# Boat hum



## Guest (Sep 18, 2009)

one of my students asked a stellar question regarding boat hum...
why forces of drag vs. positive effect... keel issue
skiff issues... say a 420 or Laser

some say a boat is singin' & in perfect sych when she hums 


any thoughts?


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## smackdaddy (Aug 13, 2008)

Actually, that is a great question. I've heard people talk a lot about it - but know the physics behind it.


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## St Anna (Mar 15, 2003)

I can vouch for the hum. Our Peterson sings when when going to windward (probably in at least 20kn apparent) and the sails are well trimmed. 

The Peterson designs are a work of art so your question is valid. It has been reported that the 46 with a wider stern has some weather helm in comarison to the 44, but with headsail and main traveller trim, I havent found to much weather helm. Weather helm characteristics, (CE vs CLR & mast rake etc etc) may be worth looking at for the design causatives of 'hum'.

Its a beautiful sound - its just like she is communicating & having fun and its implanted in my memory.


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## imagine2frolic (Aug 7, 2008)

I was on a test sail with Imagine. We were sailing from the Atlantic between St. Maarten, and St. Barts. The spinnaker was up, and as we got on the crest of the wave we got a good gust. We were doing 14 knots, and the boat seemed to dig in, and accelerate quickly. Just as we were peaking at 18 knots there was a small hum. I was in love, and my knees were weak. I was standing there thinking, and SHE'S MINE!........*i2f*


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## saildork (Feb 20, 2007)

imagine2frolic said:


> I was on a test sail with Imagine. We were sailing from the Atlantic between St. Maarten, and St. Barts. The spinnaker was up, and as we got on the crest of the wave we got a good gust. We were doing 14 knots, and the boat seemed to dig in, and accelerate quickly. Just as we were peaking at 18 knots there was a small hum. I was in love, and my knees were weak. I was standing there thinking, and SHE'S MINE!........*i2f*


AHHHHHHHHHHhhhhhhhhh!! That special moment when man and boat are as one.


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## zz4gta (Aug 15, 2007)

I was under the impression that the hum was caused by imperfections in the bottom and or foils. Fair the bottom, and sand to 600 grit, hum should go away.


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## philsboat (Oct 16, 2006)

Boats hum cuz they don't know the words....hehe

Phil


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## US27inKS (Feb 6, 2005)

When I was working on my friends keel I ran across a great video showing the cause of keel hum. Unfortunately I can't find it now. Basically it has to do with the width of the rear edge of the keel or rudder. As the foil moves through the water, an eddy forms on the rear edge of the foil. This eddy swirls first from one side then the other. As boat speed increases the eddy switches sides faster and faster until a harmonic vibration is set up in the foil.

The eddy on the back edge of the foil creates drag, and can be eliminated by reducing the trailing edge to a point. This is impractical at best, so reducing it as much as possible is the goal without sacrificing the ability to withstand a small impact. Besides, getting a pointed edge straight is a *****.


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## Crapaud (Jul 31, 2009)

In my experience, the hum is a function of speed and exciting the natural frequency of the rudder, keel or combination of the two. My "Fiz-ix" knowledge dealing with fluid flow is rusty at best, but to add to John's comments it is a vibration brought on by flow inconsistencies.

One of my thoughts has been - could this vibration be a result of flow separation (or eddies) coming off the keel that the rudder then contacts and starts the process? Almost like a flag flapping in the wind generated by a fan?

The nat frequency is determined by the object's geometry and stiffness. So, the frequency would be changed by changing the length of the rudder or the materials it is made from. Stopping the eddies completely would probably require NASA to design and build the foils to have no flow separations at the higher cruising speeds.


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## Guest (Sep 30, 2009)

sanding with grit! gee I'm hopin' he was being comedic...
I've sailed BRAND new high performance skiffs & if u took a piece of sandpaper within 20 yards of one the maintenance director would have your head on a platter, a seafood platter

thanks for the foil discussion right above, VERY interesting
& Chapman's discusses this a bit too

cheers
-JD


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## zz4gta (Aug 15, 2007)

stpetersburgsailor said:


> sanding with grit! gee I'm hopin' he was being comedic...
> I've sailed BRAND new high performance skiffs & if u took a piece of sandpaper within 20 yards ...
> -JD


No I wasn't.  How exactly do you think they get a mirror finish on racing boats? They wet sand the bottom to 800 or 1000 grit. Most only take it to 400-600.


