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Old 22-03-2017, 05:33   #1
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Which is trailing edge of propeller?

I have a singing prop which people think can be quieted by filing a rounded bevel on the trailing edge, sometimes also called the suction side. To me these mean the side facing the rudder or stern the boat, do you agree? I ask because I think that is the side driving against the water so the bow side is kind of the trailing edge as far as the thrust is concerned.
Any advice?

Video and audio here:

Dan
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Old 22-03-2017, 06:06   #2
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Re: Which is trailing edge of propeller?

That does not sound like the propeller to me, it sounds like bearing/shaft noise, accentuated through harmonic vibration through that particular RPM range. Specifically, sounds like an issue with the cutlass bearing. That said, I don't know what a "singing propeller" sounds like as I've never heard one.

The trailing edge of a propeller is where the two blade faces meet. The "front" face is the low pressure side and the "rear" face is the high pressure side, at least with respect to functioning foils. As the propeller spins, it creates low pressure on the front face which "pulls" the water through the prop. So I believe the front face is the "suction" side.

I'd suggest before you have your prop modified that you simply try a different prop and see if the noise goes away. If it does, then your problem is not the prop.
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Old 22-03-2017, 06:20   #3
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Which is trailing edge of propeller?

I have not heard many, but that sure does sound exactly like a singing prop to me.
If you slowly stick your finger into a fan that is blowing air at you, your finger will first touch the trailing edge of the blade.
The leading edge of an airplane wing is the edge that hits the air first, the trailing edge is where the air slides off of the wing last if that helps visualize.
The suction or lifting surface of a wing is the top of the wing and the curved surface of a prop blade that is facing forward toward the bow. Look at a prop blade, it is shaped exactly like an aircraft wing, assuming a normal fixed pitch prop.
I have not filed a prop to cure singing, however I have always heard that is the cure. I think I might go ask a prop specialist even if on,y by phone before I broke out the file, although I don't think you could hurt anything unless you removed a massive amount of metal.
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Old 22-03-2017, 06:27   #4
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Re: Which is trailing edge of propeller?

Think of the individual propeller blades as wings or sails. The "leading edge" is the edge that encounters the flow (air or water) "first". On a sail that is, of course, the luff. The edge the flow departs across is the trailing edge (or leach) of course.

The following is abstracted from Lake Lanier Propellor:

Quote:
Singing
Some propellers in service produce a high-pitched noise, often referred to as Singing. This sound typically is a clear harmonic tone much like a humming or ringing wine glass.
More of an annoyance than anything harmful, the causes of singing are not completely understood. Many theories have been put forward to account for the phenomenon of Singing, but it appears to be affected by critical factors for which the theories make no allowance. For example a twin-screw vessel has one propeller that sings and the noise is eliminated just by switching position of propellers. Or a singing propeller is replaced by an identical spare Propellers which is found to be silent. Also the lower the number of blades on a prop the less chances of "singing".

Anti-singing Edge
The singing is a result of propeller diameter and R .P.M. , Boat speed and Trailing-edge thickness and shape or roundness. In most of the cases not much can be done on Diameter, Rpm's or speed, but we can modify the Edge Geometry. This has been the Strategy for all efforts to eliminate singing.
Most Propeller professionals (and others) are familiar with the Anti-singing Edge a Chamfering of the Trailing edge, typically on the Suction side. This shape avoids the creation of curving flow eddies by cleanly separating the water flow off the blade.
The following graphic shows the Chamfering that goes from 0.5 or 0.4 Radius to the tip of Suction Side.
However the adoption of unduly thin edges can result in erosion or fracture of the blade near the edges. Anti-sing Edge may be considered as a last resort to minimize the Singing of an existing Propeller.
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Old 22-03-2017, 07:07   #5
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Re: Which is trailing edge of propeller?

Good to know, but I think I'll need to re-caffeinate for this to "stick". Thanks!
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Old 22-03-2017, 09:38   #6
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Re: Which is trailing edge of propeller?

Singing props alright. I have this problem on both props on my sailing cat since it was hauled last year. And I have only this last weekend consulted a propeller machine shop on how to fix it.

It is caused by a thickening of the trailing edge usually from anti fowling. He said it was very common with Prop Speed treatments as the coating would sag to at least one trailing edge whilst drying. His answer was to file a bevel on the low pressure side of the trailing edge of the prop. This would be the front of the prop.

