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 Post subject: Re: Rudder hum
PostPosted: Wed Aug 26, 2015 11:08 pm 
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mespig wrote:
I love the hum off my 18... but then, I'm just screaming (literally) around a lake with the kids and not racing. I'm going to name the boat "Caterwaul" :-)



It's also really fun to pass someone one a quieter boat. The humm adds and intimidation factor ;)

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 Post subject: Re: Rudder hum
PostPosted: Sun Oct 11, 2015 12:56 pm 
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Location: Orange Beach, AL
There is a lot of research on blunt trailing edges for rudders, propellers, foils etc. for both hydro and aero applications that you might find interesting if you are unsure about filing the trailing edge to square it off

On rudders, a blunt trailing edge will not only reduce fluttering, but greatly increases the effectiveness of the rudder at low deflection angles. Divergent trailing edges (e.g. schilling rudder) that look like a fishtail on the top-down perspective are very effective at low speeds.

I don't think I can link directly, but read the google books link to "The Development of the Rudder: A Technological Tale" that comes up as the second group in this search: https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=blunt%20trailing%20edge%20rudder

https://en.wikipedia.org/wiki/Schilling_rudder

edit: maybe this link will work: https://books.google.com/books?id=9fj2L3Xbpl0C&pg=PA58&lpg=PA58&dq=blunt+trailing+edge+rudder


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 Post subject: Re: Rudder hum
PostPosted: Sun Oct 11, 2015 5:51 pm 
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Location: Omaha, Nebraska
Oh my! Such a cocktail party I stumbled into!
I used the scissors trick to shave off just a little bit. No sanding. Made all the difference in the world.
Hum equals drag. Fix it and you're going to go much faster, even if you don't sound as cool.


Sent from my iPhone using Tapatalk


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 Post subject: Re: Rudder hum
PostPosted: Mon Oct 12, 2015 11:35 am 
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Location: Harsens Island, Michigan
So, while racing in a new fleet of very old H16s over the weekend, I was crew on and during an especially high speed section going around A mark, with the rudders humming like crazy, the captain said that he lost steering control with the rudders for about 3 seconds, and he suspected it was due to the humming (lost laminar flow). Is this possible?

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 Post subject: Re: Rudder hum
PostPosted: Mon Oct 12, 2015 12:03 pm 
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Very typical around A mark. You increase speed, loads move forward and the bows drive down and lift the rudders... while the skipper tries to turn down hard... the rudders ventilate. He has to reduce sail power (travel out / sheet out), move aft, steer lighter / smoother. You can reduce rudder steering load on the tiller quickly and allow the rudders to regain flow. Basically steer straight for a quick moment and then ease back into the turn.

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 Post subject: Re: Rudder hum
PostPosted: Tue Oct 13, 2015 10:36 am 
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Joined: Wed May 25, 2011 3:15 pm
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Location: Buffalo, NY
It's worth noting that the hum is not what caused his loss of steering. The hum is caused by vortices shedding off the trailing edge of the rudder at near the natural frequency of the rudder blades, causing them to hum. It creates excess drag, but doesn't cause loss of steering. What he experienced was a partial loss of flow or loss of attached flow over the rudder blade surface on the low pressure side (upwind side).

Laminar flow only exists at the leading edge of the rudder. At near 20 mph, the flow over the rudder transitions to turbulent flow within the first 3" of the blade surface, and that's in a perfect world... any surface imperfections or eddies off of the daggerboards, etc. will cause it to transition sooner. When you steer the rudder beyond it's maximum effective angle of attack (around 12 degrees) the rudder begins to stall - which means the angle of attack is so steep that the flow separates from the low pressure side of the blade entirely and begins swirling rather than flowing past the blade. In the case of rudders, because the surface of the water is so close, air will actually get sucked down into the low pressure area as the flow separates - thus, the rudder ventilates, and rather than lift, the rudder just generates excessive drag. Ventilation may be either the cause of a stall, or the result of it, depending on the circumstances.

Basically, his sails were overpowering his rudders, so they couldn't provide enough lift to turn the boat (or at least not quickly), so he probably pushed the tiller a little too far, and the rudders started to stall or ventilate. Something always worth remembering is that the sails are foils, just like rudders and daggers. Because of the way the sailplan is designed, they intentionally want to gently turn the boat upwind, unless trimmed otherwise. If the sails are trimmed to keep going upwind, and you use the rudders to try and force the boat downwind, it's not going to cooperate.

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 Post subject: Re: Rudder hum
PostPosted: Wed Oct 14, 2015 10:27 am 
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SabresfortheCup wrote:
At near 20 mph, the flow over the rudder transitions to turbulent flow within the first 3" of the blade surface, and that's in a perfect world... any surface imperfections or eddies off of the daggerboards, etc. will cause it to transition sooner. When you steer the rudder beyond it's maximum effective angle of attack (around 12 degrees) the rudder begins to stall


Interesting read for sure.

