Vibration in rudder

[Scott Dunsworth]

I have a vibration in the rudder when the boat is sailing fast, or fast for me. The you can feel the vibration all through the boat. The boat is Arch Davis, Penobscot 14. I have only had her out three times. Does anyone have any ideas? It is not something that I cant live with, but if there is a simple cause that I can fix I would like to.

[John B]

It's something I have heard of. The people I have met with this problem ( only 2)have all put it down to a too fine trailing edge( too sharp , I mean). The fix is to plane the trailing edge to a flat of about 5 mm.

[mmd]

If the rudder leading edge is open to the flow of water (as opposed to being completely skeg- or transom-hung), make sure that the leading edge is rounded and fair and symmetrical about the vertical axis. That and what John B. said, too, 'cept I usually like to have the trailing edge rounded to about a 1/4" - 3/8" diameter on a boat this size.

[Todd Bradshaw]

This is usually due to the fact that the sail is so beautifully cut and shaped that the hull is actually trying to leave the water and achieve "warp speed"...
Anybody want to buy a bridge?

I dug around in the book pile and couldn't find much solid information. It would seem that it would be caused by one of two things (or maybe a combination of both). One would be that it's moving - flexing, with the tip oscillating from side to side. Fittings that are slightly loose or sloppy, allowing it to wander a bit under pressure, could do the same thing and it just gets worse as the boat goes faster.

The other possibility is that it is continually stalling at high speed, re-starting and stalling again in rapid succession. A too-narrow trailing edge, as John mentioned, could contribute to this (the flow separates sooner and doesn't stay attached as far back on the blade), as could a too-narrow or unfair leading edge as mmd mentioned. It could also be that the blade is too flat or too thin (thicker, more curvy foils are harder to stall at dinghy sailing speeds).

The first thing I'd do is hang the rudder with the boat on the trailer or sawhorses and push sideways on the tip of the blade to see how much side-to-side play there is in it and how much pressure it takes to generate it. No wooden dinghy rudder is going to be perfectly stiff, but if you can easily move the tip an inch or two off center, it may be the source of much of your vibration and it might be worth trying to stiffen the blade itself and/or remove all slop from the gudgeons and pintles, kick-up system or what have you. On the other hand, if the tip seems pretty stiff and the mounting hardware tight, it's time to look at the foil shape.

[Scott Dunsworth]

I measured the leading and trailing edge and they are both about 1/2 inch rounded over. It is made of cherry. The rudder is tight in its gudgeons infact so tight it is hard to put in and take out. There is quite a bit of flex out at the tip. I have not put a finish on the rudder as of yet, its just got two coats of epoxy. Would adding fiberglass help. Also the plans called for a kickup and I made a solid one with plans to build the kickup one next winter. I didnt get the edges as tapered as designed and there may lie the problem. I built the rudder in a hast to get in the water. I am sure I am the only one thats ever done anything like that.

[Buddy Sharpton]

Had EXACTLY this problem on a Cape Dory 14 "whitehall" whose factory rudder was cut out of a piece of 5/8" mahogany ply, then the edges rounded over with a router bit all the way around. Read the same these guys above have said. Took a belt sanded and worked the trailing edges into a taper about 2 1/2" broad and finished to a 3/16" square trailing edge because it breaks the flow cleanly and doesn't set up eddies and swirls(the loading and unloading of these changes in pressure are what's causing the vibration.) Worked like a charm and I haven't "looked back" since.

[TomRobb]

Had the same effect from a flat plate centerboard on a formerly owned dinghy. Turbulence. A nicer foil shape may help.

[ahp]

Scott,

Your rudder is shedding what is called a "Prendl Vortex Sheet". Turbulance eddies alerternately roll off each side of the trailing edge and drive vibration of the rudder. Vibration frequency will depend on speed and the trailing edge radius. (Dear old Dr. Prendl assisted with the development of aircraft for the German Air Force in the 1930's.)

The cure is to taper the trailing edge rather than leaving a significant radius. How sharp? It depends on boat speed. For a Hobie Cat, pretty sharp, I know. For a slower boat like yours, Maybe 1/16 inch radius or larger.

I believe we have discussed this before.

