Subject: Re: [harryproa] Re: Asymmetric Bi-directional Rudders Again
From: Rick Willoughby
Date: 8/10/2010, 9:26 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Dennis

The 30kts was for the full size boat.  In racing trim it would be possible if you cater for all the issues you get with high speed control surfaces in water.  

I am building the VPP for the little test one.  Initially I will base it on a rigid wing because it is easier to do the polars than multiple soft sails.  After I have that I can get some idea of what speed you might get with it.  

If you get above 15kts the dynamic lift will be significant on such a light hull with flat bottom and then the flat bottom will start to pound a little.    On the big one the lift will not be significant until over 20kts.

GODZILLA has ability to dispel many myths about the virtue of various hull shapes.  Think of how much time you would have spent building models to tank test to get the insights you have in a few days playing with Michlet.  I got to hull V10 over about 4 years before I realised Michlet was freely available.  I coveted it for about 2 years before that because I had read some of Leo's papers.  I even tried to get back into partial differential equations so I could do my own wave analyser.   One thing I did have was 10 hulls already tested to validate the data that Michlet provided.  That gave me a lot of confidence with it before I built V11.

So you get to the lake and you look at those other nicely sculptured hulls with lovely curves and can be confident that your square box with pointy ends will be as good or even superior.  It is only 8% more drag (2% slower) off the very best it could be if it was the best possible shape for the design speed.  In practice you are going to work it over a range of speed and that it where the flat bottom gets less drag than a round section.  You probably won't mind a few bumps if you are skimming along at 20+kts.

Rick
On 10/08/2010, at 10:25 PM, Dennis Cox wrote:

 

Rick,
 
Well, I do appreciate you helping to feed my bad habits and speed tops the list.  I finally decided that on the street was against the public health.    Fortunately, I came to myself before something bad happened.  Doing it with wind (vs a big honk'n motor) and on the water appeals to my green nature, wallet and intellectual curiosity.
As far as 30 knots... you're the first that hasn't immediately scoffed at the idea.  And surprisingly, I'm probably less optimistic than you.  But, if you go hunting for elephants, you bring the biggest gun you can find.  The big guns will still work on the wildebeest if that's all you find.
 
Although I can't quantify it (yet) I do believe your flat bottom hulls can overcome their home-made decor (and low speed inefficiency) and sing on the top-end.  I look forward to raising a few eyebrows in a particular race I have in mind.  It'll be all that much sweeter... being in a boat made for penny's on the dollar compared to them and sailed solo vs a team.  Can't wait! 
 
Dennis

From: Rick Willoughby <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Tue, August 10, 2010 7:46:14 AM
Subject: Re: [harryproa] Re: Asymmetric Bi-directional Rudders Again

 

Dennis

My original interest in the 07 series was looking for something with constant pressure distribution because I played with the design with the aim of getting a constant pressure.  This was based on you target of 30kts target and the potential for cavitation.  

I have done a few boat prop designs for people using little outboards or electric motors on small boats where cavitation is a concern.  I mostly use 16 series foils for my props and did not realise the 07 series existed until I saw that Martin Hepperle had included it in JavaFoil.  I then wanted to find its distinction with the 16 series and found that NACA paper.

The 16 series accommodates the compressibility of air.   With water the issue is cavitation so the 07 series might be marginally better for water.  I have played around and they are too close to make any distinction for the conditions I have considered.

The submergence of the rudder varies so they need fences every so often to ensure the majority of the blade is working.  At design condition there is virtually zero pressure on the pressure face so all the lift comes from the suction face.  If it ventilates the full depth you suddenly have next to no grip.  The is the same for any rudder that can get air.  Used to happen on my keel boat when pressed hard to windward.  The rudder gets air and you round up.

Aircraft use various tricks to keep attached flow.  The fences are the most commonly seen on yacht control surfaces.  I personally like rudders under the hull but it creates issues.  On a well balanced sailing boat the keel or board does most of the work and the hull the rest so if the rudder ventilates it is not noticed.  If you are using the rudders as leeway preventers on a proa then it is a different case.

The size of the fences will depend on the shape of the nose and speed.  The number will depend on how much blade area you want to lose.  I guess it is an argument to go for high aspect deep rudders.  You may only have a couple so the rudders give up at some angle of heal.

