rudder for sharpie hull Proa

Any designers out there that have a good handle on what the best rudder foil shape, size and design would be for a 7.70m/26’ sharpie hull Proa with 0.16m depth and 23m2 sail area.
Would very much appreciate your comments

Best Tsan

 

Try asking the question on the proa_file Yahoo group.

If you need bidirectional foil sections they can be found here:
http://www.basiliscus.com/ProaSections/ProaIndex.html

-Thomas

 

Yes, but I don't have a lot of confidence in how those sections will actually perform. The computer thinks they will work OK, but Mother Nature may have different ideas. In any event, they will be very sensitive to small differences in shape near the edges.

If it was my boat, I think I'd go with sharp-edged sections.

 

Hi Thomas and Tom,
Thanks for your answers.
I don’t think I would be happy with these ogive sections. I can imagine though that they might be a good solution for a center/dagger board.
The fact is that a proa while shunting comes to an absolute halt/stop. The steering ability should be therefore provided especially for slow velocity. For lower speed potential the higher NACA sections i.e. 0012 - 0015 are more suitable than the sharper sections 008 - 0010. Please Tom, correct me when I am wrong. When the proa speeds 10 -20knts a not very effective rudder can be disregarded imho.
I have found a convenient solution to turn the rudders 180° (while or after shunting) and even raise the front rudder partly that it can be used like a front dagger board in order to generate more lift.
My idea is to use a 30mm carbon tube balanced at 17% of the rudder chord. That would give me probably a total rudder thickness of 35mm at 30% chord. When I use a NACA 0012 it would ask for a chord of 292mm. With a minimum aspect ratio of 2.2 the span of the rudder would be 642mm.
The rudder area would be 292mm X 642mm = 187464mm^2 that is about 0.19m^2.
As the total lateral plane area of the sharpie hull is only 1.087m^2 I hope I will get away with the rudder size ;-))
The rudder size should be in the range of 8% to 10% of the total lateral plane area which would be only 0.109m^2 (10% from 1.087m^2).
The problem, and especially with proas as they have two small rudders yet go fast, is that the rudder may not be thick enough to fit the required diameter stock.
I wonder if I should think about the Rudder T/E Cut-off Effect: One can “cut-off” up to around 10% of the rudder trailing edge chord with no appreciable effect on drag but with a significant increase in lift. For example, a 5% trailing edge chord cut-off on an NACA aero foil produced a 10% increase in lift and a higher stall angle for the foil.
(Ref: Marchaj C A, Aero-Hydrodynamics of Sailing, Adlard Coles, London 1988, ISBN 0-229-11835-6. – p 223)
The properties of this kind of construction are satisfying. The breaking strength of the carbon tube is 280kp, the force F for the NACA 0012 is 11kp at 20km/h = 5.6m/s.
It could work when my calculations are right ;-(( not very sure about it. I would love to hear your opinion.
Best Tsan

 

If you're flipping the rudders when shunting, I agree that a NACA 4-digit section would be a better choice. You could modify the section by moving the maximum thickness forward. That would let you use a somewhat thinner section and still have the same rudder stock. You could use a program like XFOIL to see the effects on section drag.

 

Hi Tom,
Thank you very much for your advice; it would make a lot of sense.
I could manage a rudder thickness of 32mm and go with a NACA 0010 with chord 320mm.
Could I cut off 5% of the trailing edge as suggested by Marchaj in order to increase lift and make the rudder area smaller?
In that case the chord will be 304mm, the span 669mm and the rudder area 0.20m^2.
I wonder what will happen with a modified NACA 0010 section when the thickness is moved forward to 17% of the chord. Unfortunately I am not familiar with the XFoil program. I had a look on their website and it looked quite complicated to me. Mark Drela where are you? Perhaps you could help?
I found even a Java Applet on Dr.Martin Hepperle's Homepage , but unfortunately I could not download it http://www.mh-aerotools.de/airfoils/java/ws/remoteapps.htm (the applet cannot be started…)
Sometimes these things don’t work here in Greece ;-(( that doesn’t imply that other things are working here ;-))
Perhaps somebody else has a solution?
Best Tsan

 

Whether cutting off the trailing edge is beneficial or not depends on what you want to achieve. The attached polars show the stock NACA 0010 (solid line), the NACA 0010 with 5% of the chord cut off but normalized by the original chord, and the NACA 0010 with 5% of the chord cut off and scaled up so it once again has unit chord.

