NACA airfoil

Can someone give me a general description of the difference between an
NACA 0007 and NACA 0006 airfoil, and any advantage/ disadvantage one has over the other? lift/ drag, etc? When one performs better versus the other?

this is for a daggerboard on a catamaran.

thanks,
Rob

 

these are identical foils but one is 7 percent (of chord length) thick, the other is 6 percent thick. They will behave very similar but the thinner one will have slightly less drag and yield a very slightly higher Cl max before stall. You usually choose the thinnest section that you can make strong enough. the only reason to choose the thicker foil section is to make it stiff and strong enough for your chosen materials.

Of course you can always make it strong enough even with a thinner section, but that means more cost, and/or more weight. So like everything else, it is a trade off. Often much thicker sections are used to keep weight down, the effect just depends on how fast you expect to go, the faster your average speed, the more noticeable the extra drag of the thicker section.

One other thing is the thinner section usually has a more abrupt stall, with less warning, than the thicker section. However, there will not be much noticeable difference between 6 and 7 percent, you will notice it if you went to a 12 percent thick one.

 

Here you can find a lot of data for different profiles: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090976_1993090976.pdf

NACA 0007 is not included, but it will be between 0006 and 0009.

Why do you want to use the 00-series in the first place? It is typically only used when good stall performance is needed and then it is typically much thicker. NACA 0012-0018 is often used for rudders for this reason.

Keels often use NACA 63 (or even 64-66) series, which have lower drag at small angle of attack. Since you have a catamaran with daggerboards, an asymmetric profile would be clearly better providing low drag bucket even at normal beat angle of attack.

 

Thanks for the excellent reference!

My understanding is the drag bucket requires maintaning laminar flow over a substantial portion of the airfoil. This requires that foils be accurately fabricated with the desired section, particularly near the tip, and with a sufficiently smooth surface. The surface also needs to be kept clean in use. Otherwise the drag will be about the same as a similar thickness 00XX foil.

 

usually typical laminar flow foils with surface roughness (such as growth on it) will have VERY poor performcance. Their L/D drops off to well below that of a simple 00xx foil. On a cat kept out of the water you can usually keep the surface clean and polished, it would be okay, but if you have less than optimum surface, the 00XX foils are still desirable.

 

I know that laminar flow profiles are more prone to surface roughness and fouling, but is it really typical that they are worse than 00-series. I don't know any keelboat designed in the 80's or later, which would have a 00-series keel. Most seem to have 63-series keels. Are all the designers just ignorant about the roughness and fouling effects or are the 63-series most often better? For rudders 00-series are still often used.

This article claims, that 63-series will not be worse than 00-series after slime/roughness at normal angles of attacks, but 66-series gets a bit worse. I don't know how the curves are made and how bad was the roughness.
http://www.sponbergyachtdesign.com/Keel and Rudder Design.pdf

 

Agreed. Any designer who knows what they are doing would not use the simple 00xx sections for their keel designs. Some of the better guys used something other than 63-series for their keels, but not simple 00-series, since the '70s.

 

How close are the sections of the actual keels as built to the section shape the designer claimed to have used?

 

This depends a lot on the yard. I faired the keel of my boat (C/R from 2005) last spring. It had too blunt leading edge and the trailing edge was about 15 mm thick. Otherwise it was very close to 63A-series. A few friends of mine have done the same to their boats and it seems that the too thick trailing edge is the most common problem. However some keels from the 80's can be really bad and asymmetric.

Before the fairing I did quite a few simulations with xfoil and some also with CFD. The too big leading edge radius wasn't really that bad according to xfoil. I also tested some other possible errors in 63/63A-series and it didn't seem to make a big difference. You really have to make big changes in order to make it worse than 00-series at low angles of attack. However the 15 mm thick trailing edge is really bad (and hard to simulate: http://www.boatdesign.net/forums/hy...es-blunt-trailing-edge-xfoil-xflr5-40889.html)

 

I would say yes. They use the information they are given, and much of the testing done for boating yields somewhat different results (oversimplified) than found for aircraft. There have been many aircraft designed with a nice low drag laminar foil, and than found after they flew it around, and it got dirty, that the crew was nearly killed because the laminar flow was unreliable and trim and control was marginal. So much more realistic testing was done to account for the loss of performance. If a keel foil has slightly higher drag, and less lift, on a cruising yacht I doubt anyone on board would notice.

that is not exactly what it says, Fig 10 "its entire low drag region is eliminated, and the drag actually exceeds that of more modest foil shapes" (the 00xx foils). Though the 63-series is not as severe, it still show higher drag than the 00XX foil shape. Also it shows the best L/D for both laminar and turbulent flow is the 00XX foils in both conditions. This results is different than often found in aerolab testing, but if you are designing for best L/D, according to this data, there is no reason to use the laminar flow foils at all.

