Rudder Vibration

[RobertMuller]

I am getting a bad very loud vibration in the rudder.  There was some in the Fall Fling in Hampton, VA above about 10 kts boat speed but it is now much worse.  I'm fairly sure it's because we tightened the lower gudgeon bolts which were pretty loose.  I think the wiggle in the gudgeons kept the vibration from getting as bad and didn't transmit to the hull as much.  I put a chamfer on the port trailing edge and now it vibrates some from 4-5 its but is very bad from 8-10 kts and then it stops between 10 and 11.5 kts (I haven't been above 11.5 since I sanded it).  Should I keep sanding to get some more chamfer?  Suggestions?

Thanks!

[Lee Shuckerow]

Generally you can chamfer Out the vibration. Try to take a picture of your chamfered edge.

Have you looked at your keel and make sure that isn't a source of vibration?

[Peter Beardsley]

Here is what we did on 161 and then 222 to deal with keel and rudder hum.  Don't think we've published this before.  It'll probably inspire some replies about how we are idiots. 

Keel and rudder fairing are generally not permitted under the Class Rules except under limited circumstances and procedures.  Read Section 3 of the Class Rules VERY carefully on this, and when in doubt, I strongly advise consulting with a member of the Viper Tech Committee in advance of doing anything for them to bless any thoughts you have -- much easier to get advance permission before changing the boat and having it not be a Viper. 

We had pretty severe keel and rudder hum at various points.  My limited understanding of the physics involved is that the hum is caused by the water "releasing" off both sides of the foil at the same time, causing the water molecules to hit each other and create turbulence.  Some of this humming would occur at speeds as low as 5 knots and even happen upwind.  I had Tech Comm chair Dave Nickerson hop into 161 in between races at the 2012 Wickford Regatta to experience this for himself, where he confirmed that it was not normal and greenlighted the approach described below. 

We did some amateur research on efficient trailing edge shapes that minimized vibration/hum and came across the diagram attached hereto.  Only the shapes in # 4 and 8 have 0 relative amplitude vibration.  We didn't want to do # 4 since we thought that would make the trailing edge too fragile.  So we went with # 8.  To do this, we put some 120 grit sandpaper on a power sander and lightly went over the starboard trailing edge of the keel for no more than 60-90 seconds.  Then we wetsanded the area with 220, 600 and 1200.  That took the hum out of the keel, and theoretically allowed us to generate very slightly better lift on starboard tack as an unintentional benefit (at the expense of port tack performance), though honestly we've never noticed a major difference. 

To deal with mild rudder hum that would only appear at higher speeds, we did the same thing to the rudder, except this time, we did the port trailing edge of the rudder to balance out any effect created by doing the starboard side of the keel.  Smart?  Who knows - I'm sure someone will say it's not, but again, we haven't noticed a major difference.  What we have noticed is that we have zero hum from keel or rudder even when going 15+ knots, so if the hum bothers you or if you think the vibration must be leading to some sort of turbulence/loss of energy, consider this approach.

My understanding of one of the things that Tech Comm unofficially looks for when examining the trailing edge of the foils is that any tapering occur only in the aft most 3mm or so of the cord length, so don't go crazy with any of this - everything was done very lightly since it's easier to go back and sand a bit more if needed a couple days later than it is to rebuild the trailing edge.

[John Leyland]

Thanks Peter.  Although I haven't done it to 191 yet, I understand that another way is to use the the diagram of the shape to make a sanding block out of wood and then hand sand the trailing edge with it.

[Dave Nickerson]

There is a link to a pdf of the TC's keel measurement process in section 3.4 of the class rules.  Peter is right, the measurement process is designed to allow beveling and rounding to take place in the aft-most 3mm of the cord length of the keel.  What I see most often in this class is what Peter did. Have also had success putting a small 45 degree bevel on one side of the trailing edge. 

There are no specific guidelines on the trailing edge of the rudder in the rules but the same profiles should address the issue.  The foils should not vibrate.

[RobertMuller]

Thanks!

See photo attached, I have put a 3/16" drill bit on the edge for reference... Is this the factory width for the trailing edge?

I'm sure it's the rudder, we have a bad keel hum on the J22 and this is different from that, plus when I wiggle the rudder it goes away momentarily

You can see the slight, maybe 1/16" chamfer, that I made on the "left" (port) side in the photo, I was being really careful.  My son who sailed in F18's, and currently a Carkeek 40, was home for Christmas he has now sanded it 45º to 1/2 way across the trailing flat.

I haven't sailed it with his sanding job yet, he said and as the other posts show, it should be sanded at an angle all the way to the other side, but we thought we should try 1/2 way first.

Robert