Hobie Forums :: View topic - Still screaming

I was surprised how out of fair my rudders were. If you make a rigid, semi-flexible sanding block, and run it over the rudders you see how out of fair they are. I'm not sure a belt sander will adequately fair out the shape. A bright light angled properly will also let you see where the concaves are. Not sure if you tried this, or not.

After I got it sanded a bit, I still had some concaves towards the trailing edge of the rudder, and didn't want to take any more off the trailing edge, as the rudders would become more flexible. So, I built up and filled in the areas with West System epoxy with a little bit of fairing compound, and graphite powder. After getting the rudders shaped right, they did several layers of epoxy and graphite, they barely hum at all now, at high speed.

I think part of the problem with the older rudders is the surface becomes less smooth as the plastic oxidizes (or atleast that's what is seemed like on mine), and gives the rudders a high surface roughness. The flow of water over a rougher surface is more like to create small turbulence that could propagate and lead to some real noisy rudders. This is just another idea, if you are sure the rudders are faired properly. The epoxy and graphite give the rudders a very low surface tension, and help let the water break off more easily.

just another thought on it.

Author:	mdgann [ Tue Jul 31, 2007 2:14 pm ]
Post subject:	Still screaming
My rudders still make so much noise that I can't talk to my crew when going over 13 knots. I have done the scissors thing and also used a belt sander and I thought I got all the hollows out of them, but apparently not, or something else is causing the vibrations. Any ideas out there. It's nice to know that I'm going fast, but it's got to be slowing me down, not to mention wearing the gear out. I'm getting desparate. Anything but new rudders as that is not in the sailing budget this season. I was thinking about turbulators to break up the laminar flow about 1/2 of the cord back. What do you all think?

Author:	Hobie Nick [ Wed Aug 01, 2007 9:51 am ]
Post subject:
I don't know if this will work, but you might be able to stop the cavitation by angling the trailing edge of the rudder. Instead of doing the scissors thing so the scissor blade is perpendicular to the chord of the rudder blade, try angling it 45 deg or so and see if that works. Like I said, I do not know if this will work.

Author:	rbell [ Thu Aug 02, 2007 3:53 am ]
Post subject:
You could have hollows in the blade, which you can check by putting a straight edge across the surface at a bunch of places. Not sure if that contributes to the humming, but it contributes to the flow seperating from the blades

Author:	mdgann [ Thu Aug 02, 2007 8:28 pm ]
Post subject:
I've done the straight edge and cannot find any hollows in the trailing edge area. I do see some very flat areas and the actual trailing edge is about a tenth of an inch thick and very square with sharp edges.

Author:	Skipshot [ Sat Aug 04, 2007 6:39 pm ]
Post subject:
Do your rudders wobble in the rudder castings? If so, tighten them up, and try the plastic bushings as found in the Hobie Catalog.

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Author:	skipper0802 [ Sat Aug 04, 2007 9:32 pm ]
Post subject:
Have you checked the "toe-in" measure from the front edges? Just an idea. Eric

Author:	bannanahead [ Sun Aug 05, 2007 9:50 am ]
Post subject:
One problem I have noted with filing down the trailing edge of the rudders is that they seem to chip on the trailing edge more, or maybe I filed them down too much. John G.

Author:	littlewing [ Sun Aug 05, 2007 10:31 am ]
Post subject:	more sanding & epoxy
I was surprised how out of fair my rudders were. If you make a rigid, semi-flexible sanding block, and run it over the rudders you see how out of fair they are. I'm not sure a belt sander will adequately fair out the shape. A bright light angled properly will also let you see where the concaves are. Not sure if you tried this, or not. After I got it sanded a bit, I still had some concaves towards the trailing edge of the rudder, and didn't want to take any more off the trailing edge, as the rudders would become more flexible. So, I built up and filled in the areas with West System epoxy with a little bit of fairing compound, and graphite powder. After getting the rudders shaped right, they did several layers of epoxy and graphite, they barely hum at all now, at high speed. I think part of the problem with the older rudders is the surface becomes less smooth as the plastic oxidizes (or atleast that's what is seemed like on mine), and gives the rudders a high surface roughness. The flow of water over a rougher surface is more like to create small turbulence that could propagate and lead to some real noisy rudders. This is just another idea, if you are sure the rudders are faired properly. The epoxy and graphite give the rudders a very low surface tension, and help let the water break off more easily. just another thought on it.

Author:	cronometro [ Sun Aug 05, 2007 12:32 pm ]
Post subject:	Re: more sanding & epoxy
littlewing wrote: I was surprised how out of fair my rudders were. If you make a rigid, semi-flexible sanding block, and run it over the rudders you see how out of fair they are. I'm not sure a belt sander will adequately fair out the shape. A bright light angled properly will also let you see where the concaves are. Not sure if you tried this, or not. After I got it sanded a bit, I still had some concaves towards the trailing edge of the rudder, and didn't want to take any more off the trailing edge, as the rudders would become more flexible. So, I built up and filled in the areas with West System epoxy with a little bit of fairing compound, and graphite powder. After getting the rudders shaped right, they did several layers of epoxy and graphite, they barely hum at all now, at high speed. I think part of the problem with the older rudders is the surface becomes less smooth as the plastic oxidizes (or atleast that's what is seemed like on mine), and gives the rudders a high surface roughness. The flow of water over a rougher surface is more like to create small turbulence that could propagate and lead to some real noisy rudders. This is just another idea, if you are sure the rudders are faired properly. The epoxy and graphite give the rudders a very low surface tension, and help let the water break off more easily. just another thought on it. A high quality wax would also help if the surface roughness is part to blame.

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