, Navier-Stokes, etc.) and testing resources (wind tunnels, hydrodynamic tunnels) are therefore often used to handle subsonic aerodynamic and hydrodynamic problems. However, aerodynamics and hydrodynamics differ in their specific effects.

The particularities of hydrodynamics:
  • high fluid mass (gravity and inertial interaction between the fluid and structures),
  • presence of a free surface (diffraction-radiation of the swell by obstacles),
  • existence of two phase mixing (cavitation phenomenon in particular).

In aerodynamics, particularities relate to high speeds:
  • effects of air compressibility (shock waves),
  • thermal effects,
  • physico-chemical effects at high temperatures (combustion, hypersonic flows) and high altitudes (rarefaction).
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Old 01-02-2013, 08:04 PM   #94
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I hope you aren't suggesting that aerodynamics and hydrodynamics use all of the same physical principles....if so...not even remotely close....

here's a website clip....

Many complex flow problems are common to aerodynamics and hydrodynamics as they relate to fundamental phenomena such as turbulence, separation, and wake vortices. The same digital models (Euler, Navier-Stokes, etc.) and testing resources (wind tunnels, hydrodynamic tunnels) are therefore often used to handle subsonic aerodynamic and hydrodynamic problems. However, aerodynamics and hydrodynamics differ in their specific effects.

The particularities of hydrodynamics:
  • high fluid mass (gravity and inertial interaction between the fluid and structures),
  • presence of a free surface (diffraction-radiation of the swell by obstacles),
  • existence of two phase mixing (cavitation phenomenon in particular).
In aerodynamics, particularities relate to high speeds:
  • effects of air compressibility (shock waves),
  • thermal effects,
  • physico-chemical effects at high temperatures (combustion, hypersonic flows) and high altitudes (rarefaction).
What i meant was that the same physical laws of nature that apply to an airplane propeller also apply to a boat propellar. Guess thats why they are both called propellers?
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Old 01-02-2013, 09:18 PM   #95
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What i meant was that the same physical laws of nature that apply to an airplane propeller also apply to a boat propellar. Guess thats why they are both called propellers?
I guess the article pointing out that not ALL the same physical laws apply is slipping by you?
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Old 01-02-2013, 10:27 PM   #96
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I guess the article pointing out that not ALL the same physical laws apply is slipping by you?
I'm sorry, i guess my comment that propellers all must conform to the same laws of science and physics passed by you?
Each propeller has different design criteria but all have to obey the same laws of science. Because of the viscosity of the operating medium some factors will have a lessor effect upon the airplane prop than a boat prop but they will still be there and will effect both props. Engineers often develop what they call approximations for specific applications to simplify calculations. What you are used to seeing are these approximations so you conclude that each are governed by different laws of science and that is not true. What is true is that some factors have more of an effect upon a boat prop than one for aircraft but they are still both props and must conform to the same laws of science.
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Old 01-03-2013, 07:05 AM   #97
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So they compromised and used a 600 hp engine but put a gearbox on it to produce a higher horsepower at the output shaft so it could swing a fatter propeller at the same rpm than it could with only 600 hp.
A gearbox doesn't increase horsepower, it increases (or decreases) torque and rpm proportionally, the horsepower remains the same.


P&W made the 600 hp 1340 in geared and direct drive versions. The extra 50hp above the standard 550 hp rating came from an additional 50 rpm at take-off and a higher speed blower.
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Old 01-03-2013, 11:46 AM   #98
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There are more similarities than dissimilarities re the two fluids .. air and water.

Re Ricks post I thought surely Marin couldn't have made that mistake. I read back and see that he actually did. Got him fair and square Rick. He said it but I'm sure he knows the difference ... pretty sure. Marin are you taking advantage of the new state law about smoking?
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Old 01-03-2013, 11:51 AM   #99
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The difference between a 5' diameter and a 25'' diameter prop are HUGE .a Trawler prop is mostly likey turning slower as well. No issue with blade tip speed
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Old 01-03-2013, 12:17 PM   #100
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A gearbox doesn't increase horsepower, it increases (or decreases) torque and rpm proportionally, the horsepower remains the same.
You're rignt, of course. The horsepower of the engine itself remained the same. But by increasing the torque available the gearbox made the powerplant seem like it had more horsepower than it did. The same propeller fitted to the geared engine could have been swung without a gearbox by a more powerful engine.
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