new rudder needed?

Hi Theo

For starters looking at this a bit more methodically.

What's the rudder plate thickness? And what rudder angles do you find yourself applying when you think you are getting too much drag from the rudder ?

 

its a 3mm plate. I'll need to do a bit more sailing to really determine the issue at hand!- but it tends to be at slower speed, the boat doesn't really turn much.

 

So any delayed stall mechanism at high angles of attack under high foil loading is not going to help ..... If a flat plate with t/c of .025 isn't providing enough lift at reasonable angles then the rudder is simply too small. Flat plates are good at producing lift. And drag can be reduced by using a bigger rudder at lower angles of attack.


How much effort do you want to go to to make a new rudder ? The simple solution is to cut another profile (better from 6mm plate than 5), make the chord length say 400 rather than 200. Round the edges and knock the corners off.

A timber cored rudder sheathed such as PAR posted is a good one to fabricate if you want a bigger project. If you build a low drag bucket chord shape You'll get a little less drag providing you fabricate it really accurately but for your purposes I'd keep it simple.


The next step might be the sail area

 

Just for fun, I'd really like to see the center board.

 

Folks,

Having happened across the thread dicussing leading edge modifications to a flat plate rudder, I expect you would find the following on-line SAIL magazine article interesting:

http://www.sailmagazine.com/sailboat-centerboard-upgrade

The final editing of the on-line article (and the in-magazine version) somewhat corrupted the message, but the pre-edit long version (which I still have), includes scales on the lift and drag curves, and is consistant in identifying the problem as a lee helm and associated big leeway.

Regards,
Mike Raleigh

 

Thanks Mike! The article does a fine job quantifying the improvement. I have a couple of places I want to try this -kayak rudder and a Jet keel.

 

Original Article on leading edge mods for a float plate

Folks,

I'll try attaching the original article where the graphs have scales! It would be very interesting see experimentally, if a leading edge add-on would increase the control authority of a flat plate rudder. Xfoil thinks so, since the stall angle greatly increases.

The suggestion to fit turbulators (zig zag tapes) to a flat plate rudder is intriguing. Expecting flow separation to occur right at the nose, fitting the tapes aft of the nose might already be too far downstream. If I was to try this, I would scratch vertical grooves in the rounded nose of the rudder (one heavy pass with 40 grit paper?). I would not expect (based on Xfoil results) as much improvement as with an elliptical add-on nose.

The experimental evidence that I have for improvement to my centerboard is somewhat indirect (GPS ground tracks, helm balance, ability to "hold a lane" against other boats). One of my colleagues want to rig the centerboard out the side window of a car, put tufts on it to observe the flow, and drive down the road at ~80 mph to hit the right Reynolds numbers! There are some things we won't do for science.

Mike Raleigh

 

Thanks again. The article is much clearer in the Doc.

Kicking the flow turbulant can help avoid laminar separation but when the foil reaches full thickness so soon (like in the flat plate) it can't bring the flow around and the boundary layer just grows in a big wedge anyway. The zigzag leading edge tests earlier in this thread were interesting in showing the relation between size and the reynolds number they were optimum for.

The surprise for me in this article was that the majority of the improvement was in reduced drag not increased lift -this is particularly true at the lower angles of attack you expect a keel to operate at. For a rudder this will be a dramatic improvement in both lift and drag.

The only thing I think the article is lacking is guidance for the dimensions of the nose. It states that turbulent flow is reached before the max thickness but was it checked at different speeds? What Ncrit? What I wish we had was a simple (calculable) relation to cord length.

 

Well, 3 months have passed since Theob started this thread.
Hopefully he has done something about his rudder in the meanwhile - so it would be interesting to hear what modifications did he make and what is the final result.

 

Hi Skyak,

Some profiles are attached. They won't make much sense until I get the writeup to successfully attach ( a 1/2 MB .docx, should be no problem!?!?).

Mike Raleigh

 

Hi Skyak,

I have attached the document that offers more explanation of my efforts.

Mike Raleigh