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Thread: Lee board flutter/vibration

  1. #1
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    Default Lee board flutter/vibration

    My son and I have completed our first two sails with our new SD Expedition High Spec 35 and Expedition Lee Board (with full lee board thwart) and rudder. Our first sail was very light winds, but we did sail. Our second sail had some excitement good wind and some gusts that got us really moving. Our next sail will include a capsize test, and then we will be ready for a larger body of water.

    During our last sail, in a couple instances when we picked up speed, there was a noticeable flutter and vibration coming from the lee board. The vibration was felt through the whole boat. When I angled the lee board back, the flutter stopped. The lee board thwart was secure, and the bolt that attaches the lee board to the thwart was tight.

    Is this normal? Any comments on what could be causing this, and how to eliminate the 'flutter/vibration'

  2. #2
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    As with paddle blades a thick or a square blunted edge can give rise to flutter, so can an uneven profile. The water leaving the edge is uneven on both sides and gets confused. So if angling back solved the problem, maybe it's due to a squared off edge somewhere along the lee board or an uneven profile.
    Last edited by TGB; 11th-June-2012 at 02:41 PM.
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  3. #3
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    I have made and used many boards over the years and sometimes i have had a board that does this. I have never got to the bottom of this and usually it only happens occassionaly so i have put up with it. I have heard that the trailing edge is best kept square with sharp corners. I tend to make them like this and then sand off the sharp edge so as to avoid splinters. If it continues to bother you you could try planing the trailing edge square again and see if that helps.
    We finish the board with Danish oil as it is easy to apply and keep a finish on. This tends to leave the open grain showing on the Ash board, and maybe if you apply more oil, the grain will fill. Rubbing the finish down occasionally with wet and dry paper with a bit of oil on will fill the grain and build the surface up to give a smoother surface which should also help.
    The board needs to be vertical when going close to the wind upwind, but it is normal to angle the board back at perhaps 45 degrees when you come off the wind or run downwind.
    If you do solve the flutter i would pleased to hear from you.

  4. #4
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    My dinghy centreboard hums when it gets to 5.2 mph (GPS reading at slack tide).

    Last week, I sailed a wayfarer, which I think had a slightly smaller board. This one started singing at 5.4mph.

    I read somewhere that you can stop it doing it by either flattening the rear edge with a wetstone, or chamfering it at an angle.

    I heard it was caused by the water sort of colliding as it rushes back together as the board passes.

    Regardless what everyone else thought, Ive always thought it was caused by cavitation.

    However, Ive just watched a vid of cavitation. This board starts to hum at 1.04 minutes.

    The board doesnt start cavitating until much later - unless its usually caused by the boards tip.

    If the lee board stops if you tip it back, I would hazzard a guess that, rather than stopping the vibration, by widening the presented board to the water, you have moved the speed at which it resonates.

    This would indicate to that the vibration and noise is not cavitation, but is directly linked to width/length/speed/board density (density would change resonance)

    Looks like I was wrong.

  5. #5
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    In Holland the traditional wooden fisherman vessels are succesfully equipped with boards since centuries.
    In addition to substantially weight, the boards are profiled.



    The outside is "hollow" with the deepest part approx. one-third of the width (measured from the front) while the in(boat-)side is "rounded" .
    By this profile a "wing" is created like the wing of an airoplane; in addition the angle of the board should be 5 to 7 off the midline of the boat.
    This configuration, together with the correct (length-)position of the board t.o.v. the mast should give maximum effort and flutterfree working of the leeboard when in action.
    Last edited by Scandibus; 11th-June-2012 at 11:58 PM.
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  6. #6
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    Quote Originally Posted by Scandibus View Post
    In Holland the traditional wooden fisherman vessels are succesfully equipped with boards since centuries.
    In addition to substantially weight, the boards are profiled.
    I think I'm right in thinking that these fisherman boats had a leeboard on either side. If so, it makes sense to profile each leeboard like an aerofoil and change the angle of attack to help push the boat to windward. A single leeboard needs to be set neutral so that it works on both tacks.

    Tri-guy, the vibration you are experiencing is very common on Laser sailing dinghies. It is called Detached Laminar Flow. Basically the water can't decide which side of the board to "release from" first. As Dave has suggested, you could sharpen the trailing edge of the leeboard so that there is a pure separation point.

    I used to get this on my Enterprise until I replaced the centre board rubber closure seal. Somehow the flow of water was swirling around inside my centre board casing and this was disrupting the flow further down the centre board causing it to vibrate.

    See this image of the how a sharp trailing edge can reduce the turbulence, which should also reduce the vibration.


    Also a smooth surface will hold the water flow along the leeward side of the leeboard for longer (Called the transition point to turbulent flow) before it breaks away from the leeboard and this will generate smaller turbulence and again less vibration. Again as Dave suggests, have a close look at the surface of the leeboard. If it is rough, try to make it smoother with more oil to fill the grains.

    See image of laminar transition to turbulent flow.