Art

[Buddy Sharpton]

No point in a foil shape if you can't get a thickness of at least 7% or so to the chord, say 3/4" in a 10" chord rudder. So a rounded nose say over the first inch and a tapered trailing edge, say 2 1/2 is plenty and easy to do on flat plates. All the racing guru's swear a squared off flat trailing edge, DEFINETELY NOT ROUNDED, creates a clean break and less drag than a nicely radiused on. I just barely sand over the square edge on my varnished dinghy boards and rudders so the finish holds up better.

[gert]

maybe it's the same problem Chuck Yeager had?

[Scott Rosen]

Could it be something as untechnical as a warped rudder?

[ken mcclure]

We did have this conversation in recent memory, and one of the solutions I remember bieng proposed was to have holes in the rudder to provide a little disturbance in the flow of water past. Anyone else know about this?

[Tom Lathrop]

The only explanation that makes any sense is the one Art gave. It's the same thing that makes a tight rigging wire or line sing. People who have lifting swing keels can tell you all about that since they often make really loud "music". Although many suspect a lack of symetry caused by a distorted foil as the culprit, it's really just the opposite. The more symetrical and perfect the foil is, the more likely it is to sing. The laser daggerboard is a really good foil shape and it will sing right up through the seat of your pants. The solution is to do something to damp the oscillations. The problem is to find just where they originate and that is not always easy.

[paladin]

use your sander to make a shallow groove on each side of the blade at about half way back from the leading edge, epoxy in several layers of 2 or 3 inch wide carbon fiber tape and re fair the blade and then shape the blade as above.....The groove goes full length top to bottom of the rudder...it will add rigidity to the blade and reduce tendency to vibrate by changing it's natural resonance in the water.....

[J. Dillon]

Ken might be on the right track. The Chinese have holes in their rudders for that reason.

JD

[Tom Lathrop]

Some of the Chinese junks did have holes in their large rudders but the reason is up for argument. Drilling holes might stop the vibration though. Stiffening the board with carbon fiber will not but it may change the frequency or speed at which it occurs.

I have a luff foil on the forestay of my sailboat to contain the luff of the headsails. When docked in a strong wind, the vortecies (sp?) mentioned above could shake the whole boat by whipping the foil back and forth. I wrap the foil a few times with one of the halyards and that breaks up the symetry enough to totally stop the problem.

And Jack, can you drive your boat fast enough with the yuloh to make the rudder vibrate?

[J. Dillon]

No way Tom. The best I could get out of it was 2.5 knots, and that with great effort. :rolleyes: My conclusion for my boat with steep dead rise and light weight was that the yuloh shook the boat too much reducing efficiency. :( I think they work great for the sampans of the Yangzee or like designs. :D Never saw one while there in 2000. :(

Did you ever build yours ?

JD

[Peter Jacobs]

Scott, your boat is singing to you ... she's happy sailing fast. My M*&^%$#y 17 does the same!

[Scott Dunsworth]

I took her out again today in 15mph winds and tried to pay more attention to the vibration. I can feel it in the tiller but after more investigation I found that it is actualy the centerboard that is the source of the problem. I should have noticed this before but I have enough to do just keeping her on her feet. Am I right in thinking some of these same problems that can cause a rudder to vibrate can also do the same to a centerboard? Sorry for not investigating more before poping off a question to you all. Thanks; Scott

[mmd]

Yup.

[ken mcclure]

But don't drill holes in it.

[Tom Lathrop]

Jack,

None of my present boats are really suitable for the yuloh. They are too light, transom is too high, kayaks or they are sailboats. Just finishing an 8' tender that I may try some experiments on that are based on the yuloh.

I was just kidding about drilling holes in the rudder or centerboard.

[ 04-17-2002, 09:02 AM: Message edited by: Tom Lathrop ]

[Buddy Sharpton]

Same same fix for centerboard. On my Cape Dory it is a 54 pound rectangular piece of 1/4" steel with "square" edges with considerable side to side play in its slot. It thrummed. I fixed it first. Ground a bull nose on the leading edge, and about a 1 1/4 tapered flat on the trailing edge, finishing about 3/32 flat. Worked for me.

[bainbridgeisland]

This is from a previous posting:

Actually there are 4 possible reasons for a hydrofoil to vibrate and make noise.

Flutter – When a hydrofoil is already vibrating, it induces the surrounding flow to vibrate as well. The vibrating flow can either increase or dampen the vibration of the hydrofoil. This phenomenon can add to a rudder or centerboards problems.

