The Hobie 33 has a notoriously shallow rudder. When pressing hard, the boat is narrow and is a bit low on the righting moment so the rudder root gets exposed and voila, ventilation and stall. It happens fast with little warning sometimes - the wrong wave angle in a bit of chop, a puff and it's into recovery mode.
I've been thinking about a rudder upgrade for a few years - not an uncommon thing to do on H33s, and came across two interesting concepts. The tubercle design is intended to prevent ventilation - I've had some email conversations with the designer and builder of this rudder and read the white papers it was modelled after:http://ptsail.org/20...rudder-designs/
It looks like the new Riptides will be featuring (standard or as an option I don't know) the tubercles/bumps on their rudders now based on the success they've had.
Separately, I've been talking with the Swiss designer of the ONYX (http://www.onyx-yach...ge.asp?seiid=14
) who also modelled their rudder on a whale flipper to achieve the same result, but a different approach. The good pics of the rudder are on p48-49 of this PDF: (http://www.onyx-yach...X_Heft_2011.pdf
) and if you read German you can get an idea but it's basically the same overall concept. The designer said he's been experimenting trial-and-error with rudder designs for years, his goal being to reduce abrupt stalling of the rudder on overpowered boats, and he claims his rudder achieves that.
Both designs do the same thing with different methods - as you run down the leading edge of the rudder it protrudes forward (either with several small bumps, or effectively one big one). This is what redirects the ventilation aft (it becomes a path of least resistance) rather than further down the rudder blade. The Swiss design looks cleaner to me, but until someone actually goes to a university and tank tests these things I guess we won't know.
People have been poo-pooing the tubercles for years, but testing in several research studies is pretty conclusive that this concept allows a higher angle of attack to avoid stalling at the cost of minor drag at low speeds. What does 'minor drag' mean? What is the optimal shape (eg # bumps and their size/shape, or the size of the forward leading edge, curve, angle, etc)? I don't think anyone has it figured out to the point that you plug in some boat dimensions and out pops a blade shape.
Here's some light reading for anyone interested: (http://upcommons.upc...le/2099.1/13814