Sharpie 14 centerboard motor...

[troyg54]

Has anybody ever had a problem where the centerboard makes a strange noise while going fast? My centerboard makes a rather annoying hum like an engine. I don't know what is causing it or how to stop it and am just wondering if anyone else has had the issue. I think it is vibrating back and forth but why? or how? i don't know.

[Rick]

Does you centerboard have a wire or rope used to lower and raise it? Those can hum (unless they know the words, then they sing <rimshot>).

I always think of humming centerboards as the sound of speed. Is it really loud and obnoxious?

[ks8]

Rudders can vibrate or *shimmy* also, as speed increases.

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If its really loud, it may be a thin board banging against a wider opening... but even with a snug fit, they can hum, which is far better than a crack. Fluid dynamics stuff. Heady. But if it is a loud clunking and not a *hum*, how loose is the board in the slot as it exits the hull?

I was sitting by the dock one day as the tide was coming in, and was mesmerized as I watched the turbulent eddies change sides and direction around a piling every 1.5 seconds on the trailing edge of the piling in respect to the current flowing past it. It was a perfect rhythm of change, oscillation. It is called a turbulent flow as the water would break away from flowing smoothly around the piling, as opposed to a laminar flow where the water would cleanly cling to and exit the surface of the material without creating a turbulent eddy. This oscillating turbulence is also what makes the mouthpiece of a flute generate a sound so that a flute can be something you play rather than throw. Sort of the the same thing may be going on at the trailing edge of the board. I have heard some tell me that a flat board with simple rounded edges can minimize it (probably makes a more chaotic turbulence disrupting a more regular oscillation, but also increases drag and reduces lift), while others say you must taper the trailing edge and fair it even more (which may only change the pitch or amplitude of the hum unless you can achieve a purer laminar flow that never breaks away from the surface in a turbulent eddy, but you still may get a resonant hum from the stiffness and length of the board reacting to hydrodynamic energies that match the resonance of the boards physical characteristics, ie, the lower end flexes from lift, the flex slightly reduces lift, and the materials natural flex then pulls it back again setting up a resonance that gets it buzzing back and forth, having nothing to do with the trailing edge turbulence. I think high aspect ratio, long and narrow, boards and rudders are most susceptible to this. By nature they may have the most flex, and they are the most efficient, thereby generating more energy that could result in more hum or resonant flexing.). Regarding the trailing edge phenomena... even if you get an oscillation free laminar flow, it may only occur in a very narrow speed range, depending on the thickness of the foil. Faster, and the flow breaks away again, closer to the thickest section of the board, and hums. It is the old play of compromises again in the foil shape, and the materials of the board itself. I'm not the one to give you formulas in this matter though! And technically, I am speculating, admittedly. I did experiment with airfoils years ago and saw some of these effects at work. I bet there may be some computer simulations available to demonstrate them, and a foil in water is the same as an airfoil, except in a different viscosity fluid. Foil Shape, material stifness, composite materials engineered to stop (or dampen) resonant energies at certain frequencies, speed of the flow over the surface, surface texture (smoothest may not be best?), water temp and salinity, angle of attack of the foil in relation to flow through the water ... for most it isn't worth the details if one parameter or more is not controllable as you sail your course.

When so many report that their boards do this, on so many popular designs and materials, you know it could be tricky to try and make it go away (just lower the sail!). As Rick said, *It is the sound of speed*.

GPS your speed and you may find that the hum is telling you when you've got things trimmed in a way that the boat performs best! It may be what you want to hear! I do not know enough to know if there is a concern that the board is so loose in the case that it is getting beat up at the exit slot. How does the paint look there?

