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PostPosted: Sun Jul 10, 2005 5:02 am 
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I just bought a very slightly used 1996 Miracle 20 and the rudders hum louder than I can talk! The old plastic rudders on my H-16 did that and I fixed it by sharpening the rudders with a machete (it is easier to control than a pair of scissors). On my new boat the previous owner had trimmed the trailing edge of both the rudders and the daggerboards to a very rounded profile. I assume that this causes the hum. He trimmed one of the daggerboards so much that the core was exposed and it had started to delaminate. I fixed it with epoxy and so far the fix has held.

Question: Are my rudders also constructed with a core like the daggerboards? Do I need to be careful when reshaping the trailing edges so that I don't expose the core, or are they a solid material like the old H-16 plastic ones?


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PostPosted: Wed Jul 13, 2005 8:30 pm 
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Foam core, so build up before you file and sand the trailing edge to a nice smooth exit point. To get rid of the hum, no hollows (bumps on the trailing edge), no round edges. A squared off edge is easier to maintain than a sharp point, either will eliminate the singing foils.


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 Post subject: Rudder Hummmmmmm
PostPosted: Thu Jul 14, 2005 1:03 pm 
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Rudder Hummmmmmm

There is a sketch in the support area that shows the best trim shape for the trailing edge.

Image

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PostPosted: Fri Jul 15, 2005 3:56 pm 
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Now that I've reshaped the rudders they're quiet enough for me to hear the daggerboards humming. Same problem. Previous owner rounded the trailing edges. With the foam core I'm concerned about shaving them down too much to get the correct shape (he REALLY rounded them off). What do I use to build them up before reshaping them?


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PostPosted: Sat Jul 16, 2005 8:54 am 
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Location: 315 N. Hwy 79 Panama City Beach, FL 32413 850-235-2281
When the boards start singing to us we just take a sanding block and wet sandpaper out on the water with us, sail a bit, pull a board up and just a bit and then see if it helped, we do this until it is gone.

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Brad Stephens
www.sunjammers.com
Hobie Division 15 Chairman
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brad@sunjammers.com
850-235-2281
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 Post subject:
PostPosted: Mon Jul 25, 2005 10:10 am 
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The flat on the trailing edge is the key.


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 Post subject: Shaping Tool
PostPosted: Sun Jan 08, 2006 1:24 pm 
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Does Hobie still sell a rudder shaping tool? I bought one about 15 years ago. Two passes and the hum is gone.

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PostPosted: Mon Jan 09, 2006 6:46 am 
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The flat trailing edge is exactly right. If it is rounded, the fluid flowing over the rudders and daggers will not separate cleanly and will cause cavitation (the hum). If you want futher explaination let me know. Otherwise I won't bore everyone here with a fluid dynamics lecture.

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Nick

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 Post subject: Shaping tool.
PostPosted: Mon Jan 09, 2006 8:44 am 
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Rudder edge shaping tool? Nope. That really hacked the trailing edge anyway. You should use a file or sand paper on a block.

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 Post subject: Re: Shaping tool.
PostPosted: Mon Jan 09, 2006 11:16 am 
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mmiller wrote:
Rudder edge shaping tool? Nope. That really hacked the trailing edge anyway. You should use a file or sand paper on a block.
Au contraire mon ami. The shaping tool creates a smooth, consistent trailing edge. A number of friends tried scissors, knives, files, sand paper on a block, machete and guillotine with no improvement. They used the shaping tool and voila, no more hum.

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PostPosted: Mon Jan 09, 2006 11:44 am 
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I think Matt is referring to using the shaping tool on the fiberglass (racing / 20) rudders.

The shaping tool really only works well on solid plastic (ABS / Lexan / PCG) rudders.


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PostPosted: Mon Jan 09, 2006 12:09 pm 
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MBounds wrote:
I think Matt is referring to using the shaping tool on the fiberglass (racing / 20) rudders.

The shaping tool really only works well on solid plastic (ABS / Lexan / PCG) rudders.
The tool did a most excellent job on a number of EPO rudders.

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 Post subject: Shaping Tool
PostPosted: Mon Jan 09, 2006 5:47 pm 
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The tool (the original one) was better, but even that can chatter along the edge causing little divits and chips. Best done with the file or sanding block.

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 Post subject:
PostPosted: Wed Jan 11, 2006 10:42 am 
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Could you use a wood plane? Just make usre it's really sharp when you start each rudder.

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Nick

Current Boat
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Previous boats owned
'74 Pearson 30
'84 H16
'82 H18 Magnum
St. Louis, MO


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