Return to Hobie.com
Hobie Forums
It is currently Thu Mar 28, 2024 4:42 pm

All times are UTC - 8 hours [ DST ]




Post new topic Reply to topic  [ 11 posts ] 
Author Message
 Post subject: Rudder vibration
PostPosted: Fri Jun 22, 2012 5:36 am 
Offline
Site Rank - Captain

Joined: Sun Mar 18, 2012 8:27 am
Posts: 32
Location: Coastal NC
When I get up some decent speed, my rudder commonly gets a high-frequency vibration that audibly hums. Should this be a concern? What should I do about it?

_________________
Fair Winds and Following seas,
Scott

Image


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Fri Jun 22, 2012 6:55 am 
Offline
Site Rank - Old Salt

Joined: Sat Nov 06, 2010 8:10 am
Posts: 681
Location: Satellite Beach, FL
Image

I need to do this on mine too. Does anyone know what the best way to do this is? Just scissors or would a sharp knife work better? I'm worried I'll take too much off.

_________________
Zach


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Fri Jun 22, 2012 7:37 am 
Offline
Site Rank - Old Salt

Joined: Sat Jul 25, 2009 11:19 pm
Posts: 340
Location: San Diego
Also if both rudders are not in good alignment with respect to each other they will hum. You may need to turn one rudder in or out a few mm. I had a hum on my 18 that I corrected buy turning a rudder out a few mm.

_________________
ALLEY CAT 1984 RED LINE HOBIE 18 MAGNUM
Sail # 10505 or 277
Image Image


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Fri Jun 22, 2012 8:09 pm 
Offline
Site Rank - Old Salt

Joined: Fri Aug 18, 2006 1:11 pm
Posts: 313
Location: West Point, Utah
I think that the tuning guide calls for 1/4 inch of toe in on the rudders. I did not scrape my rudders at all. I used a belt sander. you have to square up and mark the center of the trailing edge first though so you can see how much material you have taken off the trailing edge. I think that the instructions also say to leave at least 1/10 of an inch thickness to the trailing edge and make the trailing edge nice and square. I also used a straight edge to check the surface for hollows. You just rock it along the surface from the leading edge to the trailing edge and look for space under the straight edge. What you are trying to do is take all the hollows out of the rudder surface so that you do not get cavitation. When you reach about 10 knots the water is passing over the rudder fast enough so that in the hollows it creates low pressure areas and the water will boil to create bubbles. This is a draggy situation and not good. Mine still hum at about 15 knots. They do not scream like they used to before the fix. If you are not a racer, there is no harm done and it lets you know that you are moving.


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Fri Jun 22, 2012 8:29 pm 
Offline
Site Rank - Old Salt

Joined: Fri Aug 18, 2006 1:11 pm
Posts: 313
Location: West Point, Utah
Went back and read some of my old posts and this is not cavitation, but "ventilation". Also bluff body vortices are probably involved here. Several years ago I did some calculations that determined that the Reynolds numbers (unitless number that compares speed and viscosity) that our rudders are seeing is high enough to be in a transitional regime between laminar flow and turbulent flow. This means that any roughness or protruding surface feature will trip the boundary layer and change to turbulent (high drag) flow. Cool stuff, just in case anyone cares.


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Sun Jun 24, 2012 1:27 pm 
Offline
Site Rank - Captain

Joined: Tue Jul 19, 2011 1:31 pm
Posts: 31
Location: culver city CA
I was so tired of the hum on my boat I replaced them with EPO2's


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Sun Jun 24, 2012 8:32 pm 
Offline
Site Rank - Old Salt

Joined: Sun Dec 23, 2007 1:20 pm
Posts: 493
Location: Clearwater, FL
I have the original lexan rudders and I enjoy it when my rudders hum. To me it means I am going at a decent clip and my boat is happy.

_________________
Tim
84 H16
82 H16
87 H14T
Tortola Sails: 115222
Blue Prism Sails: 88863
Clearwater, FL
Image


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Mon Jun 25, 2012 11:03 am 
Offline
Site Rank - Captain

Joined: Sat Apr 16, 2011 11:53 am
Posts: 99
Location: Florida Panhandle
My boat seems to be happiest when it's pitchpoled me through the rigging, yes that's when I can hear it giggling it's mast off!

_________________
1999 Hobie 20, Sail #1005
2001 Hobie 16, "Spirit of 76 sails" #18515. Sold
1981 Hobie 18, Dead!
Image


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Mon Jun 25, 2012 7:45 pm 
Offline
Site Rank - Old Salt

Joined: Sun Oct 03, 2004 8:45 pm
Posts: 648
Location: Saskatoon, Sk. Canada
The only time my 16 would hum was in a good wind going real fast on a reach, I loved the hum, it was like a speed detector, when it started to hum I know I was really flying. If it hummed all the time I would fix it.

_________________
06 getaway -- always remember, man with both feet in mouth have no leg to stand on.


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Tue Jun 26, 2012 6:58 am 
Offline
Site Rank - Deck Hand

Joined: Mon Jun 04, 2012 11:14 am
Posts: 8
Despite how it all might sound.. going fast, humming and all.. the main problem with "Hum" is that it takes energy to produce it, and it creates drag. In actuality.. you are not going "as fast" as you might.. without it. In aircraft, they call it flutter.. and it can literally remove control surfaces in flight (not so good). Shaving the plastic rudders makes a huge difference, but it's also important to shim the rudders snug in the castings, or to replace loose hinge pins, and/or worn or sloppy gudgeons. Any room for movement can accentuate the likelihood for vibration.

If there is any room for movement (in the complete rudder/tiller system).. the rudder(s) can act more like flags.. oscillating in the water.. fast enough to make the sound.

Rudder shims can be cut from any slick thin plastic (including gallon jugs). Use real stainless hardware instead of the older plastic nuts, so that slight pressure can be applied by tightening the rudder and tiller pivots. And replace any other worn or loose parts.

Jerry
H16 Lake Monroe Indiana


Top
 Profile  
 
 Post subject: Re: Rudder vibration
PostPosted: Wed Dec 23, 2015 10:17 am 
Offline
Site Rank - Deck Hand

Joined: Tue Dec 22, 2015 10:09 am
Posts: 1
Gentlemen and Ladies,
My Hobie Sports Cruiser ('94) came with Delrin rudders. At speeds of 10 - 11 knots the rudders almost consistently would start to hum.

At first I did not mind the humming because it was an indicator of speed, but after awhile it started to become bothersome, and probably impacted speed. I installed shims and spacers to reduce the play of the rudders but this did not reduce the humming.

I attributed the humming to low frequency side-to-side flexing of the rudder tips in the water at high velocities, but consulted the Hobie Forum for other opinions. Several forum contributors suggested shaving the the trailing edge to get rid of the humming. Although I was skeptical, it was worth a try before investing in EPO type rudders (which are less flexible). I used a sharp edged knife at right angles to the plane of the rudder and a file to bring the trailing edge to a fine taper.

I can report that it worked well, which appears to support the cavitation theory.

Thanks,

Hobie SC 21 Sail 156


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

All times are UTC - 8 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Jump to:  
© Hobie Cat Company. All rights reserved.
Powered by phpBB® Forum Software © phpBB Group