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## US27inKS (Feb 6, 2005)

Crapaud said:


> In my experience, the hum is a function of speed and exciting the natural frequency of the rudder, keel or combination of the two. My "Fiz-ix" knowledge dealing with fluid flow is rusty at best, but to add to John's comments it is a vibration brought on by flow inconsistencies.
> 
> One of my thoughts has been - could this vibration be a result of flow separation (or eddies) coming off the keel that the rudder then contacts and starts the process? Almost like a flag flapping in the wind generated by a fan?
> 
> The nat frequency is determined by the object's geometry and stiffness. So, the frequency would be changed by changing the length of the rudder or the materials it is made from. Stopping the eddies completely would probably require NASA to design and build the foils to have no flow separations at the higher cruising speeds.


NASA does design a lot of foils, and the NACA (the predecessor to NASA) foil sections that most boats use were designed by the same smart guys. Just like a boat, every foil is a compromise. Lift and drag are big considerations, but so is flow separation at the expected top speed. No foil is going to be perfect for all conditions, so they pick the one that works best in the expected range of conditions. My boat uses an 0012 foil for the keel, and my friends Ranger uses a 63009 foil.

All of our keels would be much more efficient if we could only get the lead to change shape from one tack to the other.


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## Guest (Sep 30, 2009)

Gemini

this beauty designed by CCS has twin leeboards for just that John
great post, thanks I enjoyed reading it
'sqawk' to the bird' is he on board all the time w/u?


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## Waltthesalt (Sep 22, 2009)

My experience with the vibration has been a flow noise common with keels, daggerboards or centerboards that have flat instead of foil crossection. It's usually noticeable at higher speeds so I've seen it in racing dinghys and a fix there is to reshape the board or if that's not possible tightening up the fit in the trunk. Vibrations hurt boat speed. The March Practical Boat Owner ( a British sailing magazine) had an substantial article on the problem and how to streamline bilge plates to cure it. They attribute vibrations to rounded leading and trailing edges for the board or keel which causes unnatural flow which causes turbulence and hence vibration.


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## US27inKS (Feb 6, 2005)

stpetersburgsailor said:


> 'sqawk' to the bird' is he on board all the time w/u?


No he only makes it to the lake on rare occasions. Hard to believe, but he gets seasick if I take him out on the boat. At the clubhouse he's everyone's new best friend whether they want to or not. Very few parrots are too friendly, but he's one of them. He will latch onto someone's hand or shoulder and insist on being petted, then bite at me if I try to take him away from them. They aren't called "Psycho Chickens" for nothing.


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## imagine2frolic (Aug 7, 2008)

I am not going to get into changing the shape of my rudders, or keels. So I will just keep at 18 knots, and less.:laugher  .......*i2f*


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## Crapaud (Jul 31, 2009)

US27inKS said:


> My boat uses an 0012 foil for the keel, and my friends Ranger uses a 63009 foil.
> 
> All of our keels would be much more efficient if we could only get the lead to change shape from one tack to the other.


Interesting. . .

I am guessing the foil # has a meaning - do you know what the numerical code for the foil is?

Many moons ago, I was part of a research design team that looked at resonating frequencies to control eddies in order to increase fuel atomization. Before I left, another project had been submitted by NASA to reduce noise generated by airplane prop blades. The idea was to reduce the effects of the eddies coming off the leading blade so when contacted by the trailing blade the noise would be reduced. . .theory. One idea was to introduce trailing "streamers" that would break-down big swirls into little swirls to reduce impact on the trailing blade, but induce more drag.

I have only experienced the hum on dingys, racing cats and high performance monos - never a full keel. Is this a case of the full keel not being capable of reaching speeds high enough or the fact that the distance between the rudder an keel is nil or the flow is interrupted by a prop? Would a folding prop give different results? I have never been on a boat with a folding prop.


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## RichH (Jul 10, 2000)

Crapaud has it. 

The hum comes from the flex due to the viscous drag forces causing the thin plate centerboard to flex. If these forces excite the natural frequency of vibration (or a harmonic) of the keel/centerboard, then the board will vibrate quite excitedly. 