Typically you are designing to a particular lift coefficient, and you pick the best foils for those Rn and Lift conditions. sailboat operate all over the map at different points of sail in terms of required lift coefficent, so it would be difficult to say which is best, type of race and weather conditions would make any choice a gamble. But typically the laminar foils when operating in turbulent conditions (high surface roughness), they will have more drag for the same amount of lift, and at very high angle of attack, the laminar foils have much less lift and far more drag. I.E. the laminar foils are almost ineffective at very high angles of attack, and are are much more prone to stalling at lower angles. Would this be good for a sailboat in varying conditions?

If you look at the curves of lift even in the laminar flow plots, at high lift coefficients the 00XX foils are better. It would take very low wind conditions, and low loads, to take advantage of the drag bucket of the laminar foils narrow drag bucket, even if you could maintain laminar flow (always cleaning and polishing the keel and rudder at each race).

The essay, while interesting, is a rather superficial look at the demands of foils on a sailboat hull, there are a lot of variables not explored in detail. Some of the statements can not possibly be accurate unless they have found a way to suspend the laws of physics. I am not saying there is not a place for laminar foils on keels and rudders in certain types of specialized racing sailboats, but it always struck me as a poor choice for most applications.

I spent many years doing aerodynamics design for an aircraft manufacturer in the past, and there was a lot of time and money spent on the wing desin and selection used on modern aircraft. Not a single manufacturer has built an aircraft using a "stock" airfoil shape in the last 40 years. Everyone has used costly specialized CFD programs building up three dimensional wing shapes that continuously varied from root to tip. A lot of agony and arguments are spent over not only optimizing the pressure distribution over the whole surface, but also compromising it at various angle of attacks, speeds, and flow conditions during yaw. Stall and drag was only part of the considerations, not only is the surface curvature continuously varying, but so is twist, LE radious, and TE shape. There might come a time when keels are designed the same way, but that has not happened yet. Probably because sailboats have no commercial or military application, but are built only for sport or recreation. Perhaps if someone developed a 3-D pressure distribution program that is either cheap or free, you will have some serious hobbyists playing around with better keel shapes.

 

As Joakim says, depends on who built it.

A good builder will have accurate templates and will check the plug against those. Then the keel will be faired using the same templates.

For a production boat you would almost always need to have the keel faired after the delivery, since most production builders don't factor in the cost of fairing a perfect keel into their price.

Some keels are cast oversized and milled to the designed shape. This is very costly.

 

True, but I think on racing yachts the benefit of 63-series etc. over 00-series is clearly proven by the fact that 00-series are not used at top level racing. Using 63 series on production boats and pure cruisers could be just fashion, but I'm not ready to believe that yet.

If you look at the Fig 10, you'll see the 63-series being identical to 00-series in the "after bottom paint" conditions until Cl~0.7, which is already a very high value for a keelboat keel. Typically you would have 2-5 degree leeway on a beat and you have to remember that keel is there also during reaching and running with close to zero leeway. For a keel stalling angle is very seldom a problem (it may be for some new high aspect ratio keels), since you just don't sail at high leeway angles. For rudders stalling is important and I have heard that TP52's went back to more traditional (00 or 63??) rudders after trying something more radical. Most production boats have 00-series rudders.

I don't know how the Fig 10 values are derived, but I don't quite believe adding bottom paint is enough to get Cd=0.011 at zero angle. Of course there is no limit how badly you can apply a bottom paint...

You are quite limited for using the best L/D for keels, since changing leeway also changes the drag of the hull and rig geometry. Also a sailboat has many operating points and you can't adjust the keel like you do on most airplanes.

