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.
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| Rudder vibration http://www.hobie.com/forums/viewtopic.php?f=14&t=43442 |
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| Author: | TarHeel 16 [ Fri Jun 22, 2012 5:36 am ] |
| Post subject: | Rudder vibration |
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? |
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| Author: | PurdueZach [ Fri Jun 22, 2012 6:55 am ] |
| Post subject: | Re: Rudder vibration |
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. |
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| Author: | jmecky [ Fri Jun 22, 2012 7:37 am ] |
| Post subject: | Re: Rudder vibration |
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. |
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| Author: | mdgann [ Fri Jun 22, 2012 8:09 pm ] |
| Post subject: | Re: Rudder vibration |
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. |
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| Author: | mdgann [ Fri Jun 22, 2012 8:29 pm ] |
| Post subject: | Re: Rudder vibration |
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. |
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| Author: | bryan b [ Sun Jun 24, 2012 1:27 pm ] |
| Post subject: | Re: Rudder vibration |
I was so tired of the hum on my boat I replaced them with EPO2's |
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| Author: | Tim H16 [ Sun Jun 24, 2012 8:32 pm ] |
| Post subject: | Re: Rudder vibration |
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. |
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| Author: | tmason [ Mon Jun 25, 2012 11:03 am ] |
| Post subject: | Re: Rudder vibration |
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! |
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| Author: | Roy [ Mon Jun 25, 2012 7:45 pm ] |
| Post subject: | Re: Rudder vibration |
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. |
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| Author: | IndyJones [ Tue Jun 26, 2012 6:58 am ] |
| Post subject: | Re: Rudder vibration |
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 |
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| Author: | PSZacarias [ Wed Dec 23, 2015 10:17 am ] |
| Post subject: | Re: Rudder vibration |
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 |
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