Scows with very thin bilge board hummmmmm easily and regularly when at certain high speeds, if you pull the board ever so slightly up (slightly changing the natural frequency of the board) the hummmmmmm will occur at a different speed. Ditto when a mast starts to pump at relatively high windspeeds. Ditto why your diesel engine 'develops the shakes' at a certain specific rpm. 

All solids will have a specific natural frequency of oscilation for their 'geometry' .... excite that item at its natural frequency and listen to the hummmmmm. 

The same is what makes a violin string vibrate when rubbed with a rosined bow.


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## St Anna (Mar 15, 2003)

Dear Rich,
I believe you are perfectly correct in what you are saying.

However, I am not talking about a vibrating harmonic frequency;
Therefore, your hum and mine are different. _Rushcutter (a KP46),_ hums on a beam reach and slightly forward of beam in 20-25kn of breeze and we are hooting along at a few kn greater than hull speed. No prop noise (folding) and the girl just sings. Cant explain or describe it.


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## US27inKS (Feb 6, 2005)

Crapaud said:


> Interesting. . .
> 
> I am guessing the foil # has a meaning - do you know what the numerical code for the foil is?


The foil numbers do have meaning, although I'm sure they mean more than I understand. Foils are broken down into 4, 5 and 6 digit foil numbers. The 4 digit foil numbers are very basic foils, I'm guessing developed first. Foils developed later have a "series" number that likely has something to do with a percentage or ratio, but I don't know what. The last part of the foil number is a ratio of chord length vs width. My keel has a 12% ratio, my friends ranger has a 9% ratio. Every foil profile has a slightly different lift to drag ratio, and will stall at a different angle of attack. My friends keel will have a slightly lower drag due to the finer entry, but will stall at a lower angle of attack than mine. The Ranger uses a rudder profile that stalls at a much higher angle of attack than the keel.


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## zz4gta (Aug 15, 2007)

RichH said:


> Crapaud has it.
> 
> The hum comes from the flex due to the viscous drag forces causing the thin plate centerboard to flex. If these forces excite the natural frequency of vibration (or a harmonic) of the keel/centerboard, then the board will vibrate quite excitedly.


I have a hard time imagining keels 'flexing'. Turbulant flow around or off the trailing edge of a foil yes. But having thousands of pounds of lead or cast iron 'flex', even under 15+ knots, I just don't see that happening.


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## RichH (Jul 10, 2000)

zz - the original poster implied/stated dinghies (usually with thin flat plate boards) such as 420s and lasers.

Nontheless, EVERY solid is by definition elastic to some degree and will vibrate when induced at its natural frequency or a harmonic of the natural frequency of the object. The 'flex' (amplitude of vibration) doesnt have to be all that great as the 'heard sound' may be further amplified by the adjacent shapes/geometries .... like the 'sound box' around which a guitar, etc. is built. It can also happen because the turbulance @ the edge/tip is in harmonic sync with the natural frequency of the object thats vibrating ... many many causes for a solid object to 'vibrate'/resonate, I only listed the principal and most common causes.


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## US27inKS (Feb 6, 2005)

zz4gta said:


> I have a hard time imagining keels 'flexing'. Turbulant flow around or off the trailing edge of a foil yes. But having thousands of pounds of lead or cast iron 'flex', even under 15+ knots, I just don't see that happening.


When I was in the Marine Corps, our Boeing rep told me a story about the company the developed the torque gauges for the H-46 helicopter. The torque is determined by measuring the twist in a 1.5 inch solid shaft over a span of about 8 inches. When the Boeing rep toured their facility there was a 10 inch thick piece of marble with sensors on it. They could measure the deflection in that marble when he set his coffee cup on it. Everything flexes. When the frequency of the flex matches the items natural resonance, the flex compounds itself, and causes a hum.


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## ckgreenman (Aug 22, 2008)

My limited knowledge of physics says that pretty much everything has a resonant frequency. When something passes over the surface of an object at a specific speed it will cause that object to resonate. The hum you're hearing is the result of that resonance.


Of course I'm probably WAAY off but at least it sounds good


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## artbyjody (Jan 4, 2008)

That hum can be reproduced just at the docks. It usually means some aspect of the rigging is tighter than the rest and as the result - the mast vibrates while being more tightly secured in some areas than the others. You'll never hear the mast vibrate it will be channeled through the tightest or loosest part of the rigging just depends on direction of pressure. Usually it will be the loosest but not always.

