Rudder length

[mutt]

does anyone have a feel for what's the right rudder length and chord for sea sailing. my Annies Apple rudder is too small - I know that much!.
Matt
N3486

[Phil Brown]

Matt

Are you coming to Spinnaker this Saturday? Graham and others will be there so you could compare rudder sizes

[philipcosson]

From what I have read...

Longer the more stable going downwind, but more drag upwind.

Thicker section gives more lift in the sea (12-15%).

The NACA 63 section is theoretically the best, with the thick point further back from the leading edge, but this only works when angle of attack is zero degrees. If angle of attack is bigger than 3 degrees the NACA 00 profile's higher lift makes it optimal.

So if there is a long downwind leg in windy conditions, use a high aspect rudder, with a thicker NACA 00 12 section.

But i'm just relaying what i have read - not from real experience
Philip

[mutt]

accepted but what is length is good. I can presumably have a high aspect ratio rudder of 50cm length but that won't let me turn effectively in waves.
As for spinnaker I can't d/t child care responsibilities.
Matt

[Phil Brown]

If you want a bit of theory from a fluid dynamicist who was also an enthusiastic sailor, immerse yourself in these two articles

http://www.int505.org/eck1.pdf

http://www.int505.org/eck2.pdf

Bransford was an amazing guy, full of theory and innovation (he was producing balsa cored carbon sheathed foils long before anyone else). As part of his work he had access to some NASA research and modelling techniques and used a great deal of other peoples computer time into his research into foils. ( The theory of aerofoils in air at subsonic speeds readily translates to dinghy foils in water as the Reynolds numbers are similar)


NACA 63 is a laminar flow section that does have a lower drag at zero angle of attack but it has a very narrow low drag bucket ie as angle of attack increases, it rapidly become high drag, one reason being that the laminar flow round the foil soon becomes detached and turbulent on the low pressure side thus inducing drag. Possibly acceptable in a board but not in a rudder.

The NACA 0000 series has much wider low drag bucket and is therefore more forgiving and for a rudder, 0012 is generally accepted as a norm.


For the historians amongst you, the NACA sections did not come about from any research but from the work of NACA in the 1930's to codify some of the sections that were being created by the aircraft builders of the time so they're getting pretty long in the tooth

Bransford did model another section E1161-24 which had a slightly sharper leading edge, the max thickess at 24% (compared to the 30% of a 0000 and 40-60% of a 63 section) and a hollow trailing edge  that he maintained  was better (see the second article).

I built a fixed rudder to that section that was absolutely bullet proof. I used a straight leading and trailing edges tapering to about 40% of the top chord as it was easier to produce (that's a relative term, it took ages). No matter what any foil shaper says, the only way to get real performance out of a foil is to use templates and not shape by eye.

All that was based on then. The trend now for high speed craft such as skiffs, 505s and Moths etc where the planform is not fixed in the rules is to use very high aspect ratio foils, both rudder and board, with the advantage in the former that the foil is less likely to cavitate (aslo countered by putting fences near the surface and parallel to the direction of flow to reduced the amount of air dragged down the foil when it does)

However, a significant disadvantage of high aspect foils is the lack "paddle" effect of a lower aspect rudder in light winds.

Is anyone using high aspect ratio foils in the 12?

Happy reading

Now, what was the question??

Sorry you can't make Spinnaker, maybe we can canvass some opinion for you

[johnk]

To be pedantic, most foils 'ventilate' when the pressure on the low pressure side of the foil drops below the local hydrostatic head. Cavitation is when the pressure is below the vapour pressure of the liquid and it boils. Cavitation usually only appears around propellers in power craft and I am not aware of any (legal) N 12s with engines and propellers.

[Phil Brown]

Noted, couldn't think of the word when I did the post

[cookie (Guest)]

Phil, The 63 series Naca foil also has the chord at 30 percent (like the 0010). A 6 series foil with the chord at 60 percent would be a 66010. This is a 10% thick foil with 0 lift applied (hence symetric). Moths use the 66014 for foiler daggerboard vertical sections. I reckon laminar sections are not really suited to 12 rudders. which rarely go very quickly enough to make the benefit and really need to work at high AoA's

Mr Mutt, I usually do 12 dagerboard rudders with 70-80cm below the waterline and a 20 cm chord, 0010 section. Terry thinks this is a bit small for sea use but I never had a problem with it.

[Jimbo41]

Just approaching Chapter Nineteen of Frank Bethwaite's "High Performance Sailing". Looks interesting for the basic physics, but not much in the way of practical examples for a 12. The foils on my Tasar, esp. the daggerboard have a high aspect ratio and I suspect that if you're approaching ideal weight for a 12 I'd go for something that cookie uses, even in waves (just make sure he puts enough meat on it around the pintles though ). Either way, flat water or waves, I've found that the ideal situation in the 12 is not to use too much rudder and to steer the boat using crew with/position.
Cheers!
Jim. N3470, 3130 and T1293 (now being refitted)

[DavidG (Guest)]

Ref. the Annie Apple rudder.  The tip is cut off and square because there is a bit if theory by a guy called Betz who indicates that an elliptical tip does little to benefit lift, so you might as well crop it off, look at a Hurricane wing vs. a Spitfire wing.  This has added benefits that you can sail in shallower water with the rudder down and it reduces weight in the ends.

You might consider going for a bigger thickness to chord ratio which will reduce stalling.
The shape of the Annie Apple rudder was quite carefully considered to acheive an elliptical distribution.  You could consider scaling it if you are not getting enough grip.
The thinking behind the Annie Apple foils is that the rudder should only be big enough to steer the boat, since the centreplate is a far more efficient lifting device, since it has the benefit of the hull acting as an end plate, rather than a free surface.
The 505 articles on foils are very good, but I think that Bethwaite generally talks a load of tosh and rarely backs it up with any data or facts.
NACA 4 digit sections are rather easier to reproduce and are less critical than the 6x- series sections.  The offsets are in Larson & Elliason and Gutelle, but if you require a copy let me know.
Best Regards,
David
S4789 Y177

[mutt]

DavidG,
yes you are probably right. I'm more than happy with the AA's performance in light airs and on flat water but I guess because of my poor sailing skills I've found the rudder insufficiently reactive in waves. Think I'll build on aobut 20-25% larger in area. Fixed rudder this time to save weight and make docking more errr ... interesting.
thanks for all your ideas everyone.
Matt

[davidg (Guest)]

If you are going for a fixed rudder, how about attaching one side of the mainsheet bridle with a carabine clip, that way you don't need to feed the tiller through the mainsheet bridle whole leaving the beach?
Regards,
David

[MikeDay]

Matt
 
Interesting thread.  Just wondered whether you really want a fixed rudder?  Apart from making launching and landing more "interesting", how are you going to ensure that your rudder never makes contact with an underwater obstruction?  It seems to me that if you hit something with any degree of force, something has to give - either your pintles or the transom.  Either way, you retire from the race, maybe the series and you have some cost as well.  I can't see that the weight and/or cost saving is worth this.  Other fixed rudder devotees might want to take issue here too ...
 
Mike D
N3496
 
 

[Phil Brown]

Fair point Mike.

I've taken a transom out landing at Weston with a fixed blade, it does get a bit shallow. In my defence, m'lud, I wasn't the nut on the tiller that day!


The stock as decsribed in the "How to" section looks easy enough to build and can't weigh that much on top of the blade and tiller

[tedcordall]

Fixed rudder. Don't do it.
 
A lot of the Finns at our club have fixed rudders and they regularly knock them off.
Best result - race over.
Worst result - pintles ripped out of hull.
Makes launching in a blow an unnecessary trauma, too!