I just recently upgraded my Hobie 20 from EPO 2s to EPO 3s. At the same time, a sailing friend/competitor did the same. I drilled the holes identically when I switched the rudders.

We both found the rudder performance very different than the EPO 2 and we both needed to rake the new EPO 3s more under the boat. Helm is now 0 or slightly positive. But, the biggest thing is that I now have a hard time keeping the new rudders from loosing laminar flow when going to weather in big wind.

SabresfortheCup wrote:
thus, the rudder ventilates


Wowowowow, scary with in big seas and big wings. I find I have to turn the boat up violently to reattach......to the point of loosing the crew on the wire (Very unhappy wife).

SabresfortheCup wrote:
Basically, his sails were overpowering his rudders, so they couldn't provide enough lift to turn the boat (or at least not quickly)


OK, Now what. Depower the boat? That wouldn't make (business) sense as we were flat and driving very hard in race conditions. Wrong rudder? The boat wasn't designed with EPOs in mind. I know the surface is perfect as the EPO 3 is in new condition. What could I effect to minimize this possibility/habit?

Mast rake, different rudder rake. toe in or out........ All I can say is go sail a H20 in 20-25 and have this happen. Ohh $h!# is first response.... Hold on to your @$$ is the second....... :o

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 Post subject: Re: Rudder hum
PostPosted: Wed Oct 14, 2015 10:39 am 
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Thanks for the quote credits "wscotterwin" but, those statements were by "SabresfortheCup" so I have corrected your post to reflect that.

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 Post subject: Re: Rudder hum
PostPosted: Wed Oct 14, 2015 10:55 am 
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wscotterwin wrote:
What could I effect to minimize this possibility/habit?


The EPO3 has a small rubber pad that protects the rudder from impacts in the casting. That means bolt holes have to move a bit to get the same rake, so not the same as another rudder if the pad is in place.

If you reduce weather helm too much (near neutral going up wind would be even lighter on a reach and downwind) you can too easily over-steer the rudders (causes ventilation). I suggest maybe moving the rudder rake a bit aft again. You should always have a bit of weather helm that would round the boat up when you release the tiller.

Steer very smoothly in wider arcs when driving off around the mark.

And yes... sometimes you have to reduce sail power.

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 Post subject: Re: Rudder hum
PostPosted: Wed Oct 14, 2015 11:20 am 
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mmiller wrote:
The EPO3 has a small rubber pad that protects the rudder from impacts in the casting


The current EPO 3s have pads that line up for the H16 casting. I removed them as they served no purpose and fell in a odd location for the H20 or H18 castings.

mmiller wrote:
Steer very smoothly in wider arcs when driving off around the mark.


This happens in an open lane pointing pretty close to the wind. Wave chop might be contributing. I will take the rake advise and consider tho.


mmiller wrote:
Thanks for the quote credits "wscotterwin" but, those statements were by "SabresfortheCup" so I have corrected your post to reflect that


Not sure why the system did that? I didn't make any changes. Just highlighted and quoted using the button provided.

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 Post subject: Re: Rudder hum
PostPosted: Wed Oct 14, 2015 11:33 am 
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You may be too far forward as well. That can reduce rudder depth and control.

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 Post subject: Re: Rudder hum
PostPosted: Wed Oct 14, 2015 12:32 pm 
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wscotterwin wrote:
OK, Now what. Depower the boat? That wouldn't make (business) sense as we were flat and driving very hard in race conditions.


I was just trying to imply that to make drastic course changes, you need to change sail trim simultaneously to suit - aka travel out the main as you steer to bear off, and don't try to take the turn too sharp. Rudders hard over and main sheeted hard is going to cause a lot of drag and a slow turn.

Aside from that, I think all you can do is try to keep up speed and take the turn more gradually, or maybe even sheet out a little as you turn down.


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 Post subject: Re: Rudder hum
PostPosted: Thu Oct 15, 2015 11:29 am 
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Two different situations being discussed on this thread. Steve is talking about rounding A-mark. Scott is talking about just driving upwind.


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 Post subject: Re: Rudder hum
PostPosted: Thu Oct 15, 2015 3:30 pm 
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On the H20, we always had issues with the flow detaching at high speeds. Typically this was when going downwind in high wind, trying to bear off in a puff - definitely made for an exciting ride. Upwind, I don't recall having problems with the stock rudders going upwind.

I had a set of EPO II rudders for a while on my H18 and I couldn't stand them. They never balanced out properly and would get very squirrely helm when sailing upwind. They were noticeably thinner than the original EPOs and I think that was the problem. Maybe it's that the EPO II & III are more optimized for the H16. Personally, I still find the original EPOs to be the best rudders I've used. The things just grip the water and handle very well. If you're spinning out and sailing with inconsistent helm, you're not going fast.

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 Post subject: Re: Rudder hum
PostPosted: Thu Oct 15, 2015 5:06 pm 
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srm wrote:
If you're spinning out and sailing with inconsistent helm


You have probably gone too far under and have neutral helm. You need some weather helm for easier steering control.

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