Rudder questions

I built a wing rudder for an O'day 25. I took off about 7 inches so that the rudder would not hit before the keel. I added a wing to make up for the 7 inches . I built it to a naca 0012. It works well but there is slighly more vibration at top speeds. The original vibrated too at 6 knots but not as much.


Also there is a little more weather helm. Any ideas?

TIA,

John

 

Try to chamfer the trailing edge a little. That's a common advice against vibrations. Don't know about the weather helm. Does it mean that the center of effort has moved back, so there is more of moment on the tiller?

 

Weather Helm

The early models of the Catalina 34 had a problem with weather helm, especially with the shorter rudder for the wing keel. They solved it by increasing the width of the rudder to make it more an eliptical shape. I'm currently in the process of modifying my rudder to the new shape. Adding a strip along the trailing edge 5" wide for the bottom 2/3 rds then tapering to a point at the top. Might consider adding some more width to yours.

I had a Catalina 22 rudder that vibrated at higher speeds. It had the swing up design and open hinge pins for easy removal. Always figured it was the slop in the hinges and swing mechanism that caused the vibs.

 

Be careful adding material to rudders, it is very easy to make the helm forces unacceptable.

Vibration is caused by un-even shedding of vorices, Laser 13 rudders do it, LARK centerboards do it. The normal culprits are flexibility and sharp corners. Obviously, slop in the system will not help.

Tim B.

 

I thought the trailing edge should be sharp and flat. Should I round it off?

John

 

in a perfect world it should come to a perfectly sharp point. Trailing edges in practice, tend to have some thickness.

The corners I was refering to were in planform and on the tips. The corner at the trailing edge of the section should probably be left sharp. I doubt you'd notice much difference either way (as long as you only removed the sharpness).

Tim B.

 

Re: Rudder Mod

I totally agree with you, rudder mods need to be done carefully. In the case of the C34, the OEM did the trial and error coming up with a better design. It is now standard on the new boats. A resourcefull C34 owner happened to have his boat with the old design on the hard next to one with the new design. He made tracings and compared. The only difference was to the trailing edge. It has become a fairly common DIY modification. Roughly $100 instead of $1600-1700 for a new rudder.

PS. Another thought about vibration. If you have wheel steering make sure the cables are properly tensioned. If you have an Edson pedestal, the older models have a problem with the cable run sheaves. If I remember correctly they had brass pins the sheaves turned on. Tend to wear out causing binding problems. New models have stainless pins. Should be easy to change if you have the older pins.