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Daggerboard humming....kinda fixed

mixmkr

Well-Known Member
So...busting along today on KY Lake...and the centerboards starts to sing. For some reason it was a little annoying....so I only pulled it up about 5 inches... and it stopped humming.

So...I've got the original '69 board, which admittedly was in sad shape when I got the boat. So I sanded and faired with West Systems back to what I thought the shape used to be. Basically taking on dings and fairing the edges to be consistent along the length. The bottom 2/3rds of the board I went ahead and gelcoated, because it required enough filling in some gouges, that the wood grain wasn't so pretty. But the top 1/3rd looks nice...all woody lo0king.

So...where do I start to fair a little more (or add, etc) to nix the hummers?

oh...the round, wooden board. I'll get pics later.
 
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fhhuber

Member
This kind of thing can be difficult to deal with if you aren't 100% sure of the cause.

In aviation its called flutter. An interaction of the airfoil that causes cyclic structural flexing, which can self-amplify eventually leading to structural failure. In aircraft it usually starts due to the hinge line or trailing edge if its not due to a lack of structural stiffness.

Aside from the noise... the board is vibrating against the daggerboard trunk and can eventually shatter the fiberglass just as if you were rapidly tapping it with a hammer.

ALWAYS when you have symptoms of flutter: SLOW DOWN. Get the flutter to cease. Don't repeat the conditions, since flutter can be extremely destructive.
Flutter is always related to speed.
In theory, going faster you could have the flutter cease due to leaving the speed range where it occurs. In aviation, the aircraft structure will fail before you go fast enough.

Factors such as this usually apply similarly to boats as to aircraft. Airfoil vs "waterfoil"... Water is just "thick air"

Applying the aircraft flutter sources to the daggerboard:

We'll start off assuming your daggerboard is stiff enough.

Are the corners at the trailing edge SHARP or rounded off?
Sharp, nearly square corners tend to resist flutter better than a well rounded trailing edge.
Intuitively you'd think the rounded trailing edge is better, but without a sharp line for the separation of flow from the trailing edge the separation "wanders" over the rounded edge which will cause the whole "flying surface" to see rapidly changing angle of attack.
That's usually one of the simplest things to try to reduce or delay the onset of flutter.

A well rounded leading edge will help resist the issue.

Its possible that in sanding and gelcoating you altered the "airfoil" shape in a manner that makes the board subject to the issue. Try light sanding the back 60% on just one side. The idea here is to cause the board to ALWAYS load slightly to one side.
Note that if altered cross section is the issue, picking the wrong side will make it worse before making it better.
 

mixmkr

Well-Known Member
I wasn't super critical in fairing the old rounded wood board back into condition. To the eye, it looks pretty good...and that's really all that I did...no intense measuring, etc. I think I'll revisit and make things more precise. I wasn't expecting it to hum along when originally fairing, but I'll work more on it. I do LOTS of fiberglassing and gelcoating...so I kinda whipped this one back into shape. I was just using the daggerboard trunk to be my guide, that it felt the same going in all the way down....even turning the board 180. I presume the trunk is fairly symmetrical as well. I get very light resistance pushing it in...but there is SOME.

It's a rounded board...all edges and symmetrical.

I found it interesting that pulling it up 5" stopped it and wondered what kind of clue that might provide.
 

L&VW

Well-Known Member
I've experienced "board flutter" in fiberglass (Hobie 16, 18, Tornado, catamarans), iron (Hunter "Venture" cruiser), mahogany (class "A" catamaran, four different Sunfish), plywood (Folbot sailing kayak).

Each time, I was zipping right along! :)

I think it's a "harmonic" that we'll just have to live with--or pull the board up a little to put a load on the board. ;)
 

Rudder

New Member
The wood boards hum, usually slight imperfections, warped. When going fast they really hum, even pulled up some and that was a sign you were really moving, everyone after a race would tell how their board was humming.

Easiest way is to replace with a new glass one. Foil shaped, longer, won't warp, you won't sideslip after tacking, better upwind speed.

Raced a long time and have not seen fiberglass trunk shatter on a sunfish.
 

oldpaint

Active Member
Back when my fish was new in 1969 the daggerboard fluttered at a certain speed. Your reconditioning efforts are not the culprit. If you haven't already, add an eyestrap to the top of the board and use it to fix a bungee that runs around the mast to help hold the board up. Also lining the inside top and bottom of the daggerboard trunk where the board hits the fiberglass might help to prevent damage. I used the soft half of velcro tape for this.
 

wjejr

Active Member
Having the daggerboard fit snugly in the trunk will help, and that is recommended in a lot of what I have read. If you are racing, check class rules as to what you can do. I do not race my Sunfish, and I used the "fuzzy" side of velcro tape, around the top and bottom daggerboard slots. That worked really well for me. An added benefit is that the board stays where you put it even without the use of the bungee retainer. If you go that route, you may need more or less tape depending on how loose the board is.

While the humming isn't particularly fast, assuming the board is reasonably fair, you won't do any damage. Boats just don't go fast enough. As a teenager my Laser hummed for hours at a time without the slightest problem.

You do not want an asymmetrical foil side to side.
 

andyatos

Well-Known Member
After dealing with the leaks on my old Sunfish, I finally got around to focusing on the daggerboard. It had an old, wooden one but it wasn't the original. And, like so many daggerboard set ups, it vibrated when I started to go faster.