The main point is that you realise what to look for so you can analyse issues with some understanding as you see them rather than just accepting that they did not work well.

Rick  
On 10/08/2010, at 8:47 PM, Dennis Cox wrote:

 

Rick,
 
Woah!  Anyone investigating for the application of rudders for a sailboat would have skipped over this paper based solely on the title.  Obviously, you have a very interesting day job!  OR are you reaching supersonic flow speeds with your human powered props?  (dead serious question... no sarcasm)
 
And you are so right!  When "people" generally think of high tech, they naturally think it has to have happened in the last ten years (if not last ten minutes).  What "people" tend to overlook is the very significant pinacle of propeller driven aircraft culminating with the P-51, Bearcat and TA-152.
 
Also Interesting. .. that for a theory that has singularities at the nose and tail and has to be "adjusted".. . provides a superior airfoil with reduced drag and higher critical speed... AND the critical area of modification is at the... of all places... the NOSE. 
 
The key take-away is that the more modern airfoil (16) is ONLY superior at critical speeds... otherwise the 07 is superior.  Or did I miss some finer point(s)???? ?
I'm assuming (please correct if I'm wrong) that the fences must be at some certain minimum depth or the "suction" would pull from right around it.  And that the multiple fences shown in the second picture are solely to account for height changes????  Have you seen any numbers for placement of the fences... say as a function of chord or speed or ????
 
Dennis

From: Rick Willoughby <rickwill@bigpond. net.au>
To: harryproa@yahoogrou ps.com.au
Sent: Mon, August 9, 2010 8:51:31 PM
Subject: Re: [harryproa] Re: Asymmetric Bi-directional Rudders Again

 

Todd

Not My foil.  Here is the story on the series 07:

Lots of useful stuff in old NACA reports.  Sailing boats are about 70 years behind aircraft development when it comes to these bits.  Structurally only 20 to 30 years although some of the development in high speed sailing boats is narrowing the gap.  They are getting closer to aircraft.

One of the issues with surface exposed rudders is the ventilation on the lifting face.  This is not flow separation as normally defined.  It is a water/air surface interface issue.  At high speed where high negative pressure develop it becomes hopeless.  The big foilers use fences on the leading edges to avoid ventilation of surface piecing foils.  With these high lift sections you really need to fence the top of the lifting face to avoid the problem at high speed.  You may observe this problem at higher speed.

The best example of full fencing is shown in a photo down the bottom of this page:
http://www.macquari e.com.au/ speedsailing/ design.htm
Again shows the merit of asymmetric foils if you only need to lift one way.  The multiple fences are needed where the foils are going to move relative to the surface either through boat lifting or with  waves.  Another solution is to operate the rudder below a little faired pod or below the hull if you can tolerate the draft issue.  

If the rudders are not fenced then they become deflectors at high speed and the lifting face is useless.  The Cl drops dramatically.  If I ventilate one of my high lift props it is the same feel as breaking a shaft - nothing.

Rick 
On 10/08/2010, at 3:36 AM, tsstproa wrote:

 

Sorry sent before finished.

I see the evolution to a low displacment low draft designed hull would benefit from Your highly defined foil shapes. Send me set $$$

Time for me to build a different leeward hull.