    It is also worth checking if the vibration happens on both tacks or just one. If it is on both tacks, the trailing edge could be sharpened as described above, but if it happens on one tack only, it could suggest that the leeboard is slightly out of symmetry.

    I assume that the leeboard is parallel to the centre line? It shouldn't cause the vibration if it's not, but would allow you to tack better in one direction than another and results v speed would differ also so vibration may not be noticed unless you were sailing faster.

    Changing the angle of the leeboard will adjust how the water flows across the surface of the leeboard and this should stop any vibration, but you still want it fully down when close hauled, so little room to maneuver if it is vibrating when fully down.

    Unless you are racing though and need maximum efficiency, I wouldn't really worry about it. Think of it as a speedometer that lets you know you are going well!

  7. #7
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    I would agree with the "don't worry about it" approach - quite a few of the many boats that I have sailed over the years have hummed at certain, usually higher, speeds. I just take it as an indication that I'm going fast and enjoy it!

  8. #8
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    [QUOTE=Steamerpoint;409080Unless you are racing though and need maximum efficiency, I wouldn't really worry about it. Think of it as a speedometer that lets you know you are going well! [/QUOTE]

    i like this.

  9. #9
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    Quote Originally Posted by Steamerpoint View Post
    .... It is called Detached Laminar Flow. Basically the water can't decide which side of the board to "release from" first. As Dave has suggested, you could sharpen the trailing edge of the leeboard so that there is a pure separation point.

    See this image of the how a sharp trailing edge can reduce the turbulence, which should also reduce the vibration.


    Also a smooth surface will hold the water flow along the leeward side of the leeboard for longer (Called the transition point to turbulent flow) before it breaks away from the leeboard and this will generate smaller turbulence and again less vibration. Again as Dave suggests, have a close look at the surface of the leeboard. If it is rough, try to make it smoother with more oil to fill the grains.

    See image of laminar transition to turbulent flow.


    Unless you are racing though and need maximum efficiency, I wouldn't really worry about it. Think of it as a speedometer that lets you know you are going well!
    With Respect, may I suggest that sharpening the edges of a board is a serious no no.

    If you hit someone in the water with a loaded up board, it will break bones. If you sharpen the edges on a swap sides board, you have now got a sword instead of a club.

    You are also going to handle the board daily with soaked skin and if, like the wifes boyfriend you ever managed to let your boat run you over.....

    The laminar flow thinking makes sense to me, even if I can prove to myself its right. Tilting the board backwards would lengthen the chord without increasing the width, suggesting that a board of, something like, a 25 by 160 by 1000mm, instead of a 25 x 100 x 120mm would not have turbulance so early and have its resonance in a different frequency that maybe you couldnt hear or feel.

  10. #10
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    Whilst at Barton Turf, I browsed a fair few of Simon and Sheila's books - a great collection

    Re: foils - one text (perhaps early 1980s) talked of the problems of turbulence at trailing edge of of the board. The key point was that a long, shallow (sharp) trailing edge would be (rather counter-intuitively) really bad news!

    What was needed (according to that text) was a narrow, flat section of maybe 2-3 mm... squared off with 90 degree corners to allow the water to detach cleanly without creating turbulence.

    All made sense as I read it... but I'm afraid I can't recall the detail.

  11. #11
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    If you hit someone in the water with a loaded up board, it will break bones. If you sharpen the edges on a swap sides board, you have now got a sword instead of a club.
    I accept that a swap sides leeboard doesn't want sharpening, but I thought we were talking about a SD pivoting leeboard that stays on one side? If so making sure that the trailing edge of the board is thin enough to allow the flow of water to rejoin itself without too much turbulence is all we are talking here. Not turning the board into a razor sharp knife!

    Vibration indicates that unstable forces are at work here. It can only be down to a few things, so its a case of work it out through a process of elimination or live with it.

  12. #12
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    Quote Originally Posted by GregandGinaS View Post
    What was needed (according to that text) was a narrow, flat section of maybe 2-3 mm... squared off with 90 degree corners to allow the water to detach cleanly without creating turbulence.
    Greg this will work to a point as long as the width from the two sharp square edges are not too far appart otherwise negative pressure will form and drag/ more turbulence will take place. Like following a truck, you get all the buffeting of turbulence behind it.

    The problem with Laser dinghies rudder & centreboard vibration was due to very flat trailing edges which are a good quarter/ half an inch width. People would sand away some of the thickness of these trailing edges to stop the buzzing.

    My leeboard has buzzed a few times when I have been going quite fast, but it's not an issue. I just think ye-ha and wonder if I can get the canoe up onto the plane!!!

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    Quote Originally Posted by Steamerpoint View Post
    .... It can only be down to a few things, so its a case of work it out through a process of elimination or live with it.
    That "few" can very easily turn into phew! - in addition to the hydrodynamic considerations discussed above - are the structural flexure issues - and then the interactions......

    If it's going well enough - just enjoy the ride.

