Rudder tuning

I'm rebuilding my rudders over the next 2 weeks.

Rudder blades:
I am reading that sharpening the leading edge helps with rudder hummmmmmm and a bit o speed. - anyone have thoughts? True? BS? Waste of time?
Can I just hit the rudders with a sander while wearing a mask???

I'm tightening with a rudder kit when I reassemble...

Rudder control arms:
The guy I bought the boat from went insane with silver paint over all the metal parts the rudder castings have fused the adjusteers from the over spray! I noticed these are plastic - can i replace with metal ones?
I'm stripping it all off to bare metal, and then I want to paint the control arms BLACK... any advise on doing this so the paint doesnt flake off??? SHould i use a high heat paint?

Thanks Hobiests...

The method used to work the trailing edge depends on what material your rudders are made from.

In any event, I wouldn't use power tools for this work. A half-round bastard file works well for the coarse work.

On Lexan rudders, you can use a sharp edge of a pair of scissors to scrape the edge down. Sandpaper is pretty useless on Lexan.

Look in the FAQ's.

A photo would help with trying to figure out the rest. Your terminology is a little mixed up / not making sense.

What you are trying to do by shaping the rudders is to take out the hollow that forms from shrinkage when the rudder comes out of the mold. The hollows create cavitation when the boat speeds up. The bubbles forming and bursting creates vibration, thus the humming. It can get pretty extreme and actually scream at real speeds (17-20 knots). It also improves with the washers used to take out the slop and also new connectors on the crossbeam to tiller arm connections.
As you can probably tell, I have been down this road. When I first got my boat, it was so loud I thought that I would have to start using ear protection. As I have made all the adjustments and improvements the humming (screaming) has just about gone away. I very seldom feel the telltale drag begin anymore.
Do not sharpen the leading edge of the rudders anymore than they are. If they are smooth and without major dings you can leave them alone. If you sharpen them, you will lose your down speed control as the sharp leading edge will encourage early separated flow and stalling of the foil.
When tapering the trailing edge to take out the hollow, be careful to not take too much material off the trailing edge. You want to have no less than about a tenth of an inch thickness with a nice square trailing edge. My rudders are plastic and I scraped the material off and then sanded with a belt sander to smooth it all off and make sure that it was even. Be real careful with the sander and use greater than 120 grit. Good luck and don't let it stop you from sailing.

The first statement (in red) is incorrect. Cavitation occurs a flow region when the pressure is below the vapor pressure of the water - literally creating a steam bubble. This is a problem on propellers and can actually eat away the metal as the bubbles form and collapse:


The flow across Hobie Cat rudders isn't fast enough to cause cavitation on the foils. They will ventillate (draw air down from the free surface of the water) long before they cavitate.

The humming is caused entirely by the raised profile of the trailing edge, not by hollows in the foil section. Von Karmann vortex shedding is what causes the hum. This is how you fix it:



The original EPO rudders hummed badly out of the box until the bulbous trailing edge was removed. They do not have hollows in the foil section.

The statement in green is correct.

A 1/10" wide trailing edge is . . . wide. 2 mm has less drag. The key to getting rid of the hum is a fair, clean exit from the foil - sharp corners on the squared-off trailing edge.

If the rudders still hum after all this, try angling the "squared-off" trailing edge slightly to one side.

The method of material removal (and how aggressive you can be w/ power tools) depends on what your rudders are made from.

Awesome feed back guy - thanks. I have burned up the printer with lots of info and pictures. Most importantly.. I wont be sharpening the leading edge!

What about painting the grudgeons and the tiller arms?

Mine are silver (metal).. can i spray them black? Any reccommendations? I was going to use acetone to clean them first, then I was gonna use a high heat black rustoleum spray paint for a nice flat black. Will this peel?

How does one refinish tiller arms? (mine are silver)

If I where refinishing the metal, I would take them to anodizing shop, and get them done there. I think Paint would look nice, but would peel easily.

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It all depends on how much you want to spend and how durable you want the finish.

Anodizing can be done on all the extruded parts (tiller arms, tiller crossbar), but not the castings. (Hobie Cat uses a dye on the castings). It's probably the most expensive route.

Epoxy appliance paint will look good initially, but will chip and scratch. To do it right, you need to use a conversion coating on the aluminum (phosphoric acid / chromate solution) to remove oxidation and prepare the surface to accept the paint.

well,... i dont like being cheap, but a can of spray paint can be done yearly pretty easily.

I bought acetone last night. Someone said hit the tiller arms with that, clean with soap and water and paint.
Then next year I should be ablt to just hit the scratches again with a fresh can of paint? This was my thought, but i am un sure so im asking.
I will look up an anodizing shop in my area to test prices.

The cams sound "special" - can they be painted? or am i better to by some used black ones?

tks

I painted my upper and lower castings, rudder arms and tiller crossbar over the winter with two coats of black Rustoleum spray. Mainly because I had a can sitting on a shelf in the garage. I did not use a primer reasoning that Rustoleum is formulated for metals. It is not quick dry paint but there is absolutely no sign of peeling or wear after three months of fairly heavy use on the beach. I wiped everything down with acetone first. My parts were originally black anodized that had faded over 26 years to a gun metal gray color.

Most commonly you get Karmann street vortex shedding from bluff bodies in low to medium reynolds number flow. I have not done the CFD for any foil shapes to investigate this regime, but have done some tests of an experimental keel shape with turbulence promoting bumps near the trailing edge. The fastest we could get the tank to flow was around 12 knots. Around 10 knots we would begin to observe bubbles forming just after the turbulators at the lower end of the foil and then climbing up the foil as the velocity increased. Instead of decreasing drag by delaying detached flow, we increased drag from the formation of bubbles and vibration. I wish I still had access to the facilities to continue the investigations. I also no longer have access to the programs to do any computational fluid dynamics which would be cheaper and faster (though not as much fun as empirical testing). I would love to stick my Hobie rudders in the tank and play away. Also love to confirm the vortex shedding as Matt describes.

You'd need a facility like the US Navy's Large Cavitation Channel in Memphis, TN to do a full size test.

The circulating water channel at the University of Michigan is too small.

Foils at 0° AOA can behave like small diameter bluff bodies, especially when there's a bulbous trailing edge.

I believe you Matt. I was just reading a paper online that was using a NACA 0009 blunted(truncated) foil to create vortices in low Re number flow (42000 to 70000). In the abstract they stated that "When the pressure is low enough, cavitation develops
in the vortices cores. Young and Holl8 investigate
the case of a flow around wedges and observe
that the cavitation development in the wake increases
the vortex shedding frequency by up to 25% and
therefore affects the dynamic of the von Kármán
street. "
With a chord length of 100mm(8in.) and viscosity of 1.004 @20c you have a velocity of only .7028 m/s or about 1.5 mph. This seems ridiculously low to create cavitation. I am going to recheck the numbers (Units) and do some more reading.
I haven't had my calculator out in months. This is fun. Maybe I could use one of the beautiful clear flowing rivers around here to do some of this work, like in the movie "Wind". Only problem is the temp is around 45 F.