Vibrating Rudder

[Don Maurer]

I launched my Tammie Norrie sailing dinghy this past weekend and noticed that there was a slight vibration in the rudder. It was not enough to have an effect on control, but it was noticeable. Actually, you could hear it more than feel it. I'm just wondering if anyone has experienced this and what can be done to stop it?

[Ian McColgin]

Vibrating when beating or only when running?

Also, some remarks on the rudder construction and design, like swinging (sailfish style), deep and narrow v broad and shallow, fit of the pintles and gudgeons, etc. would help focus the guesses.

[John E Hardiman]

Is the trailing edge cut off square? (It should be). Is the bladed ventlating? These are two very common problems with dinghy rudders.

[Ian McColgin]

Googled to the plans. Assuming you followed them still need a bit on the fit of the pintles and gudgeons. Can you shake it at rest?

And a bit on the exact shape of the rudder. Is the trailing edge finely tapered or half rounded or slightly square sectioned? The last should be least likely to chatter.

But mostly, going to weather does she have a little weatherhelm and still chatter or does a bit of lateral pressure sooth her troubled breast?

If the p&g fit is snug and if the trailing edge is not got a little flat to it, I'd suspect trailing edge vortecies.

[Ian McColgin]

I think what John means by ventilating is the space between the trailing edge of the keel and the leading edge of the rudder.

A different problem is cavitation where air from the surface is sucked down, usually on the weather side, but that usually does not cause chatter, only weathercocking and general loss of control.

[cs]

Just an obersavation. On my little sharpie my rudder used to shake a little. I took a plane and cut the taper on the trialing edge a little bit finner and the vibration stopped.

Sounds like this is completely opposite of what others are saying, all I know is it worked in my case.

Chad

[Don Maurer]

I followed the plans. The rudder is wide and relatvely shallow. The trailing edge is squared off, about 1/4" thick, although the edges are rounded slightly. No movement between the pintles and gudgeons when I shake it. There is some space between the skeg and the leading edge. Maybe 1/2 inch. It seemed to vibrate when beating. I did not notice it running, although we spent a lot more time beating than running. I didn't really notice much weather helm, but winds for the most part were light.

[Ian McColgin]

I bet John got it in one.

Try getting the rudder in closer.

I usually put an upward facing pintel at the wormshoe - bottom trailing edge of the keel. That way the gudgeon can be set back even with the leading edge of the rudder and the excess pintle that goes past the gudgeon can land in a hole running up just inside the rudder.

Top p&g will be above the water anyway but you might be able to let the gudgeon a bit into the transom with a bit of a slot below so that again the rudder is sitting nice and close.

Ed to add: If that upper p&g can't go into the top of the transom, put the pintle on the rudder still but set it back from the leading edge. Leave enough of a notch below so the gudgeon can clear and get in.

Never hurts of have the lower pintle longer so you can get that one started and then fudge in the upper. If the pintles are of even length, either shorten the upper or, letting the weight be entirely on the lower (no harm here) lower the upper gudgeon or raise the pintle a tad, whichever, so long as you engage the lower first.

The leading edge of the rudder needs to be radiused so it will turn, of course. Down on the skeg, if you can fair that back to make a hollow partial round, that will also close the gap nicely.

[ 08-09-2004, 01:52 PM: Message edited by: Ian McColgin ]

[John E Hardiman]

Was it a high frequency vibration (humming) or a low frequency (pulsing)? Humming is an indication of a high flow problem such as vortex shedding, while plusing can be indictive of large scale fluid motions in the hull/seaway. Also, the size of the rudder area and the location of the center of lift to the pivot axis can amplify normal flow fluctuations.

EDIT

Ian is being too kind, but he is right that if there is a significant gap (say the thickness of the rudder blade) between the rudder and the skeg/transom that flow through this space can unload the rudder. This is usually a low frequency vibration due to the size of the forces though.

[ 08-09-2004, 02:35 PM: Message edited by: John E Hardiman ]

[John E Hardiman]

Originally posted by cs:
I took a plane and cut the taper on the trialing edge a little bit finner and the vibration stopped.Sounds like the shape was too fat aft to begin with. My rule of thumb is that the enclosing angle between CL and the aft taper should be approximately 13 degrees and the trailing edge should be cut off square at 5-10% foil thickness. This has got to do with boundray layer energy growth considerations and trailing edge stall from vortex rollover.

[JimConlin]

Some multihulls develop gadawful high frequency hums at speed. A common fix is to shape the trailing with a bevel of maybe 60 degrees (Not square). I gather that a square trailing edge provokes an oscillation of the flow separating first on one side, then the other.
YMMV

[John E Hardiman]

Originally posted by JimConlin:
I gather that a square trailing edge provokes an oscillation of the flow separating first on one side, then the other.
The only reason that a square trailing edge will hum is you didn't do it right in the first place. ;) Putting in a single sided bevel destroys the symmetrical stall characteristics and a double sided bevel shows that the foil was clipped at the incorrect place. Also, the Rn between this hull and a racing cat is about an order of magnitude, so the foil would be thinner, meaning the trailing edge would need to be thinner. If it is a production foil that has the problem, I can most likely tell you why (has to due with the minimum thickness of the trailing edge obtainable from the most common production processes), and if it is a custom foil, they missed the critical separation point.

[Don Maurer]

This is definitely not a racing hull. It is based on an 18th century work boat. I would prefer not to do any non-reversible modifications unless I know they will work. It sounds so far like the separation between the skeg and the rudder is a likely culprit. My inclination is to come up with a coved spacer of some sort that I can tack to the trailing edge of the skeg to see if that helps.

[Smacksman]

Its quite handy to tell you how fast you're going.
Doh, ray, me is 3, 4, 5 knots; la and tee and you're into 'underwear changing' speeds.

Avoid major surgery until it drives you mad.

You don't have a propellor near by do you?

[Stiletto]

This post reminds me of once when sailing in a race on a 30' fin keeler, the skipper said "the tiller is feeling funny" we all started speculating as to why it was so, when out popped a dolphin astern of us. I'm sure the bugger was grinning.