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Rudder shudder

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hillerji
1st Mate

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USA
37 Posts

Initially Posted - 10/14/2014 : 21:03:39

I've been noticing a bit of 'shudder' in my tiller and its stumping me as to just what might be causing it. I do have the newer rudder and I don't think the gudgeons are the cause. It's a pretty balanced helm but the shuddering has me concerned. Thoughts, input always appreciated.

J

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 10/14/2014 : 22:06:24

Too much "balance", perhaps... You might just be feeling the normal turbulence off the trailing edge of the keel, working against both sides of the forward part of the rudder that's intended to balance the pressure from weather helm. A little weather helm is good for performance and safety, and adds some pressure that might smooth things out at the tiller. Just a thought... How much is your mast raked (inches aft of the mast that the main halyard hangs at the boom)?

Edited by - Stinkpotter on 10/14/2014 22:10:14

Derek Crawford
Master Marine Consultant

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USA
3312 Posts

Response Posted - 10/15/2014 : 10:45:21

It could also be just "junk" on the rudder.

Edited by - on

hillerji
1st Mate

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USA
37 Posts

Response Posted - 10/15/2014 : 11:03:13

Dave/Derek,

Thanks,
I have an adjustable backstay so that will vary, as far as a junky rudder I've been able to keep it pretty clean as recently as a couple of weeks ago to install a new tiller. We'll see what happens at haul out

Edited by - on

WesAllen
Navigator

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USA
222 Posts

Response Posted - 10/15/2014 : 17:18:02

Mine also has a shudder and it is a balanced rudder. It is very minor under sail with the keel down but more noticeable with the keel up and the motor running. I figure it is from turbulence from the swing keel, but I'm not an engineer or anything close to it. I figure the ease of steering is worth the trade off.

Edited by - on

jerlim
Master Marine Consultant

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USA
1484 Posts

Response Posted - 10/15/2014 : 18:11:34

I replaced the gudgeon bushings and that took out the sloppiness...it was difficult to get them into the hd gidgeons, and even more difficult to get the pintles into the new bushings...but having done that, it's a whole new experience.

Edited by - on

iwillnotsubmit
1st Mate

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64 Posts

Response Posted - 10/16/2014 : 01:35:16

I also have a bit of 'shutter' in the rudder. I built a balanced rudder and just assumed the vibrations were from not having a perfect foil shape. It works just fine with a very light feel. I don't worry about it too much.

Edited by - on

hewebb
Admiral

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USA
761 Posts

Response Posted - 10/16/2014 : 03:45:25

Mind does it at speeds above 5 knots and I figured it was from the propeller disturbing the water. Sometimes I forget to put the transmission in reverse. Pintle bushings are new and tight.

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islander
Master Marine Consultant

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USA
3992 Posts

Response Posted - 10/16/2014 : 05:20:31

I too have a very slight pulsation/shutter. Sometimes I would look over the transom to see if there was something caught on the rudder but there never was so I just put it down as turbulence. I have the balanced rudder with the new pintle/gudgeons so thats not where it comes from. Seems more noticeable when motoring than sailing.

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 10/16/2014 : 06:26:50

Assuming my theory is right (a big assumption), I'm not sure your backstay adjuster will make the difference. Depending on how your lower shrouds are set, when tightened, it bends the mast a little, pulling the midsection of the mast forward to flatten the main, and tightens the forestay to flatten the headsail.

To move the CE aft by increasing the rake, you'll probably need to ease the forestay and then tune the lower shrouds (easing the forwards and tightening the afts) and backstay to tilt the whole mast aft and end up with the same function from your backstay adjuster. I think my SR/FK had about 5" of rake to produce a light weather helm.

My "theory" might be supported by those with balanced rudders who report more shudder under power--that will give a virtually neutral helm such that any turbulence will be magnified by the balance of their rudders. It's a kind of "shimmying" or "squirmy" feeling that I can recall with mine under power. If that fits the description, then some weather helm should make it feel right under sail. IF it doesn't sound like your case or you already have some weather helm, disregard...

Especially with a balanced rudder, weather helm is measured by the degrees off center for the tiller on a beat--you only want up to about 5 degrees in moderate wind, such that if you let go, the boat turns up.

Edited by - Stinkpotter on 10/16/2014 06:32:10

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 10/16/2014 : 08:05:56

I'd look at the keel, especially the trailing edge, to see if it is fair, or if it is creating turbulence. In Wes' case, as I recall, the leading edge of the swing keel is blunt when raised. That could create enough turbulence to be felt in the rudder. Also, if the rig is severely out of tune, I think it could cause a stream of turbulence to flow off the trailing edge of the keel.

It could also be caused by any significant imbalance, regardless of whether it is too much weather helm or too much lee helm, but I think it's more likely that it would be too much weather helm, because the helm would feel squirrely and uncontrollable if it had that much lee helm.

<u>Something</u> is applying a force to the rudder at a resonant frequency. Something is causing the rudder to vibrate. That represents wasted energy. If you're a serious racer, it could reduce boat speed. If you're a cruiser, it might be only a minor irritant. I suppose that force could be caused by an imperfection in the shape of the foil itself, or it could be caused by an external force, such as turbulence. Good luck in the search for the cause!

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 10/16/2014 : 08:19:43

One more thought... You can simulate weather helm and neutral helm while under power (sails down) by adjusting the outboard's direction with the steering damper set to hold it steady. When the outboard is pushing so the helm is completely neutral (no correction needed to hold course), do you get the shudder? Then if you turn the outboard very slightly to require some correction with the rudder to hold course (simulating weather helm), does the shudder diminish or disappear?

Edited by - on

ftworthsailor
Captain

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USA
279 Posts

Response Posted - 10/27/2014 : 14:24:33

The C-25 pintle inspection port sold by Catalina direct is only 3 1/4", so I decided to make it a little larger. I installed a 5" inpection port so that my BIG hands can fit to do whatever I need to the pintle nuts and bolts.

I did learn that if you intend to put in a 5 inch inspection port, you need to buy a 5 1/4 inch hole saw.... They are not cheap!! So, I had a little grinding to do with my drill grinder.....

Edited by - on

islander
Master Marine Consultant

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USA
3992 Posts

Response Posted - 10/27/2014 : 15:42:57

Funny that you wanted a bigger hole. I was looking to make it as small as possible. From looking at the thickness of the transom I figured the nuts were going to be only an inch or two inside once the hole was cut. The hole also only had to be as big/wide as the bolts on the gudgion. The bottom gudgion you can access from inside the boat.

Edited by - on

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