Hobie Cat Forums

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?

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.

Rudder Hummmmmmm

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

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Matt Miller
Director of Parts and Accessory Sales
Hobie Cat USA

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?

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.

Thanks,
Brad Stephens
www.sunjammers.com
Hobie Division 15 Chairman
Authorized Hobie/Vanguard/Hunter Dealer
brad@sunjammers.com
850-235-2281
Panama City Beach, FL

The flat on the trailing edge is the key.

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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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

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

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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Matt Miller
Director of Parts and Accessory Sales
Hobie Cat USA

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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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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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.

_________________
Matt Miller
Director of Parts and Accessory Sales
Hobie Cat USA

Could you use a wood plane? Just make usre it's really sharp when you start each rudder.

_________________
Nick

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