Todd

--- In harryproa@yahoogrou ps.com.au, "tsstproa" <bitme1234@.. .> wrote:
>
> The design of the deep V is suppose to do that lift hull to windward when slightly heeled. Taking that into consideration small boards low aspect rounded edges flatish profile low lift low drag.
>
> --- In harryproa@yahoogrou ps.com.au, Rick Willoughby <rickwill@> wrote:
> >
> > Todd
> > On you last statement regarding the lifted tack. That is what I am
> > talking about with the trim adjustment. The rudders are set up to
> > give you more lift then you need for some conditions. They are
> > dragging the hull sideways into the wind. So you have the rudders
> > fighting the hull, creating unnecessary drag. Inevitably it will be
> > faster to go where you point rather than ending up higher than you
> > point.
> >
> > Rick
> > On 09/08/2010, at 11:06 AM, tsstproa wrote:
> >
> > > Knowing you need aoa for best flow on different types of foil
> > > shapes. The foil that needs the least amount of aoa for best flow
> > > is again a compromise to over come the drag you describe as
> > > opposing force from each board if in deed this is the case.
> > >
> > > The whole design goal of the linked boards hung on center chord was
> > > just that zero aoa for best possible flow, speed, along with a
> > > neutral helm. How controllable the set could be and what if any
> > > negatives there where. Everyone I talked to said the rudders would
> > > be over balanced and the set up would be uncontrollable.
> > >
> > > So far in most all my shunts leeway is not even an issue. I'm am
> > > almost always on a lifted tack after a shunt.
> > >
> > > I think its about refining the compromises. Not replicating the all
> > > ready known.
> > >
> > > Todd
> > >
> > > --- In harryproa@yahoogrou ps.com.au, Rick Willoughby <rickwill@>
> > > wrote:
> > > >
> > > > Todd
> > > > With your AR around 2 they will have a best lift to drag just under
> > > > 10. If you were to increase the AR to 4 then the best L/D is just
> > > > under 15.
> > > >
> > > > To get the full benefit from the asymmetry you need to be able to
> > > > trim them with respect to each other. You could set them up for
> > > > close hauled but then they will have more lift AND drag than needed
> > > > off the wind.
> > > >
> > > > They have similar performance to a NACA0010 section. The minimum
> > > > drag is about 20% lower than a NACA0010.
> > > >
> > > > Rick
> > > > On 07/08/2010, at 4:25 AM, tsstproa wrote:
> > > >
> > > > > RG14 mirrored...
> > > > >
> > > > > --- In harryproa@yahoogrou ps.com.au, "willoughby_ rick"
> > > > > <rickwill@> wrote:
> > > > > >
> > > > > > I have posted the co-ordinates for the best L/D rudder section I
> > > > > can come up with for AR of 4 and Re# 10E6:
> > > > > > http://www.rickwill .bigpondhosting. com/NACA_ 07-310.txt
> > > > > >
> > > > > > It is worthwhile noting that if the AR is not limited
> > > > > structurally or by draft then the best L/D section will get
> > > > > thicker. So if someone wants to play and can handle higher AR then
> > > > > I can go through the exercise.
> > > > > >
> > > > > > The best L/D for this section occurs between 0 and 0.5
> > > degrees AoA.
> > > > > >
> > > > > > It is VERY important to realise that they need to be able to
> > > have
> > > > > relative angle trimming. If the boat goes best on the wind with
> > > the
> > > > > combined trim greater than 1 degree into the windward side then
> > > the
> > > > > blade area is likely too small to get the best L/D. Typically
> > > these
> > > > > rudders will have the combined area similar to a centreboard and
> > > > > rudder of other boats with the same sail area. With the Harry
> > > > > configuration there should be a tendency to round-up with no
> > > > > rudders until the ww hull unloads but these rudders are not going
> > > > > to have much feel so there may be no indication of weather
> > > helm. So
> > > > > it may require actually observing the angle of trim between the
> > > > > rudders.
> > > > > >
> > > > > > The aim should be to trim the rudders to eliminate leeway.
> > > > > Current aside, the boat should go where it is pointed. So the idea
> > > > > would be to set a course and then slowly trim the rudders relative
> > > > > to each other while maintaining the course to get the best speed.
> > > > > Once this has been done a couple of times the best settings will
> > > > > become intuitive for any particular conditions.
> > > > > >
> > > > > > These rudders will not have a cavitation issue for any
> > > reasonable
> > > > > speed expectation - Cp less than 0.5 in best working region. If
> > > > > thicker section for higher AR then cavitation would need to be
> > > > > checked and the vortex shedding will be more significant.
> > > > > >
> > > > > > The vortex shedding should not be an issue on this section
> > > either
> > > > > as the separation is only on the last 1 to 2%.
> > > > > >
> > > > > > Rick
> > > > > >
> > > > >
> > > > >
> > > >
> > > > Rick Willoughby
> > > > rickwill@
> > > > 03 9796 2415
> > > > 0419 104 821
> > > >
> > >
> > >
> >
> > Rick Willoughby
> > rickwill@
> > 03 9796 2415
> > 0419 104 821
> >
>


Rick Willoughby
03 9796 2415
0419 104 821


,


Rick Willoughby
03 9796 2415
0419 104 821




Rick Willoughby
03 9796 2415
0419 104 821

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