Another Rudder Question- The Wing

[John Stevens]

I've followed the advice here and have a nice foil shaped rudder. I'd like to attach the wing now. Should I foil the top? Bottom? Both? Also why?

TIA,

John Stevens

[Andy P]

Symmetrical NACA 000x

The boat pitches, so the foil has pos and neg angles of attack.
If the horizontal foil is acting as trim angle control, then the foil should be a follower and naturally attain zero degrees angle, but in order to get there may be + or - .

For flying hydrofoils, or if you consistently want more lift, then asymmetric foils OK

John,I believe that you wrote earlier that you were making a test rudder to see how it worked compared to the normal rudder for your Precision 18. In light of that I would suggest trying the simplest thing first-which is sailing the boat w/o any wing or end plate just to see the performance difference. That would also give you a reference point when you attach a wing to the experimental rudder.Is there another Precision 18 you could sail with?
Second thing I'd try would be an endplate similar to that shown by Pierre Gutelle in "Design of Sailing Yachts" on page 52,top figure on the left. He shows an endplate that I would make approximately 2.5" wider than the foil on all sides and going all around it. I would fix it parallel to the static waterline. There probably won't be too much load on it for testing so you could probably just screw it in place.You could make it ot of 3/16" G10 epoxy glass you can get in small quantities from McMaster-Carr.Taper the TOP edge a bit.In fact, you might be able to make it so that it could be removable out on the water for testing-since you live in Fl. it shouldn't be too cold(!).
To keep it simple I would experiment with the endplate on the bottom and possibly some form of wing if the endplate is not satisfactory. But if you want to see a treatment of the top of a transom hung rudder that is an endplate faired into a hull check Guttelle page 186. Another upper device is a fence recommended by Frank Bethwaite on page 240 of "High Performance Sailing"-mainly for high speed sailboats but both these ideas work to prevent ventilation of the upper part of the rudder. I personally think you'll be fine with an
endplate or small wing on the bottom of the rudder.
Since you're doing this is to keep the rudder from hitting bottom before or as the keel does I'm betting that a relatively small endplate will work by increasing the effective aspect ratio of the rudder blade in the abscence of the part of the rudder you cut off allowing nearly the same rudder performance with a shorter span. I don't think a higher aspect wing will offer significantly greater benefits w/o also causing headaches such as imparting vertical loads to the gudgeons and pintles-the simpler the better for your purpose in my opinion.
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You had mentioned vibration earlier; this is a quote from Lars Larsson and Rolf Eliasson's book " Princibles of Yacht Design" page 127; they have a chart on page 128 showing the best cut off for the trailing edge of a rudder or any other type of foil:
"The figure shows the amplitude of the vibrations at resonance for each case relative to those of the square cut off. It can be seen that if the edge is symmetric and wedge shaped(cases 2-5) the total wedge angle has to be 30° or smaller.For 90° and 60° much larger vibrations occur than for the square ending. This is also the case if the ending is rounded in some way ,which it normally is, if the trailing edge of the keel or rudder is left without attention. An aymmetric cutoff is somewhat more forgiving, and a 45° cut-off is acceptable, provided the corner on the cut-off side is smoothed."

[wet feet]

Phil Bolger has used an endplate in a number of his designs and has been quoted as saying that since first doing so,he has come to view a rudder without such an addition as looking unfinished.The idea of a fence just below the hull is good if you can be sure that it will always remain immersed.The endplate should not suffer in this respect but it is more prone to damage.

[John Stevens]

Can anyone post or email me a picture of what Bolger's endplate (wing) looks like?

Thanks,

John

Two excellent shots of slightly different Bolgerrudder/end plate combos(scroll to find; top url has stock ads but scroll down to the 4th boat pix after the ads):
http://mkstocks.tripod.com/boats/as19/

On this next url scroll down patiently to the 17th picture:

http://www.chebacco.com/chebuild/bui...structions.htm

[sorenfdk]

Originally Posted by Lorsail

Two excellent shots of slightly different Bolgerrudder/end plate combos(scroll to find; top url has stock ads but scroll down to the 4th boat pix after the ads):
http://mkstocks.tripod.com/boats/as19/

On this next url scroll down patiently to the 17th picture:

http://www.chebacco.com/chebuild/bui...structions.htm

The shots might be excellent, but the rudders... What's the name of the NACA section used on the second one?
An end plate may be a good idea if your aspect ratio/rudder area is as low as in these examples, but if not I wouldn't recommend it.
Just think of the loads an end plate may/will experience and how this will affect the rest of the rudder structure.

[dionysis]

Perhaps you could have a look at the wingtips of aircraft. Their function is similar to rudder wingtips: to minimise induced drag.

Check out this site.

[John Stevens]

I made two different wings for the new rudder. I tried them today and they both worked equally as well. I tried it without the wing and I couldn't keep the boat under control when the rail was in the water. I really liked the ease of steering, it was effortless with the wings. I plan on building one more a little larger to see how that handles.

I've put the foil on the bottom side. Should the foil be on both?

Here is what I have now:

If both work equally as well I'd choose the one least likely to catch weeds....Didn't quite understand your last question but if you mean do you need a foil on the top of the rudder I'd say no if you have good control with the lower wing. Where you might need more help is if what you cut off was more than 20% of the original rudder area...
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In the picture it seems that the rudder is a kick up type? I presume when using it with the wing/endplate you'll have it securely locked in place?!

[John Stevens]

Yes the rudder is a kickup type but there will be no need for it to kick up. The rudder itself is NACA 0012, I was asking about the wing. I put a foil on the bottom of the wing ( facing the water). Should I have the foil on the top side of the wing instead or both sides? The Bolger picture looks like the wing is flat with no foils but it's hard to tell.

Thanks,

John

[Skippy]

John Stevens: I put a foil on the bottom of the wing ( facing the water). Should I have the foil on the top side of the wing instead or both sides?

That's an interesting question. The wing, rudder, keel or whatever is a foil. The foil shape refers to the whole thing, both sides. But the pressure difference on the low-pressure side of the rudder (or keel) is greater than the difference on the high-pressure side. So is the low-pressure side more important? Maybe it would be good for the wing section to be cambered, Convex up/concave down, so it works better on the low-pressure side. On that right-hand picture, I would make the chord length of the wing equal to the chord length of the rudder.

[Sorry, I originally said camber convex down/concave up.]

Edit: Come to think of it, Convex up/concave down would pitch the boat forward, so maybe the other way or no camber at all does make sense.

[SuperPiper]

John:

The rudder is a kick-up. Will the foil act to keep the rudder in the down (or neutral) position?

My kick-up is held down with a bungee cord. From time to time, it is necessary to pull the rudder back down: it drifts up due to the drag. I think that your foil would make a kick-up rudder self-leveling. That would be a definite advantage.

[tspeer]

Originally Posted by SuperPiper

John:

The rudder is a kick-up. Will the foil act to keep the rudder in the down (or neutral) position?...

Probably not much, if at all. The force on the foil will be more or less lined up with the axis of the rudder, especially when it starts to kick up. If anything, it will tend to make the rudder kick up because of the extra drag at the tip.

John, any more test results?