The same is true for ship propellers. I think the main reasons for this are that both have a very limited operating region where efficiency needs to be optimized and both are asymmetrical. Of course they need to operate also outside the normal operating range, but only for a minimal amount of time. Also they are most often adjustable (only blade angle for propellers), which helps a lot for operating outside optimized region.

Keelboat keels and rudders have to be symmetrical and the operating region is very wide. This very much limits the optimization. Still a lot of money and effort has been used to optimize the keels and rudders of e.g. AC and VOR yachts. I don't know what kind of keel profiles they use, but certainly not 00-series for keels. VOR yachts spend several weeks at sea thus there must be some contamination on their foils.

 

There is a lot of data and discussion about roughness in the link I gave earlier (starting from page 22): http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930090976_1993090976.pdf

When comparing this data to keels one must remember that keels operate at rather low Re (e.g. 6 knots and 1 m chord Re=3e6). E.g. Fig 19 shows that almost 0.03 inch (0.8 mm) protuberances are needed for turbulence transition at that Re while at 1e7 (still low for commercial/military airplanes) only 0.005 inch (0.1 mm) is needed.

The standard roughness used by NACA is 0.011 inch (0.3 mm) grains at the leading edge. This size of grains are used in 50-60 grit sanding paper.

A bottom paint is not going to be even close to that kind of roughness unless there are 20 years of worth old cracking antifouling or more than just a bit of slime.

In Fig 20 painting the foil with camouflage lacquer caused transition at Re 2e7 thus nowhere near sailboat keel values.

In FIG 12 measured Cd at zero angle at Re=6e6 with standard roughness and 12% thickness for 00-series is about 0.0097, 63-series 0.0092 and 66-series 0.0095. 63- and 66-series have lower drag at all the thicknesses

So where does the values of the Vacanti paper Fig 10 come from and how did he get more drag for 66-series? Why are the Cd's over 0.01 from just bottom paint while camouflage lacquer needed almost ten times higher Re to show an effect and quite rough surface is needed for the turbulent transition at keel Re?

One answer may be that NACA standard roughness is applied only to the leading edge (first 8% of the chord). Applying the same roughness for the rest of the chord would add the drag. But is bottom paint really enough to cause transition right at the leading edge at Re~3e6?

 

Any decent design office will use something other than the 00-series when drawing their keels, whether it is a race boat or a cruising boat. There are sections that are very good for racing applications, and others that are used when designing longer, shallower keels. None of them are 00-series.

A competent designer will even do this for production boats. Whether the production builder follows the design or not is another matter.

One other issue with production keels is they are often shipped laying flat. This causes them to sag and bend, making them assymetic. So you need to get out the power planer and the templates whenever you buy a production boat.

 

No doubt there are much better shapes for keels than 00XX foils. These were some of the first foil shapes developed and tested going back to the 1930s. There are other advantages to the 6XXXX foils, their thickest part is further aft, allowing more volume for ballast. Even if drag was similar, that alone would make it a better choice. And one would have to allow that for racing dinghys and trailer sailors, they would have a different surface finish and maintenance standard than deep water cruisers.

Yes, I agree, the paint alone should not be tripping the boundary layer at normal sailing speeds. It is just one of the many variables that always shows different results depending on who does the testing. There has not really been any standards developed for testing foil shapes for boats. All we have are the old NACA/NASA shapes done in a wind tunnel for aircraft use.

Also, typical yacht keels are MUCH lower aspect ratio than what NASA had in mind when they developed their profiles. I think the NACA/NASA profiles is the only readily available shapes with data, so designers tend to pick the best they can from these profiles. It would be interesting to see if some organization, perhaps a university, could develop some optimized foil shapes specific to sailboat keel design, standardized for use normal sailboat operating conditions. And perhaps eventually there will be low enough cost 3d CFD software where whole 3d keel shapes can be optimized so both professionals and us amateur builders would have something better to draw from.

It is also possible to develop foil shapes that take advantage of laminar flow when it can be achieved, but also have good performance when in turbulent flow fields. There was some serious "non-commercial" airfoils developed by skilled amateurs using low cost software on the PC that developed some new foil shapes for both model airplane builders and home built aircraft that did just that. Now we just need to get someone to do that for foils used in dinghys and yachts. There was one time that I could have done that, but it takes a huge investment in time, that I do not have, and I am not current with what is available in CFD tools for the PC.