Rudder Rake

[topcat0399]

I have the unexpected opportunity to rebuild my rudder. The 50 year old teak has finally cracked head to foot both upper and lower blades. Not to mention it is time for the prop bites to go away....again.

I've reshape the lower blade from a barn door into a naca 12 foil and glassed it over thoroughly.
I will be glassing the upper later this week. Anyhow:

If the entire blade stays inline with the pintles on the transom the rudder blade will rake forward at the bottom several degrees.

It appears that the original lower may have been cut so that it canted more to the vertical.

What is best for performance of the foil? Should it be raked to forward, vertical, or to the rear?

Any comments appreciated (I will be installing a rudder eating preventive)

 

[Daveinet]

Effectively, you are changing the profile by changing the angle. Draw a line across your profile in the direction of water flow. That is the profile the water sees.

 

[Gunni]

You will be balancing the rudder by projecting more of the foil forward of its rotational axis, you should see easier steering if it is just a few inches.

 

[EV21]

Am watching this thread as may be facing a similar project. Was thinking that if the lower half ends up a bit more forward it may evade reach of the prop too. Wondering about effect on pitch at different rake angles as tiller deflects away from neutral.

 

[Joe]

The problem with a lower end being forward raked is the likelihood of it catching stuff that would normally be swept off by water movement.

 

[SG]

I ask myself: If someone thought there was an advantage hydro-dynamically on a sailboat to rake the leading edge of the rudder forward, why doesn't everyone do it? I ask myself, if it was such a good idea, why do paper airplanes and fighter planes seem to have various degrees of flat or swept-back leading edges for their wings (and other appendages)?

To me, the answer is probably: It doesn't make sense, and won't do any good, and it probably be more likely to do harm.

 

[capta]

If you are racing the boat in a class, then changing the rudder will disqualify you from that class. If you are in a division (multi-class boats sailing w/a handicap), then she'll probably need to be remeasured.
Just something to consider.

 

[walt]

rake forward at the bottom several degrees

I dont think you will notice anything over vertical except for one possible thing. If you get the foils spanwise center of lift in line with the rudder shaft (where it rotates), there will be less effort on the tiller for any given rudder lift. You really notice this with kick up rudders that are not completely held down all the way, tiller force can go way up even when the rudder is generating the same lift. I think my rudder on my 26S slightly cants forward (only talking about a few degrees like the OP) and I dont think it had any influence on how well the rudder works. I would suspect that if you get the rudder too far forward you could run into the high tiller force problem you have if the rudder is raked back because the kick up rudder wasn't adequately held down.

Back in the windsurfing days, you wanted a swept back fin for better maneuverability and a more upright fin for upwind. I will take this to mean that a swept back foil is harder to stall and a more upright foil has better L/D. Going past vertical.. maybe it affects the stall but I personally dont know which way..

Picking up weeds.. I dont think any different than vertical (we are only talking about a few degrees). .

If the tiller feels good with the new foil, I personally would not worry about several degrees.

 

[SG]

On most cruising (modern) keel boats, I've sailed on, when you go under sail bow goes down slightly from "resting"? The rudders usually have an aft rake which is very noticeable. That different than some lateral control surfaces, but almost always the case with rudders.

There may be some aft hung rudders which are different, but the flow of water back there different. In the case of a catamaran or dingy or keel boat with aft rudder, I'm not sure what the condition should be. The need to develop any lift may be irrelevant and the necessity of minimizing whetted surface may control.

I wouldn't be advising anyone to improvise with the shape of there rudder -- certainly not to change the "cant" or underlying concept.

I remember Columbia Yachts -- which used to come-out with an "earth shaking" concept that changed everything (remember the Champagne Sterns). If it was such a great idea, why didn't everyone do it? Because they didn't want to go fast, or be balanced, or be sea kindly, or...???

I'd be cautious especially taking a "BALANCED RUDDER" and changing its characteristics. Forgetting the interaction with the rest of the under body of the boat and it's control surfaces (irrespective of the rudder itself) -- the rudder as a control surface has forces which the whole assembly need to deal with through the shaft, steering, glands, etc. There is a lot of torque and other forces involved on the whole assembly. If you have little dingy, it's probably not an issue.

In the case of the MacGregor -- that's a pretty quick boat, isn't it???