It also indicates that you are not dialed in and there is too much pressure exerted on the mast and all its components in one particular area. Has nothing to do with the keel unless you have a centerboard arrangement. The singing keel is different than singing rigging. 

In instance of example - my roller furler is sometimes a bit with slop - but where moored - the wind blows into the furler putting pressure on it. The running backs will start to hum unless I tighten them and thus tighten the mast and forestay... 


Humming / singing is an indication that you are not dialed in with your sailing plan and thus not getting the most efficiency out of it... IMHO...


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## zz4gta (Aug 15, 2007)

RichH said:


> zz - the original poster implied/stated dinghies (usually with thin flat plate boards) such as 420s and lasers.
> 
> Nontheless, EVERY solid is by definition elastic to some degree and will vibrate when induced at its natural frequency or a harmonic of the natural frequency of the object. The 'flex' (amplitude of vibration) doesnt have to be all that great as the 'heard sound' may be further amplified by the adjacent shapes/geometries .... like the 'sound box' around which a guitar, etc. is built. It can also happen because the turbulance @ the edge/tip is in harmonic sync with the natural frequency of the object thats vibrating ... many many causes for a solid object to 'vibrate'/resonate, I only listed the principal and most common causes.


I agree, everything does flex to some extent. However, I think that the finish on the foils itself and the trailing edges (non-laminar flow) will cause the hum before a "flex" or vibration hits it's natural harmonic frequency. Or am I looking at this completely wrong? Not trying to be a smart ass, just trying to learn.


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## US27inKS (Feb 6, 2005)

zz4gta said:


> I agree, everything does flex to some extent. However, I think that the finish on the foils itself and the trailing edges (non-laminar flow) will cause the hum before a "flex" or vibration hits it's natural harmonic frequency. Or am I looking at this completely wrong? Not trying to be a smart ass, just trying to learn.


The non-laminar flow is what is causing the flex. As the eddys exert a small amount of pressure on one side then the other, you get a small amount of flex either direction. As the frequency of the flex reaches the harmonic frequency, you get a hum. The water doesn't hum, just like the wind doesn't hum or whistle. The finish of the foil and the shape of the trailing edge (and the rest of the foil) will effect the laminar flow, and the resultant eddys.


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## Crapaud (Jul 31, 2009)

*Hum vs. Speed?*



zz4gta said:


> I agree, everything does flex to some extent. However, I think that the finish on the foils itself and the trailing edges (non-laminar flow) will cause the hum before a "flex" or vibration hits it's natural harmonic frequency.


If the turbulence occurs at the trailing edge, it should not cause vibration or hum - unless the turbulence wash from the keel is effecting the rudder. I would think at the higher speeds, the fluid flow is separating (Transitioning from laminar to turbulent) slightly behind the widest part of the foil. This would then effect the trailing & thinnest section of the foil - and noise (hum) could be generated.

I think you are correct in saying the surface finish is a factor, but it helps promote the vibration not the reason for it. Because of the shear stress the hull, keel and/or rudder generates while moving through the water - any disruptions on the surface finish will cause the flow to transition - resulting in the eddies and pressure variations that cause the vibration.

The word "flex" might be misleading - think of it more like the continuous shifting from tension to compression.

I do believe the vibration can and does promote fatigue in components (how much - is debatable), but does it effect speed? When I think of vibrations as mechanical energy lost into sound waves - the thought is that it could slow a boat down. Another line of thinking is it reducing the effects of friction - like the old football game with the figures moving on the vibrating game board. Any feeling one way or the other? Not that I an suggesting adding vibration generators to sail faster. . .if that be the case.

JP


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## artbyjody (Jan 4, 2008)

Crapaud said:


> If the turbulence occurs at the trailing edge, it should not cause vibration or hum - unless the turbulence wash from the keel is effecting the rudder. I would think at the higher speeds, the fluid flow is separating (Transitioning from laminar to turbulent) slightly behind the widest part of the foil. This would then effect the trailing & thinnest section of the foil - and noise (hum) could be generated.
> 
> I think you are correct in saying the surface finish is a factor, but it helps promote the vibration not the reason for it. Because of the shear stress the hull, keel and/or rudder generates while moving through the water - any disruptions on the surface finish will cause the flow to transition - resulting in the eddies and pressure variations that cause the vibration.
> 
> ...


