Your Fluid Mechanics lesson for the day . . .
CavitationCavitation occurs when the local pressure is lower than the saturated vapor pressure of the fluid - the fluid "boils". You can see the cavitation vapor trails in this photo of a cavitating propeller:
This can be a major problem on propellers and pumps, since the bubbles collapse violently, eroding metal away.
Hobie Cat rudders do not cavitate along their trailing edge. What happens more often is
ventilation, where the local pressure is below the free surface pressure of the water, and air is drawn down along the low pressure side of the foil. Since air is 1000 times less dense than water, the rudder loses its "grip" and you can't steer.
Laminar vs Turbulent FlowIn fluid mechanics, there are two general types of flow regimes:
Laminar (from the Latin
lamina - a thin sheet of material) where the flow moves in "layers" and
Turbulent, where there is intermixing between the layers.
This illustration shows both:
The general flow on the bottom of the foil is laminar; on top is turbulent.
However, that explanation is a bit simplistic if you look at the smoke trails more closely. What happens to the three smoke trails closest to the bottom of the foil? They disappear before they even get to the trailing edge! What's going on?
The answer is that there is also turbulent flow on the bottom of the foil. There is enough small-scale turbulence to dissipate the smoke but not enough to cause the streams to "blow apart" like they do on the upper surface. This is the turbulent boundary layer that starts at some distance back from the leading edge, depending on the velocity/viscosity/density of the fluid and the surface roughness of the foil. A laminar boundary layer has less drag, but a turbulent boundary layer is harder to separate from the foil. At 10 kts in water, the laminar boundary layer extends only a few inches back from the leading edge, then transitions to a turbulent boundary layer. On the rudder, the trailing edge is fully turbulent.
(This illustration also shows the foil at a relatively high angle of attack - in fact, this foil has
stalled, where the flow has become separated from the low pressure (top) side. On most foils, lift will increase linearly with the angle of attack, then fall off sharply at the stall angle.)
So what's going on with a humming Hobie rudder, at zero angle of attack, if there's no cavitation, and all the flow is turbulent?
The answer is a weird phenomenon in fluids called a "Karman Vortex Sheet":
They generally only form behind "bluff" bodies - like the cylinder shown in the animation. However, a foil at zero angle of attack and above a certain speed will behave like a bluff body and create a Karman Vortex Sheet. The hum occurs when the frequency of the vortex shedding equals a factor of the natural frequency of the rudder. A harmonic vibration will start and will persist even when speed drops below the "trigger" speed.
So how do you eliminate the hum?you need to change the shape of the trailing edge to make the foil act more like a foil and less like a bluff body. Eliminate the edge bulb by shaving or sanding. A sharp trailing edge is best, but impractical - it damages easily. A 1/16" wide squared off edge is better, but you may still have humming problems. If that's the case, then have a 1/16" wide "unsquare" edge - angle it slightly to one side (same side all along the edge). That will make vortices preferentially shed to one side and stop the oscillation.
I've left out some not-so important details like Reynold's Numbers, what happens with 3 dimensional effects (tip vortices) and the viscosity and density differences between salt and fresh water, but this covers most of the high spots - and explains why Hobie rudders hum.
BTW, for the engineering geeks - the R
e of a Hobie rudder is about 1.43x10^6 in fresh water; 1.53x10^6 in salt water.
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