After keel reparation - rudder shivers

[accept]

Hello….I need some advise from all in here

After a groud sailing last summer I had the keel professionally repaired. Fin keel, rudder with a little fin as stabilizer. Boat 42 feet, 11 tons, excellent sailor.
The surveyor found no damage on the rudder.
Now - after reparation - the stern starts shiver over 6.5 knots speed. The warf have no idea, we have checked the bearings which looks good.
What in hell can this be, have the rudder been touched during the crash just so little we can't find it by eyes. Has it been turbulence from the new rebuilt keel?

I need some experience and ideas from you out there….

And if the rudder cause the stern shiver….can we just fix that turbulence in some way…??

Anders

[DaveC]

If it happens under sail, are you sure it's not the propeller rotating? If you're not sure, try putting the transmission in gear to lock the prop shaft to see if it's free-spinning and causing the shudder.

If under power then maybe the engine-prop shaft alignment got knocked out of whack?

Good luck with the problem. Maybe an uneven fairing of the keel or rudder from side to side?

-Dave

[accept]

Thank you for quick replay, Dave….The shiver is under only sail. Prop is 3 blade prop that foldes in. The shiver was not there before reparation so no change in the motor-prop installation.

[a64pilot]

My IP although a full keel is a spade rudder. It's an unusual spade as it does have a "lobster strap" attached to the bottom of it. Older IP's apparently exhibit rudder shudder just as you describe, you feel it worst in the wheel but can feel and hear it in the hull. I believe it's caused more than anything my on older IP's the keel is pretty thick and I think there is turbulent flow off of the keel, exciting the rudder.
Shimming the bottom of the rudder takes some of the play out of the rudder and greatly diminishes the shudder to where it's pretty much un-detectable.

Non laminar or turbulent flow off of your keel I feel can most certainly excite your rudder.
Look up aerodynamic flutter to see what I believe may be going on, yes our speeds are much lower, but water is much more dense.

I'd look at the keel, it's the only thing that changed, or possibly look to see if you rudder may have excessive play


Sent from my iPad using Cruisers Sailing Forum

[DeepFrz]

What did the yard do to "repair" your keel?

[FSMike]

Originally Posted by accept

Thank you for quick replay, Dave….The shiver is under only sail. Prop is 3 blade prop that foldes in. The shiver was not there before reparation so no change in the motor-prop installation.

Is the propellor still folding correctly? After sailing but before starting the engine stop the boat and get a brave person (Brrr!) to go over the side and check.

[Charlie]

Odd answer here but it is possible that the keel is now acting like a tuning fork at certain speeds. I raced on a few boats boat like that. We liked it because we knew when the speed was going fast. I would think that the problem could be solved by making a slight change to the shape of one side of the keel or the other.

[accept]

Hi again…and you are a big support here…

The yard took off all GRP from the Pb-keel and put new layers on, so in my eyes the keel look slimmer.
I talked to my insurance guy today sand he understood the problem - which is a relief - and they are ready to help me solve the problem.
First they want the yard to measure the keel to see if it is correct in angels…so I can see that a64pilot is in the right problem area. Most probably this is turbulent flow from the keel.

I have tried to understand what you mean with "shimming the bottom of the rudder"… please try to explain this "shimming", please…

I am certain that the prop will fold in as normal….

Thank you out there….

[Terra Nova]

How about some photos of the rudder and keel, before and after?

[w1651]

I had the same problem. When I hauled out last year I found at the base of my rudder was a bracket that holds a bushing in it. The bracket itself was wore out and ate up from electrolisis I am guessing. I made a new one and replaced the bushing with a bronze self lubricating and everything is fine now.
Good luck

[a64pilot]

Originally Posted by w1651

I had the same problem. When I hauled out last year I found at the base of my rudder was a bracket that holds a bushing in it. The bracket itself was wore out and ate up from electrolisis I am guessing. I made a new one and replaced the bushing with a bronze self lubricating and everything is fine now.
Good luck

This is what I meant by "shimming" the rudder, although IP sells a kit that the shim appears to be made from starboard or some similar HDPE, but the net effect is the same as above
See attached video link to help explain flutter, frequency of the flutter can vary greatly with the stiffness of what's fluttering, I believe you can add slapping halyard as an example.

https://www.youtube.com/watch?v=qpJBvQXQC2M

[Abgreenbank]

Does the trailing edge of your keel have a sharp symmetrical section? if so this can lead to vibration/flutter. There are many sections used but the french volume producers generally use an assymetric 30deg angle for the trailing edge.

Regards

Ab


Sent from my iPad using Cruisers Sailing Forum

[accept]

Thank you…
your advises I will take with me to the yard to let them them think of your experiences.
I learn a lot from this dialogue and thanks fo the film about fluttering, scaring!

I will take some photos next time I is in the yard….and try to show them here….not so used to the possibilities of this side yet….smile….

Anders from Sweden