Rudder fwd rake and L/D ratios

I'm looking for data on the sweep angle of attached rudders and the effect on lift/drag. Particulalry data that looked at fwd sweep ( bottom of planform ahead in the flow ) of a rudder that is keel or skeg hung.

Efficiency supposedly increases with some fwd rake when so mounted. Presuambly because the flow particulalry at the bottom is altered with increasing helm angle the fluid being swept up onto the deflected rudder which counteracts the normal fluid flow from high to low pressure aking a shortcut to the nearest low pressure around the end. effectively acting as an end plate. It only really suits a double hung rudder aft of a moderate keel or skeg.


Particulalry Is anyone aware of any data or study anywahere in this ballpark. Or a better explanation of just why a fwd angle on the rudder post is beneficial when the rudder is part of the planform.


cheers

 

Mike
My CFD analysis of foils is limited to JavaFoil or Xfoil. Javafoil has an options setting to cope with foil aspect but I do know if this is just an empirical adjustment or true analytical basis.

I have talked to Leo Lazauskas about making a Godzilla type optimiser for props but he has found a simple foil requires lots of computing power so a while away before we see a true analaytical prop optimiser based on CFD modelling. Rudders would be simpler to analyse but more difficult to specify the operating conditions.

I do have a prop optimiser but it uses foil data generated from JavaFoil or actual test data in combination with empirical formulas for induced drag. It gives good results providing plan shape changes gradually. It could be used for rudder optimisation as well providing the design conditions are specified.

I have no idea about the merit of forward rake. With slender boats you can get some really nice behavior with props depending on shape and angle of shaft but I have not played with forward rake.

This site gives some discussion on wing rake on aircraft:
http://selair.selkirk.bc.ca/aerodynamics1/drag/Page6.html
Reading this it would lead you to conclude that the forward rake would cause earlier stall. Something I know for sure is that you want any rudder to work over a wide angle of attack going forward and reverse.

Rick W.

 

Thanks Rick

There are good charts from empirical testing for sweepback angles and related aspect ratios for best L/D for spade foils and keels. The increased efficiency of the swept forward rudder was for a keel or skeg hung rudder I didn't make that clear sorry.


Here's an indication of optimal foil sweep for the best L/D ratio , from tank tested models looking at keel shapes (Brewer) . Was hoping for something similar on a hung rudder.

Aspect Ratio, Sweepback
0.5, 58.00
0.6, 47.00
0.7, 37.00
0.8, 29.00
0.9, 22.00
1, 16.50
1.1, 11.50
1.2, 7.00
1.3, 4.50
1.4, 2.00
1.5, 0.50

 

Mike,

Are the numbers from Brewer for the geometric aspect ratio or effective AR?

Mikey

 

Mikey
Bear with me I smashed my wrist in an accident a few months ago and just had the last operation this afternoon. So bearing that in mind that I'm fighting the mouse with my left hand and suffering from morphia.

Definitely Geometric aspect as in Span over mean chord.

 

Thanks Mike

Left-handed mousing is tricky, the morphia part sounds nice though. Stopped smoking a week ago, went from 20+ sticks per day to nothing, some morphia distraction would be nice

My AR 0.33 wasn't even in the scope... I plan to use 45 degrees even though the tests seem to indicate 60+ to prioritise wetted surface area. Do you think that is a good trade off?

Mikey

 

Mikey
I am not a hydrodynamicist. It really is a separate calling. At uni we were taught a broad fluid mechanics component and it was frankly little use to us except to hammer home just how unpredictable the field can be when you step outside of well proven parameters.

My gut feeling is that unless you are pushing the last 5% drag that anything that looks half reasonable will tend to be, if it looks radical then it can be either brilliant or terrible and only empirical data will show you which. Unfortunately there is precious little data in the private cruising sailboat arena and often questionable conclusions are drawn from very limited data which then achieves the status of popular mythology. eg Van de Staadt's study on rudders a few decades ago.
For good useable conclusions we need a meta-analysis but we usually lack the number of studies for this since the $$$ just are not available.

Probably the best pertinent advice was given to me by my first mentor when it came to the niceties of hydrodynamics of foils stabilisers rudders and skegs ......." Find those that worked well on a similar vessel and copy them as closely as possible "

I really look forward to a time when CFD will be as simple and useful a design tool as FEA is to us now.


Good luck with the sucking habit, you will be sorely tempted at times.

 

Mike

My Sunday morning coffee outside the house proved too much, it felt so "half" without the cigarette. It's true what they say, that you should change habits when you stop smoking. The chemical addiction is not so bad actually, it gets much better after only a few days. 8 days and I am back on it again

Boat design is by definition a compromise and compromises don't take giant leaps forward. Oh yes, it can happen, but it normally doesn't. It's improvement and optimisation, and although some people posting in the Seaworthiness thread don't seem to agree with it, it's trade-offs. You gain somewhere, sometimes a whole lot, but you also lose somewhere else, sometimes a whole lot.

Using successful designs as role models is just smart and well in line with my philosophy. They provide guide lines for what is good and not, they steer the outcome somewhat but don't decide the end result. Just like with children, and they often become more successful than their parents

I checked the sweepback angle on a number of low aspect ratio boats, The Dickerson 50 has 57 degrees on AR 0.33, the Cambria 44 45 degrees on AR 0.27. Perry has quite a few models with around 50 degrees on AR around 0.35. The Swan 46 has 52 degrees on AR 0.3... OK, so I will use 50 degrees, a skillful compromise

Reading what you wrote about CFD, I can't help posting Leo's wonderful comment

Mikey