About Rudders Again?

My portside cam had a lot of slop so I decided to clean it up, and do the starboard side while I was at it. Long ago, my rudder blades were black when I purchased the boat (used). Not long afterwards, I repaired some chips on the edge of the starboard side noting the fiberglass was still intact. Portside was OK so I didn't make a closer inspection.

Until yesterday.

After cleaning-up and re-greasing the portside cam/plunger, I moved to starboard. At this point I noticed (and then measured) a difference in the width of the rudders. I have a Lexan on the port and a fiberglass on the starboard. The measured difference is a quarter-inch at the respective widest point of the blades.

I don't think this will make any difference in any way. I was just really surprised to learn about it this far into ownership (three years).

Would anyone know if this would make a difference from any aspect?

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Eric
H16
Sail# 11500

From a hydodynamics view, the thick one will be more effective at lower reynolds numbers (Speeds) and will have more form drag. The skinny one will stall (detached flow) at lower speeds and have less drag over all. You will not be able to notice the difference in either of these cases. Go sailing.

Thanks for the fast response!

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Eric
H16
Sail# 11500

Mdgann,
You walk the walk as well. You wouldn't happen to know what foil cross-section the EPO's use would you? For example, Eppler 374, NACA 2412, etc.

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Eric
H16
Sail# 11500

I have no idea what the NACA number is for the hobie rudders. Since they were designed in the late 60's early 70's they would have probably started out with the 4 digit symmetrical foils, but I'll bet a dollar to a hole in a doughnut that they have changed over the years and maybe even just came up with their own foil section. Plus with the shrinkage that they see in the molding process, they didn't really have much chance to have an accurate section. If they did, we wouldn't be spending time scraping and reshaping our plastic rudders so that they won't hum when we get up to speed. They have hollows in them that promote separation and cavitation. We try to sand the hollows out and taper the trailing edge down to a nice squared off trailing edge. Mine start to scream at around 13-15 knots. Anyway, I have never seen any mention of the NACA section used. I would definitely be interested though. There are several free programs around that will plot the sections out for you. It all may be different with the Lexan and EPO rudders. Maybe that's why they cost the big bucks.

There is no NACA section for the rudders. The foil section is thinner than it really should be due to the restrictions in the class rules.

EPO rudders were the fattest section that would fit in the rudder castings.

They hummed because of a bulbous trailing edge left over from the molding process that had to be faired out. Humming results from Von Karman vortices shedding from the trailing edge.



When a vortex is shed, an unsymmetrical flow pattern forms around the rudder, which therefore changes the pressure distribution. This means that the alternate shedding of vortices can create periodic lateral forces on rudder, causing it to vibrate. If the vortex shedding frequency is similar to the natural frequency of the rudder, it causes resonance (humming).

Thinner rudders will stall and ventilate (draw air down from the surface) more easily that the thicker rudders. The difference in form drag is negligible. In general, the thicker EPO, Racer Rudders and EPO2 rudders are preferred by racers for that reason.

BTW, the new EPO2 rudders don't hum. They are perfect, right out of the box.

Matt,
Thanks for the detailed and informative response. Just the stuff I was looking for! Time to pop for some EPO2s. Are they pre-drilled, or do I need to transfer my current hole pattern?

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Eric
H16
Sail# 11500

EPO2's are not pre-drilled.

If you are happy with your existing hole pattern (rudder rake), then transfer them over. Be very careful to line everything up and use a drill press.

If you are using adjustable castings, then have Dan Carpenter at Hobie Cats Northwest do it for you.

Love the animation of the Karman vortices. I was taught that you get the alternating vortices from cylinders or fat smooth shapes, but not a foil section. When I am flying along and my rudders start to howl, I can look down and see the bubbles shedding from the hollow just in front of the trailing edge. If I could see the other side, I'm pretty sure that I would see the same thing. I have tried to sand these out, but am afraid to get the trailing edge much thinner than about 1/10 of an inch and have not been succesful yet. I can also see the tip shedding a vortex. I really ought to pick up another set and do some experimenting. Maybe an end plate and some turbulators at the point of flow separation. It would be fun to play around with.

Mdgann,
Your bubble separation description appears similar to cavitation on a propeller. I suspect they're one in the same?

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Eric
H16
Sail# 11500

No they are not.

'Cavitation' is created when the pressure on the foil gets below the vapor pressure of the water - a bubble of water vapor (steam) is created in the low pressure zone. When the bubbles collapse, they do so violently - hard enough to erode the metal on a propeller:


Hobie Cat rudders do not cavitate - they ventilate. The low pressure zone literally sucks air down from the surface. Once this sheet of air attaches to the foil, it can be hard to dislodge, especially if there's a hollow in the foil.

The cure is to immediately straighten the rudder, allow the air to shed off the foil, then try to steer again. Often, you end up doing this repeatedly until the boat turns enough that high angles of steering are no longer required.

The cure for humming rudders is to square off the trailing edge with about a 1/16" flat. If it still hums (Tiger daggerboards are famous for this), then angle the flat slightly to one side.

I am experienceing just the opposite. My rudders howl when I am going straight and then when I wiggle them slightly they quiet down for a short time and then the vibration builds up again.
Going back to some of my Fluids textbooks tells me that Karman vortex street is indeed more common in bluff body flow, but also can be present in the flat plate flow regime, but at much higher frequencies and Re numbers. My curiosity has been piqued now and I think that I will do some research and calculations to see what Re we are actually seeing on our rudders. It wouldn't surprize me if they are high enough to be in the turbulent flow area or at least in a transitional area between laminar and turbulent. If I get anything interesting, I will report it in a new topic. Love this stuff.

Mdgann,
I will be interested in your research results. Please email me at:
skipper08022000 at yahoo dot com either way

Thanks

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Eric
H16
Sail# 11500

Just a note about transfering the rudder holes. I've had to do this several times due to swapping sets of rudders between different boats. Since I don't have a "real" jig or fixture, I make a very simple one each time.

I take the rudder that will be transfered onto and place it on a flat table. Then I take three small blocks of wood (2x4s) and clamp them each to the table so that they all contact the edge of the rudder at a single point- one at the top of the rudder head, one at the front of the rudder head, and one at the bottom front of the rudder tip. Then I take the rudder that I am transfering the holes from, put it on top of the other rudder, and push it firmly against the wood contact points. I may also clamp the two heads down to the table too. Then use a pencil to transfer the holes, center punch the center of the hole, and drill with a drill press. This simple jig makes it easy to assure that the rudders are lined up when you transfer the holes.

sm

Matt,

I have a 01 European boat that has what appear to be carbon rudders. Are they equivalent to the old Hobie Race rudders? I know they pre date the new carbon finish EPO2s

Thanks

Drej