If you search the web, you will find that many popular designs of small sailing dingies have humming CBs and/or rudders that *shimmy* at speed. Or cables humming on larger boat CBs... the cables that are used to raise heavy CBs. Same deal... fluid dynamics and resonant frequencies based on tension on the cable, etc., being pushed or pulled to one side and then the other by the same sort of eddies I watched at the piling in that current, except the piling was very wide diameter and the current only about 2 knots at the most. With smaller diameters and faster current, the frequency goes up, and as it is transmitted through the board.... hhuuummmmmmmm... unless I'm wrong in which case it is something else.

If it is *rattling and clunking*, that may be the hum turning into a clunk rattle because of lots of play at the hull exit slot. A thicker board with a snugger fit may reduce *rattle*, but it may still hum at speed... enjoying the ride through the water?

If the CB were made of solid steel, on that boat, you probably would not hear a hum anymore, but that doesn't mean that there would not be the same turbulent oscillation frequency happening if the same foil were used at the same speed. You would not hear it because the mass of the steel board would not as easily transmit that energy in an audible way. Thick steel is more resistant to resonant energy from a weak source than wood and glass, well, sort of... there are parameters. The wood/glass board transmits, maybe amplifies these low frequencies partially due to less mass and possibly a physical resonance of the materials. An acoustic guitar is much louder than an unamplified solid body electric guitar, playing the same notes. And with a steel CB on that boat, you'd lose some freeboard as well...

Experiment, but know that your boat is not some odd minority example of something wierd and unnatural. Keep inspecting its surface finish for stress cracks, for the sake of prudent seamanship, but plenty enough voices make it clear that it is fairly common. Enjoy the sailing!

My longwinded hot air post of the week.

[Frenchpete]

Hi Troy,

Just wondering how deep do you drop your board? At first, I tried experimenting with dropping the centerboard on my sharpie to almost vertical and noticed unwelcome 'clunking' sounds (the board moving around from all that pressure on it). I quickly realized that it wasn't necessary to drop it that deep. Now I don't go any deeper than the 'Jacques-point' (i.e. the recommended depth in the plans...the depth of the bite of the rudder). No more clunking.....or humming. This may not even apply to your problem but I thought I'd just throw it into the mix.

Richard

[troyg54]

The centerboard was down slightly lowere than normal. I'll try raising it a little today and see if it stops anything. If I pull on the raise/lower rope just right I can get the humming to stop. While going slow it is not present only at > 3.5 mph can it be heard.

[jacquesmm]

Yes, that happens all the time.
A centerboard is profiled for a given speed. In this case, you should not have vibrations under 5 knots. If you do, check your profile.
If you go faster, lift that board at high speeds.

[Evan_Gatehouse]

This sounds like classic Von Karman eddy vortexes being shed from the trailing edge of the board.

The solution is to make sure the trailing edge is not squared off at 90° to the board, but cut assymetrically at 30° so that one side is longer than the other.

Use a file or small plane to do this. All the keels and rudders at Farr had this detail noted on their plans.

[jacquesmm]

Thanks Evan. That is certainly fine tuning. We only show a trimmed trailing edge on larger boats and never assymetrical. It would be a nice subject for a "HowTo fine tune your sail boat appendages".

[ks8]

Von Karman... yes, that's it! Thanks. It was a long time ago! I love that idea of tweaking the trailing edge to break the symmetry. One of those simple and elegant solutions. I had dealt with mostly assymetric foils and never encountered the need for the slant treatment, so it never stayed put in my head if I had encountered it. Great info Evan! Yes... thanks!

Since my boards have no paint yet, I might as well do this.

I wish I had videotaped that piling I watched in the current. Excellent example in very slow motion...

All you fishing warriors... you've undoubtedly encountered line buzz and hum also when either wind or current gets the line buzzing. I never know whether to lower the tip to stop it, or let it keep buzzing, whether it will attract big fish or chase them away.

[Cracker Larry]

It attracts fish. Some fisherman actually pluck their tight line while bottom fishing to achieve the same thing. It does work.

As to the centerboard hum, I don't think I've ever sailed on a centerboarder that did not hum, at some speed or another. Even big cast iron cbs will hum, as do the cables.