  14. #14

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    If it is a professionally manufactured board the finish and shape is probably adequate. assuming the problem persists, I'd want to know if the board is flexing, which seems unlikely with a hardwood board that looks to be a 10% thick foil profile, or vibrating all the way to the top. If the latter, some kind of profiled washer (to get a better fit) or a foam rubber pad (for damping) placed between the board and the thwart may help to eliminate the vibration. There may be interaction with the surface of the hull, perhaps a slightly different spacing may help.
    Terry Haines

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  15. #15
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    My Nova Craft Pal never had this "issue" (is it really an issue though?), my new Solway Dory Shearwater starts to buzz once I get above about 7 Knots, I like it though, it just enhances the sense of speed. When the boat starts to buzz I know I'm sailing quickly even if I don't have my GPS with me.

  16. #16
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    My leeboard never vibrates when sailing upwind. presumably because I don't go fast enough, or possibly due to sideways force on the board. Off the wind it only vibrates when fully down. I don't mind the vibration but it does remind me to angle the board back when off the wind.

    No problem.

    The rudder hums a little when I hit 8 knots.

  17. #17
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    Quote Originally Posted by GavinM View Post
    The rudder hums a little when I hit 8 knots.
    Yep mine does too!

    No really, I have never seen 8 knots in my canoe, I only wish that one day, just one day I will.
    Maybe with a 5 meter rig or a ruddy big outboard on the back!

  18. #18
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    I thought I would add a couple of comments (that I have been meaning to for a long time...) I did sail a few more times over last summer, and discovered that the lee board "flutter" only seems to occur at a certain speed. I found that when the speed of the boat increased, when sailing in windier conditions, the flutter disappeared.

    So, my thought is that there is some harmonic frequency phenomenon occurring.

    [On another note, I didn't get to do an extended canoe sailing trip this summer.... so I am hoping to do a week long trip this coming summer, and put my rig 'to the test' ... we are still in the middle of winter, here in Canada, but spring is on the way!]

  19. #19

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    The only time I experienced a noisy board was with a centerboard on a Sunfish, but the board was poorly made and a bad fit in the trunk. It may have been a replacement as it was a rented boat. Flutter started as soon as speed picked up and just got worse; I thought it was preventing the boat from achieving full speed.

    Based on that I wonder if the board is installed firmly enough?

    Some more stuff here - http://dir.groups.yahoo.com/group/pr.../message/21980
    Terry Haines

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  20. #20
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    I don't know diddley abouth the math or theory of trailing edges. I just knew that the Grumman leeboards I converted were very crude but made of good mahogany. So, I faired them down to have nice tapered entry edges and very fine trailing edges, beginning about 1/2 the width, going to a sharp deprture - no real flat. I just eyeballed the shape til it looked good. I think the reason mine don't flutter is because I applied ten coats of marine polyeurathane, resulting in a glass-like finish. Maybe some of the rumble is caused by water being roiled by a less than smooth surface.

  21. #21
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    My personal leeboard had an awful lot of epoxy layers, rubbed down to give a near perfect shape and finish and it can still vibrate. But only when i am sailing fast. The board is clamped to a very strong bracket. I have had others with just an oiled finish that didnt vibrate. Maybe a spec of dust is enough to start a vibration.

  22. #22

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    I'm wondering if the tendency to vibrate at speed has to do with the outline shape, i.e., tapered, elliptical, parallel etc.
    Terry Haines

    Boats are like rabbits: you can have one or many, but not two - A. Onassis

  23. #23
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    Quote Originally Posted by DaveS View Post
    My personal leeboard had an awful lot of epoxy layers, rubbed down to give a near perfect shape and finish and it can still vibrate. But only when i am sailing fast. The board is clamped to a very strong bracket. I have had others with just an oiled finish that didnt vibrate. Maybe a spec of dust is enough to start a vibration.
    Thus, my "theory" implodes. Maybe I just got lucky, or perhaps American waters are somehow different than those around the UK. Or, it could be that the flutter, rumble, and general cacaphony coming from the hull of my Grumman is covering the whole issue the way the Beatles were drowned out by those silly girls. Anyway, speed and a big wide grin will see anyone through this crisis.

  24. #24
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    Gentlemen,

    This is possibly unrelated because it happened with aircraft.
    At work we had a tail which was vibrating enough to be felt by the pilot.
    Everything mentioned above was examined.
    It was found that when the tail was built, the tooling was used wrong, and there was a small dip on one side. This had separate skins on top of a supporting substructure, so only one side was effected.
    The dip was filled in, the aircraft flight tested, the vibration was gone.
    Surface finish (smoothness/ fastener heads protruding/ paint scheme) were all found to have no effect.
    My conclusion is that a perfectly smooth finish is not important.
    However, the actual shape (not just smooth) is. Additionally, it was very important to know what was happening to the flow of air coming in to the tail. Given the money available to fix an aircraft there was extensive testing to see if anything in front of the tail was causing unusual flow. Pity we don't have the money and techniques to check what happens when moving fast in a boat. The fact that the vibration happens only at a specific speed probably means there is some interaction between the hull and board that depends upon specifics at the point of vibration

    I believe Outbacker is probably correct - "Anyway, speed and a big wide grin will see anyone through this crisis."

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