Prop diameters

tblshur

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Aug 24, 2011
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how does the different diameters affected prop performance thank you
 

jimmbo

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A larger diameter, may mean a larger volume of water is moved, but, it isn't as simple as that, as Blade Area is actually a bigger factor. Also as a Prop gets larger in Diameter, the speed of the Blade Tips becomes a factor. Too fast and Cavitation becomes a problem.
Let the Prop Engineers worry about Diameter. Pitch, Cupping, and Rake are the some of the other Variables you can sweat about
 

tblshur

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Thanks Jimmbo the prop list for my 83 Johnson 140 is very confusing item 80 list several props of different diameters and pitch for the motor some of which I know won’t perform on my boat don’t understand why :confused:
 

dingbat

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Thanks Jimmbo the prop list for my 83 Johnson 140 is very confusing item 80 list several props of different diameters and pitch for the motor some of which I know won’t perform on my boat don’t understand why :confused:
Other than fitting on the shaft, props are particular to the hull, not the motor.

Think of a prop as a gear.

Big motor with big boat, you need little prop.
Big motor with little boat, you need a big prop.
 

tblshur

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Dingbat now even I can understand that thank you very much for your explanation :lol:
 

Texasmark

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My uneducated figuring, after looking at several blades made for a particular HP range and gearbox size (meaning they will also clear the AV plate and trim tab also), is that the blade area remains somewhat constant in a given manufacturer's corral but as Pitch increases, and usually Rake, the blades are laid back causing the diameter to drop. Surely there are some tutorials on the www about prop design.

When I was a teen I used to fish along the Houston Ship Channel. Ships would come in high and dry, almost with their propeller shaft visible. Props had a huge diameter, and little pitch going plop, plop, plop, as each blade hit the water, low 100's of rpms capable of a top speed of maybe 10 knots, pushing 10k tons sort of thing.

Flip side were the little racing hydroplanes (8-10') running engines like 7 ½ hp, driven by "Quicksilver" direct drive lower units and 4" dia. props with pitch on the order of 20"........daddy-son projects...daddy did the boat and boy-son drove it. They had a spring loaded squeeze throttle and when out in open water, the driver would squeeze the throttle and shoot a rooster tail with little boat movement, relax the throttle and immediately squeeze it again, gaining some momentum and getting better bite, etc etc till he got up enough speed for the thrust of the prop to get the boat moving. Engines were screaming, no idea as to rpms.....I didn't know any of that technical stuff....I was a teen.....I had other things on my mind. 😎
 

dingbat

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My uneducated figuring, after looking at several blades made for a particular HP range and gearbox size (meaning they will also clear the AV plate and trim tab also), is that the blade area remains somewhat constant in a given manufacturer's corral but as Pitch increases, and usually Rake, the blades are laid back causing the diameter to drop. Surely there are some tutorials on the www about prop design.
Right here....
http://web.mit.edu/2.016/www/handouts/2005Reading10.pdf

The diameter of a prop is key to producing a prop thrust and torque.
Thrust Coef.: Kt = T/( ρ N2 D4 )
Torque Coef .: Kq = Q/( ρ N2 D5 )

Diameter D Overall diameter of the propeller Rotation rate
N Rotational speed of the propeller in rev/sec
Density
ρ Fluid density
Thrust
T Propeller axial thrust force
Torque Q Propeller shaft torque
 

Texasmark

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Right here....
http://web.mit.edu/2.016/www/handouts/2005Reading10.pdf

The diameter of a prop is key to producing a prop thrust and torque.
Thrust Coef.: Kt = T/( ρ N2 D4 )
Torque Coef .: Kq = Q/( ρ N2 D5 )

Diameter D Overall diameter of the propeller Rotation rate
N Rotational speed of the propeller in rev/sec
Density
ρ Fluid density
Thrust
T Propeller axial thrust force
Torque Q Propeller shaft torque

Got it bookmarked. Will refer to it from time to time. Noted the effeciency curve as a function of blade count. Nice paper. Expected to see triple integration to the nth degree with if being from MIT, but well presented.
 

Jakon0Kemb

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Nov 25, 2019
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[FONT=Arial, Helvetica, sans-serif]The diameter of a prop decides how much water you will disturb while the pitch determines how quickly you push the water displaced out of the way. A prop's pitch is similar to an airplane wing as well, with high and low pressure sides. So a prop with a large diameter prop with no pitch would not displace any water until pitch is introduced. The boat moves because of a combination of the differing pressures and Newton's Law that every motion has an equal and opposite reaction.[/FONT]

[FONT=Arial, Helvetica, sans-serif]So if you have a barge or a sailboat (neither are going to get on plane), then you want the biggest diameter you can swing with the flattest pitch. This allows the prop to take advantage of the torque the engine develops. For these types of boats the important thing is to have as much power (thrust) available for use, rather than trying to develop speed[/FONT].

[FONT=Arial, Helvetica, sans-serif]If you have too big of a prop you won't reach maximum RPMS. This relationship is the same at whatever RPM you are operating at, but is easiest to see at full RPM where the torque, horsepower and fuel consumption efficiency curves cross and your engine is working harder than it should to reach any RPM. It is inefficient and in the long run (very long run) can damage your engine. You'll have a terrific hole shot, but won't be able to run at full RPM-- like driving a car uphill in fourth gear when you should shift down to third.[/FONT]

[FONT=Arial, Helvetica, sans-serif]If you have too small of a prop it means that you can go over your maximum RPMS and that your engine over-revs to get to a specific speed. It also means that you may have trouble getting out of the hole or it may take forever to get up to plane and you won't be able to go very quickly as you will have the prop turning so quickly that cavatation develops. Cavatation destroys the wing lifting component. (You can actually get the water boiling on the low pressure side.) It is like driving down a hill in third gear when you should be shifting up to fourth gear.[/FONT]
 

Texasmark

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Nice analogy.

As a Junior in HS, I spent the last 2 summers as a deck hand first year, cook second year....because the cook didn't show up and I knew how to cook.......much cleaner and better hours over decking. The boat had a bent blade on the prop when I boarded the first time and they were on the way to a dry dock.

On the dock, and everything setup, dock drained and all, we were allowed to disembark where upon I was awed and amazed at just what you said. Boat was built to push barges (several at a time weighing several thousand tons) at a few knots. Prop was like 12' in diameter and I bet the pitch wasn't but a couple of feet. Engine ran at 400 rpm (6 cylinder diesel with cylinders in which a small person could stand.) ....don't know if or how much of a reduction gear it had. We stopped for fuel one day on a bayou in Louisiana, and they took on 1000 gallons of diesel......at 9 cents a gallon.

Application of your last paragraph I also experienced as a teen. Had a friend whose dad was into racing so we went to the boat races from time to time. I noticed the little hydroplanes with Merc. Quicksilver direct drive lower units were running extremely small diameter, very high pitched props.....at a screaming RPM.....once hooked up.

To get on plane from dead in the water, the kid would smack the throttle and get all he could get out of it....not much on first try. He'd release the spring loaded throttle and smack it again gaining velocity from what he had achieved on the first go at it. After maybe half a dozen of those cycles, the boat was going fast enough for the prop to get a good "solid" bite and the boat would take off.
 
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