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Old 22-03-2017, 10:06   #7
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Re: Which is trailing edge of propeller?

my prop sang until i topped off my transmission fluid...
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Old 22-03-2017, 10:08   #8
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Re: Which is trailing edge of propeller?

Holly crap! You don't know that the trailing edge of a propeller is the edge of the blade opposite the direction of rotation. The other side is called the "leading edge".
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Old 22-03-2017, 11:47   #9
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Re: Which is trailing edge of propeller?

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Originally Posted by jmschmidt View Post
Holly crap! You don't know that the trailing edge of a propeller is the edge of the blade opposite the direction of rotation. The other side is called the "leading edge".


Likely the biggest rule we have here on the forum is what we call the be nice rule. We strongly do not like it when people are not nice to each other, its just impolite and serves no purpose.
They way you said that it could appear as if you were not being nice, I assume that was not your intent?
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Old 22-03-2017, 17:11   #10
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Re: Which is trailing edge of propeller?

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Originally Posted by danstanford View Post
I have a singing prop which people think can be quieted by filing a rounded bevel on the trailing edge, sometimes also called the suction side. To me these mean the side facing the rudder or stern the boat, do you agree? I ask because I think that is the side driving against the water so the bow side is kind of the trailing edge as far as the thrust is concerned.
Any advice?

Video and audio here:

Dan
Before doing anything I would use a stetescope and I would try and probe the transmission to see if the noise is stronger or different in or out of gear. I would then go to the strut bolts inside the boat and would probe the bolts to see where the noise is the loudest. Just a way to zoom in on the problem...
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Old 23-03-2017, 03:09   #11
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Re: Which is trailing edge of propeller?

Props are also balanced,on rc boats we regularly balance our own propeller and I was taught to never remove any material from the side or face that is the rear or the part that faces the rear of the boat..normally if something is rotating clockwise & most props are if you are standing behind the boat looking forward, but I would think the leading edge is just that the leading edge,but that's not where you file! The front large surface is....& I mount the prop between centers & remove material equally on both sides or at least keeping it balanced if one side is heavier then take more off of that side but keep in mind try to maintain the shape of the prop but if you don't really understand how it works and can't understand what the Leading Edge is then you don't need to be filing anything ... as someone else said, just change the prop but I seriously doubt that's what the noise is. As someone else stated the Cutlass bearing could be the culprit. Extra paint on the prop that sags down I just can't see making any noise that's a bearing or something in the transmission Maybe or again the Cutlass bearing. It's too easy to just change out prop to be sure
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Old 23-03-2017, 16:39   #12
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Re: Which is trailing edge of propeller?

Our prop was singing like that for a while. Won't do any harm, just avoid that rev range where possible.

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Old 26-03-2017, 07:13   #13
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Re: Which is trailing edge of propeller?

For Everybody, even the rude Dude, I spent a bunch of time ruling out things like the transmission and other vibrations because it is hard to believe this is the prop when you hear it.
The noise does not happen in neutral, in reverse, or in forward thru the same rev range when tied up to the dock. Fortunately the slow speed coasting in the marina and the cruising speeds are both outside the noisy rev range but I wanted to solve it if I could.

Obviously I can figure out understand leading edge from the perspective of which edge of the blade is but I am trying to understand the face which would be trailing/leading. I should have said "which face of the blade is the suction side" so I could figure out which side of the blade to file.

Thanks for all the help and suggestions,

Dan
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Old 26-03-2017, 08:06   #14
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Re: Which is trailing edge of propeller?

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Originally Posted by danstanford View Post
.... I should have said "which face of the blade is the suction side" so I could figure out which side of the blade to file.

Thanks for all the help and suggestions,

Dan
The side that "faces" the flow. If the blade were fixed in position, the side that a flow of water would encounter first.

The suction side is curved, like a wing, such that the fluid flow (water or air) must pass over the "top" of the wing/blade more quickly than it does over the "bottom". Per Bernoulli, as the speed of a fluid increases over a point, its pressure decreases. It is the pressure difference between the sides of the wing/blade that creates the "lift" (wing) or "pull" (blade although in the case of a prop, often referred to as "thrust"). The pressure drop on the "top" of the blade/wing is what is being referred to as "suction".

FWIW...
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Old 26-03-2017, 10:10   #15
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Re: Which is trailing edge of propeller?

The front of the prop is the side to file,or the front is also the Direction the boat is going so the bow of the boat is the front,& the part of the prop that you could call the face is also the front or the side of the prop that faces the bow.
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