Hobie Cat Forums

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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Fair Winds and Following seas,
Scott

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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-Zach
West Lafayette, IN (school)
Melbourne, FL (summer)

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

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.

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.

I was so tired of the hum on my boat I replaced them with EPO2's

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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Tim
82' H16
Sail # 88863
Panama City Beach, FL

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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"Spirit of 76" Hobie Sail #18515
2001 Hobie 16, Sail #105865
1981 Hobie 18, Project Boat

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

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