I don't think alignment is the problem here. The noise is indeed cavitation and is most likely caused by hollows on either side of the stock rudders (plastic). After coming out of the molds the plastic shrinks and leaves concavities. The solution is to sand or scrape the aft third of the rudders to eliminate the hollow. There are several sites that explain how this is done. One method involves scraping with scissors. What is happening is that the water flow over these hollows creates a spot of low pressure. The faster you go the lower the pressure. It gets so low that the air in the water will begin to create bubbles (boiling) that then collapse and create vibration. Mine starts at around 10 knots and really gets annoying around 15 knots. I have sanded my rudders with the result that it changed the speed and tone of the howl. I am a little hesitant to get aggressive with removing material from the rudders as they are the only ones that I have and $300 or so for the EPO's is not an option that my sailing budget would support. It is less efficient and I can feel the drag that it creates, but the howl is kind of cool when you ripe past a monohull. It kind of gives you a sense of how fast you are going too. On a smooth day with a little wind (12-15mph) I can actually look down and see the bubbles trailing off the rudders. I can stop it by wriggling the rudders a little to disrupt the flow, but it just immediately builds back again. It also helps to eliminate slop in the entire rudder system. Washers on the sides of the rudders, cross beam connection kit, oversize rudder pins, etc...
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