Having windsurfed for decades on small gear all the way up to huge, Formula race gear with giant, expensive, pointer fins, I've understood for some time how much performance you can throw out the window with even a slightly, non perfect "under the water" foil. Because when you're going over 30 mph, just a tiny nick or imperfection on your fin is going to announce itself loud and clear.

So, I bought a slightly used but in great shape GRP Sunfish daggerboard. And as soon as I sailed with the new daggerboard, there was a dramatic performance improvement on all points of sail. But... I still had vibration, even at speeds that weren't that high. Which I suspected was simply a fit problem, now that I had eliminated the "foil imperfections" side of the vibration equation by virtue of now sailing with a new GRP foil.

So, I simply installed some soft/loops side of velcro in the daggerboard case. Two at the very top and two at the bottom so that the daggerboard was now held firmly in place. I then went for a sail. When it was pretty windy.

The result? Absolutely no vibration what so ever. I mean none. Zero. Even when I was hauling ass. Daggerboard almost all the way up, daggerboard all the way down. Didn't matter. No vibration.

- Andy
 
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mixmkr

Well-Known Member
Maybe I'll try the Velcro but I faired it to be semi snug. I migbt be able to add some. Will try better fairing too. You can see my homemade top, shockcord etc. And you can see it IS slightly different front to back. I use tbe "right" side in tbe pic as my leading edge. Seems best tbat way. But i may try flipping it as that side" looks" like it might be best aft
 

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L&VW

Well-Known Member
Maybe I'll try the Velcro but I faired it to be semi snug. I migbt be able to add some. Will try better fairing too. You can see my homemade top, shockcord etc. And you can see it IS slightly different front to back. I use tbe "right" side in tbe pic as my leading edge. Seems best tbat way. But i may try flipping it as that side" looks" like it might be best aft
I have the same board, and the left side wear is consistent with repeatedly hitting the bottom. :oops: (So the left side would be the correct trailing edge). The wood board and rudder on a Viking (Sunfish clone) have no rounded edges at all. :eek: (They're all angles). :oops:

I've bolted-on a steel angle "handle" to keep the board from warping. (Again). :confused:

As the season wears on, you can expect the board to swell—making it harder to move. :( And, as the season wears on, I use my paddle—through my rope handle—and pry against the splashboard to raise it the inch needed to pull it up fully.
 

mixmkr

Well-Known Member
Ha! When i got the boat, I had to flip it to pound the swollen board out! It had been in the water for over two years :-O
 

odegaard

cAPTN oDIE
So...busting along today on KY Lake...and the centerboards starts to sing. For some reason it was a little annoying....so I only pulled it up about 5 inches... and it stopped humming.

So...I've got the original '69 board, which admittedly was in sad shape when I got the boat. So I sanded and faired with West Systems back to what I thought the shape used to be. Basically taking on dings and fairing the edges to be consistent along the length. The bottom 2/3rds of the board I went ahead and gelcoated, because it required enough filling in some gouges, that the wood grain wasn't so pretty. But the top 1/3rd looks nice...all woody lo0king.

So...where do I start to fair a little more (or add, etc) to nix the hummers?

oh...the round, wooden board. I'll get pics later.
Back when we were racing with old woodies, we engineers at Pratt & Whitney did some serious research on the subject. We found that an elliptical leading edge on the brd really improved performance. This is better than a sharp LE cause the boat makes allot of leeway causing all kinds of angle of incidence on the LE. Also a sharp trailing edge historically causes hum. Good to blunt off the TE a bit and this is good for performance also cause the water thinks your brd's cord (width of brd) is wider. Go for it--jor bite the bullet and buy the newer f'glas brd.
 

Amfab

Member
I just bought this thing—just because I thought it was a neat thing to have.
Its this little inflatable rubber bag:


... and as I was looking at it in the store I thought, "yah know... maybe I can keep it handy on the Sunfish for those really lazy days when there isn't much wind I have the daggerboard pulled up a bit and I am making little forward motion and the daggerboard sometimes clunks a bit in the slot when the boat rocks from side to side.

Also, I wonder if it will fit in the slot low down so I can give it a couple of pumps to dampen any vibration if it is humming.

I'll try it out next time I am out in the boat.

-Andrew
 

danpal

Active Member
The only problem I see is it might be tough to pull up the daggerboard in case you get into shallow water. You'd have to release the pressure and then pull up on the board.
 

Amfab

Member
A few weeks ago I was down in Baja and one of the days was pretty light so I took the air wedge with me and it worked perfectly.
Because of the light winds, I had the daggerboard pulled up about a third of the way and the chop was causing the slight slop in the daggerboard to make it knock every few seconds as the boat rocked. With one hand I slid the wedge in, gave a quick couple of pumps on the squeeze bladder and the slop—and knock—was gone.
As I headed into the beach I grabbed the squeeze ball and could easily twist the air release valve to quickly release pressure with the same hand.
I pulled it right out and tossed it down into the footwell as I hit the beach. The little hose is short and rigid enough that it will not easily tangle with the tail of the mainsheet. It was easier and more effective than I thought.
-Andrew
 
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