Simply cutting the trailing edge off results in an increase in drag and a slight decrease in lift compared to the same foil before modification. Scaling it up to recover the same area does result in more lift at a given angle of attack. However, the drag is higher for lift coefficients below 1.0. It does have a higher maximum lift, and this is probably the biggest benefit.

The amount of lift at a given angle of attack is not very important. The lift is what is required to achieve the desired effect. If the lift curve slope is a little less, that just means the tiller will be moved a bit more.

Cutting off the trailing edge, assuming the area is kept constant, turns out to be a tradeoff between minimum drag and maximum lift. Of course, you can get the same maximum lift by adding a little more area. That would get you to pretty much the same situation as cutting off the trailing edge without rescaling the chord. The drag is higher in that case for the blunt trailing edge compared to the sharp trailing edge.

All in all, it looks to me like the sharp trailing edge is the way to go. But there may be other reasons to use the blunt trailing edge, such as robustness to damage and possibly easier construction. The difference in drag is not great, so these other considerations may dominate your choice.

 

Here's one other comparison. I took a NACA 4-digit section that was 9.2% thick and moved the point of maximum thickness to 20% chord from 30% chord. This resulted in the same physical thickness near 17% chord as the NACA 0010. A comparison between the two shapes is shown in the first figure.

The polars show the modified section has a higher maximum lift and a little lower drag at medium lift coefficients. Its minimum drag is higher, though. This is due to earlier transition from a laminar to turbulent boundary layer compared to the NACA 0010. Whether you'd see an actual difference in drag depends on whether or not you can achieve the predicted amount of laminar flow at low lift coefficients.

The second set of polars has the same two curves (long dash and dotted) plus the two sections run with the assumption that transition is fixed at 5% chord, approximating the fully turbulent case. Now there's no difference in minimum drag and the modified section still retains its advantage in maximum lift.

This, and the previous example, shows that you can make fairly substantial changes to the section shape and not necessarily see a dramatic change in the characteristics. The differences will depend a great deal on your assumptions regarding how much laminar flow is present.

Unless the rudder is pretty much unloaded, the induced drag is likely to dominate the total drag. So it's much more important to pay attention to the span of the rudder and the balance of the boat to get the right rudder loading. You have to make some decision as to what section shape to use, of course, but that choice isn't as important as the planform size and shape of the rudder.

 

Hi Tom,
Thank you very much for your illuminations and the comparisons; it’s becoming educative – I like that a lot ;-))
Now it is very clear to me that cutting off the trailing edge is not really helpful in an attempt of decreasing drag and increasing lift. I was just hoping by reducing the chord of the rudder that I could also reduce the span which would result in a smaller plan form size and therefore less drag. But as your polars show (thanks a lot for your effort!) the right way to go is with the thinnest possible section which is of course given by the rudder stock available (30mm).
So I suppose your choice of the modified 9.2% thick NACA 4-digit section with maximum thickness to 20% chord is actually the right way to go. Thanks a lot for your support; it is very much appreciated.
So let’s say this decision is taken; what do you mean by saying it is more important to decide about the right plan form size and the shape of the rudder?
Do you imply a rudder of 32mm thickness, chord 320mm and span 700mm would be too small (rudder area already risen to 0.22m^2, which is 20% of the total lateral plane area)?
OK an aspect ratio of 2.2 is not really a high aspect ratio, but the total lateral plane area of the sharpie hull is only 1.087m^2.
Or would you recommend a more sophisticated shaped balanced spade rudder, something like this here:
http://en.wikipedia.org/wiki/File:Spade_Rudder.jpg
instead of a plain rectangular shape? I think a simple square cut tip works as well as a shallow elliptical tip over the first 75% of the chord blending into a square tip (and is easier to build). Of course one could think about a little leading edge sweep angle of less than 10°, which would make shaping more complicated. Would that be worth to consider?
Best Tsan

 

Hi Tom Speer,
I stumbled over a paper you wrote to the Chesapeake Sailing Yacht Symposium about Hydrofoil Section Design.
You mention there that the compared foils have ‘lift coefficients around 0.3 to 0.5 going up at high Reynolds numbers to 0.2’. Are you talking there about the Cw value? Probably you are talking only about the lift coefficients, aren’t you?
Streamlined bodies ‘normally’ have a drag coefficient of about 0.04; what would be the drag coefficients of the NACA 0009 or the 0010 foils?
Best Tsan