rudder noise cavitation

i have a great old 72 hobie 16 but it does make noise coming from the rudders at certain speeds, it is so old there is no adjustment screw for cam, any ideas - i do notice the blades are a little out of alignment when looking from the rear - is that causing it - i would need to extend the tiller bar about another inch for perfect alignment - thoughts

You can get the adjustable end fitting off eBay, or send me a note and I'll sell you one for $5 + shipping. (hcanews(at)comcast(dot)net)

A lot of dealers have them in their used parts bins, too.

what is the measurement for rubber alignment?
I think the leading edge rudder to rudder measurement is supposed to be a half inch or so closer then the outside edge measurement?

1/8" to 1/4" inch wider at the trailing edge.

Measured with rudders down and sails up (sheeted in, too if the wind permits).

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...

Rudder humm at speed? Typical of molded plastic rudders.

http://www.hobiecat.com/support/tech/hum.html

_________________
Matt Miller
Former - Director of Parts and Accessory Sales
Warranty and Technical Support
Hobie Cat USA
(Retired 11/7/2022)

I was able to get the last pair of EPO2 white racing rudders available (apparently) from Hank at nautical ventures. I took off the ugly and rotten black nylon blades. What a difference!! This was in a 18SX but no more humm. I did not alter the rudder alignment.
Matt, why are the EPO2 white blades not made anymore ?

Take a look at the HOTLINE repair article regarding rudders http://www.hobiecat.com/hobieclass/eHCA ... 4_2009.pdf
It might give you some ideas on how to tune the trailing edge.

Production volume did not support 3 styles of the same blade, so we just offer the Black Carbon, which is most popular.

_________________
Matt Miller
Former - Director of Parts and Accessory Sales
Warranty and Technical Support
Hobie Cat USA
(Retired 11/7/2022)

Read about "noise is typical of plastic rudders".

my first 70's boat does this - at higher speeds the rudders vibrate - it's kinda cool - but - when I got my 2nd 80's boat - I found that (same plastic rudders) make NO SOUND AT ALL at any speed. (so maybe not so typical) maybe something else? I'm not sure what the differences in the two boats are yet - haven't had time to measure the rudder alighnments. (may do so this fall/winter).
While the sound was cool - I have to say that NO SOUND is cooler yet! the 2nd 80's boat is totally quiet at all speeds. I much prefer hearing the water trickle beind the rudders than the vibration noise.

i like the sound - it means it time to go flying or another opportunity to use my handy dandy righting bag

After my new EPO rudders I much prefer shpeed in silence...