Galloping – Owing to stall, a side force can amplify vibration of the hydrofoil until friction forces are balanced. For rudders and centerboards, galloping tends to be a transient problem.

Buffeting – This is turbulence induced drag. This one is actually somewhat common on boats. It means that turbulence from other parts of the boat cause the rudder or centerboard to vibrate. A good example would be a propeller in front of a rudder, or a propeller aperture in front of a rudder, unfaired deadwood in front of a centerboard and so forth.

Vortex induced oscillation – This is the big one. This phenomenon is usually the biggest contributor to unwanted vibration of hydrofoils. Von Karman, a famous fluid dynamicist, discovered the principle in the 1920s, or maybe 1930s. The common term is Karman vortex street.
So, what is a Karman vortex street? As velocity increases, symmetrical vortices at the tail of the hydrofoil become unstable. The vortices alternately grow and detach from opposite sides in a periodic manner. Since the shed vortices do not immediately dissipate energy, they create a meandering wake called a Karman vortex street. When the shedding frequencies of the vortices match the natural frequencies of the hydrofoil, then resonance will occur. In other words, noise and vibration problems will be accentuated.

Ok, but what is a vortex? We are talking about little whirling eddies here. In the case of the Karman vortex street, they spin opposite directions from either side of the hydrofoil. This is what causes the wake to meander.
The vortex shedding frequency is directly proportional to the size of the hydrofoil. The constant of proportionality is called the Strouhal number after the physicist who discovered the principle. For a cylinder, the vortices collapse on the cylinder, causing tremendous vibration. This is why your anchor line vibrates in high current. The Strouhal number for a cylinder equals frequency times diameter divided by velocity.

But a thin, flat, plate oriented parallel to the flow, operates somewhat differently. First of all, the vortices collapse in the wake, behind the plate. The Karman vortex street is not a result of flow separating from the body, like the cylinder, but is caused by an unstable wake. Vibration in this case is from impulses transmitted up the wake. A centerboard or rudder would lie somewhere between the flat plate and the cylinder. However, for most centerboards and rudders, the vortices collapse onto the hydrofoil.

What causes the flow instability that initiates a Karman vortex street? This is a bi-ig subject. Flow instability has the most far-reaching significance of any phenomenon in fluid dynamics. Trying to simplify here: speed, angle of attack, elastic stiffness of the hydrofoil and vibration of the foil are common causes of flow instability. Unfairness of the foil, or non-symmetry in just the wrong place is also a common cause. Even the finest, smoothest, most symmetrical foils you have ever seen can have this problem at times.

How do I stop the noise? If the cause is a Karman vortex street, the answer is simple. Plane a flat on the back of the foil. The flat creates a tiny amount of suction. The suction stabilizes the flow just ahead of the trailing edge. Also, the sharp corners on each side of the hydrofoil that were created when you planed the flat, tend to nail down the initiation location of the vortices. This way they can’t move forward and aft so easily, further reducing the chance of vibration.

If this doesn’t work and the cause of vibration is not a Karman vortex street, the flow instability is probably due to some large unfairness or design error in the foil. For example, a highly raked aft and highly tapered foil will stall easily. It could then develop galloping problems. Flutter could weigh in at this time. And then the vortex street develops and so forth.

[ 04-18-2002, 11:58 PM: Message edited by: bainbridgeisland ]

[Todd Bradshaw]

Yeah, like I said - the sail's so good the boat's trying to warp-out! Good stuff Bainbridge. The wire or rope pennant (if you happen to have one) that raises the board also sometimes leads down slightly below the hull and can make a pretty nasty vibration as it goes through the water. We had a trimaran that had such a system and you had to adjust the tension on the line just right to get it to stop

[TomRobb]

Perhaps because I knew no better, I kind of enjoyed the center board's hum smile.gif
Why worry about it?
Is the next Cup defense at risk?
For me it was an off/on speedometer. If she hummed we were moving past Full Impulse and about to go to Warp One :D
The steel c.b also was a good sonar transducer. High speed props sang from clear across the lake.

Very nice thesis, Bainbridge. Reminds me of how much I miss Chemist's seminars :(

[ 04-18-2002, 10:26 AM: Message edited by: TomRobb ]

[Dave Hadfield]

Michalak quotes Bolger on this topic here: http://homepages.apci.net/~michalak/1sep01.htm#LEEBOARD%20ISSUES