went out again this weekend and got the getaway going fast for me that is and there was like a lite humming or buzzing noise my crew was kidding me about cheating with an electric motor under the boat at first I though it was the cables but the crew swore it was coming from the boat, what cut cause this noise it kind of ruins the sailing ambiance.

thanks for any help. lost

Might be your rudders. Make sure the are fair and parallel. Cat rudders can hum and I understand they hum mostly because they don't know the words.
Gator:)

In addition to shaving rudders, I've also been told that you can help with the humming (cavitation) by switching the rudders to the opposite sides (port to starboard and vice-versa). Of course, it does depend on the fairness of the rudders to begin with - but this might be an easy fix.

I also seem to remember some sort of grommet or plastic fitting that helped if the humming was related to vibration of the rudder on the post.

Thanks for the help it makes a lot of sense, while inspecting the rudders I found that they have like a very slight lip along the leading edge not visible but could be felt with the tip of my finger I will sand it and polish smooth and will also switch them. will let you know if it works, thanks again. Lost

Yes, as the others said the humming is from cavitation on your rudders or daggerboards.

I recently faired my daggerboards which were doing exactly this, and it honestly feels though I picked up 2-3knots of speed on a reach.

The laeding edge should be round and smooth. the trailing edge is where the problems often occur. They should be flat, about 1/8" wide and run parallel to your cross bars. It will also help to make sure there is no slop in the rudders that will allow them to vibrate. Same goes for boards if you have them.

Seems to me that if the rudders are humming you should feel it in the tiller. Shrouds sometimes sing at high speeds. If it is the rudders then lifting the one on the windward side of the boat should cause a change in sound.
My tiller is pretty sloppy as tillers go.. no sound.

My 17 used to hum like crazy. I couldn't feel it in the tiller, it's a high frequency resonation (which is vibrations that reinforce each successive period) in the rudders, your chances of feeling it are less with play in your tiller. It only happens at speed (reaches) and conditions have to be just right. On high speed reraches, I like to have both rudders down and centerboards down at minimum for stability.

Ok just finish smothing all surfaces about an inch inwards from all edges, I went by miami cats and inspected rudders on other boats and I found that some had very nice and even surfaces and others where like mine with what semmed to be like a slight ondulation deeping about 1/2 inch from the edges in some cases all arround, it can not be seen it has to be felt with fingers, I think this rudders are molded in hetero cavity type molds and cooling time while inside mold will have an effect on the final shape of the parts as this type of plastic have no doubt will shrink as it cools down upon ejection from mold wich means that parts with different molding cycles will turn out different. I thought I mention that there is no play in the rudder brackets. I'm going sailing tomorrow and I'll let you know how it turned out and thanks again to all for your help I would have never figure this out without it. I dont want this rudders learning any songs

"I think I'll unfurl my jib"

Here's something on the Hobiecat website on rudder hum.
The page mentions the hum comes from the molding process.

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

ok back from key largo and very happy to report that the fairing or smoothing of my boards worked like a charm 0 buzzing I did not swicht them, Im not exactly sure that the boat is going faster but for sure the noise is gone.
also very interesting info on the hobie site about the molding process my experience with plastic molding came while in college I was working for a custom injection mold maker that used to make molds for the Reed molding machines, I used to work at the quality assuranse dept and although many years ago I still remember the shrink factors between nylon, celcon acetal and my favorite phenolic a plastic that does not melt it doesn't even burn you can make a knife out of it. Any how the hobie site is very nice and full of good guys but I wont ask any questions there because the people that very much misstreated me while I was in the process of buying my boat are all over that site I mean in every thread therefore I rather stay away, im not one to hold grudges but the arrogant crap I can do without so sinkyouverymuch to them. And thanks to all here for the help and for sure I will ask some more not so smart ?'s




edited by: lostsailor, Jun 10, 2008 - 08:11 PM

And here I thought the Hum was a good thing. It meant I was getting some speed :)

After reading this thread I guess I will try and see if I can correct it through all of the suggestions here Dagger/Rudder refinements.

They all seem to apply to my NACRA 18 Sq which I salvaged from a farmers field.

How are the Dagger Boards supposed to held in place?

I've connected a bungee from the stability tubes which run across the middle of my tramp. Pulling forward and inboard.

There are some eyelets on the forward tube but I use them for my Trap wire bungee.

Should there be carpet lining the dagger wells? Mine doesn't have anything there? Will this help get rid of the HUM? Should there be some?

Should i make dagger board retainers like on the new NACRAS bungee'd from the tramp to a roller outside of the daggers?

Oh My daggers are wooden Mahogany I think. I've refinished them, but the trailing edge is no longer straight it has been worn away where it contacts.

Thanks in advance!

I will post if my efforts succeed if not I will post pics.

I used to know a few girls that ''Hummed''~~~ so as far as I know~~ things that 'Hummmmm' are good

(Someone just had to say it, slow day today??)

--
~ Vietnam Vet 69-71~ 17 Hobie w/big jib, ~18 Hobie mag,~DN Ice sailor,
and other toys.......
~~ I live in NY state on the north shore of Oneida lake in
Bernhards Bay. ~~~~~~
--

Personally, I like the humming.
If my H-18 Magnum was humming, she was screaming too!

Micro adjustments on the beach do help with the rudders but not the shrouds.
All in all, it just means your moving on the water with the wind... Cool concept!