Humming is always bad - it indicates part of your rigging is not in tune and too much slack... just that simple. You can't hum a guitar string if tuned correctly nor should your rigging hum unless you want to be the next sailboat musical sensation..


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## US27inKS (Feb 6, 2005)

artbyjody said:


> Humming is always bad - it indicates part of your rigging is not in tune and too much slack... just that simple. You can't hum a guitar string if tuned correctly nor should your rigging hum unless you want to be the next sailboat musical sensation..


I think we're talking mainly about keel or rudder hum at high speed. This is completely different than the rig humming.


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## artbyjody (Jan 4, 2008)

US27inKS said:


> I think we're talking mainly about keel or rudder hum at high speed. This is completely different than the rig humming.


Only if you have centerboard keel that you can lift.. rigging is something different..Cause and effect..depends on the boat..


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## fullkeel7 (Apr 16, 2008)

Not sure of all the science here, but I loved that 'hum' in my Laser days. I just thought of the hum as an 'earily warning system' that one or possibly two things were about to happen. 1: That the real fun had just started as the boat got up on plane or 2: That I was probably going to fall out of the boat again.  If the hum stopped, that was the indicator to start back to the launch area and go home.


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## grahammcg (Oct 23, 2009)

Lots of interesting theories.
The most likely cause is trailing edge harmonics where the 2 streams of water from each side of the keel or rudder 'hit' each other.
Solution is to chamfer (angle?) one side of the trailing edge so that 'collision'
doesn't occur. Works on my Hobie - & it goes fast! (not always with me on it!)


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## L124C (Oct 4, 2007)

When my Yankee 30 is at speed on anything from a beam to close hauled, she makes a loud eerie (electronic sounding) whine in the cabin. If you are astern, it sounds like it's emanating forward. If you are forward, it's coming from astern. It really seems like it's omnipresent! I sail with the transmission in reverse (to avoid drive train wear). If I put the gearbox in neutral the sound goes away, so I assume the resonance is in the prop. However, the omnipresent quality and intensity of the sound is almost unbelievable.


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## puddinlegs (Jul 5, 2006)

artbyjody said:


> Humming is always bad - it indicates part of your rigging is not in tune and too much slack... just that simple. You can't hum a guitar string if tuned correctly nor should your rigging hum unless you want to be the next sailboat musical sensation..


Not always true Jody... There's always different tension in shrouds depending on sailing load _(think leeward shrouds while going to weather... then of course, there's the slapping headfoil at the dock when the backstay is off that is stopped by wrapping a pole lift or the like around the aforementioned flapping bits)_. One boat I sail on regularly has a runner dependent rig. When we're going to weather in a breeze and the runners are loaded to the hilt, they hum. The boat well tuned and very fast. We occasionally get some hum from a loaded check stay on my boat in higher winds as well... same thing. Boat's humming right along! But as other's have said, we're talking underwater foils, not rigging.

Haven't seen you're boat out on the Sound since you've been back in the water. Hope to see you out this summer.


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## puddinlegs (Jul 5, 2006)

US27inKS said:


> The eddy on the back edge of the foil creates drag, and can be eliminated by reducing the trailing edge to a point. This is impractical at best, so reducing it as much as possible is the goal without sacrificing the ability to withstand a small impact. Besides, getting a pointed edge straight is a *****.


Actually, though counter intuitive, a pointed trailing edge is slow. Go have a peek at any performance racing boat foil and you'll see that it will be squared, width about 1/4" inch or so, and perfectly faired in this manner from top to bottom the the trailing edge of both the foil and even the keel bulb.


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## Gust14882 (Nov 30, 2010)

I used to crew on a J-80 named "Uproar" because the keel hummed at speed. It started at around 8 or 10 knots, and increased in pitch the faster we went. Once the boat started humming, we ignored the knotmeter and trimmed to get the highest frequency. I'm pretty sure it was vibration of the fin keel itself that created the sound, but it was obviously caused by the water flow over the foil. If it had been the rudder, you could have clearly felt it through the tiller and that wasn't the case on this boat. Ah the sound of 15 knots running under spinnaker and a full moon ...heavenly!


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