More fin stuff...What's your NACA?

General Discussion
1

I’m just making a fin or 2 for the hell of it. Have read lots here about section and chord etc. Does anyone use a NACA section, if so which ones are in general use? Having access to CNC cutting it seemed like a chance to design in cad and machine a series and make a mould when I found one that works best. Not aiming at ANYTHING commercial, just interested in the process…

Looking at the fantastic fins being produced by some people out there I’ll never be able to compete but it’s a great size for student projects and having some keen surfer students it would really stimulate them…

2
Quote:
I'm just making a fin or 2 for the hell of it. Have read lots here about section and chord etc. Does anyone use a NACA section, if so which ones are in general use? Having access to CNC cutting it seemed like a chance to design in cad and machine a series and make a mould when I found one that works best. Not aiming at ANYTHING commercial, just interested in the process....

Looking at the fantastic fins being produced by some people out there I’ll never be able to compete but it’s a great size for student projects and having some keen surfer students it would really stimulate them…

For center fins I radius the front edge to half of the max width (radius is actually 1/4 of the max width to be precise). I make the fin wide point 25-30% of the distance from leading to trailing edge. And the trailing edge is blunt and 3/4 mm thick. The taper from wide point to rear edge is nearly linear, the taper from leading round to wide point is rounded.

For rail fins I do the leading edge the same, and only foil one side, the same as I would foil both sides for rail fins. The max width is generally further up, closer to 20%, which generates a little more sensitive response.

For any given width and template fin, I don’t think you will find ANY foil that generates an eye opening improvement over this foil. And if you do find such a foil, post about it here, I’ll make myself one and check it out.

Also, I do my fin template design on the computer, and the rest of its manufacture is manual. After a few fins on the learning curve, you can make very good fins very fast this way, and being able to make good fins fast lets you evolve designs quickly. When each iteration on a fin requires reprogramming a three dimensional CAD program, you’ve slowed down your iterations by an order of magnitude, and you just have to spend 10 times as much time getting things dialed in. There is a time and place for that, but I do not think it is when you are learning about fins and foils and templates and stiffness etc.

hth.

3

Hey Rikds,

Glad you brought this up. I am playing with sort of the same thing. I am currently trying to implement the Clark Y foil, here’s a pic (not sure of accuracy of the pic, I just googled it, but it’s close):

It would be intended for use in side fins. It is a rather thick section, at 11% of chord. But, it has great capabilites as an ‘all-around’ foil. It has a relatively high stall angle (13 deg for two-dimensional calculations), can generate lots of lift at slow speeds, and at higher speeds, it has a very good advantage over flat sided inside foils, as it generates less drag (it can penetrate better). If you look closely, it isn’t completely flat on what would be the ‘inside’.

Other sections for side fins I think would be useful if you would want to improve on parasitic drag from the Clark Y are the SD7037 and its offshoots, like the the SA7038. They are 9% thickness foils. The SA and SD foils were designed for low Reynolds numbers, like surfboards operate in. They offer less drag (both parasitic and induced) than the Clark Y, but have a slightly smaller stall angle, and have to be made to 1% tolerances to see a beneficial effect, according to their designer. You can find coordinates for these foils online. The tolerancing issue is what made me go with the Clark Y, as well as it’s overall good performance at varying speeds.

For center fins, the NACA 00 series would be my choice. You choose the thickness as % of chord to get the 4-digit number, like 0012 for a 12% thickness foil. between 0009 to 0012 would seem most appropriate, depending on what you want out of the fin, like drag vs stall angle.

You are fortunate to have CNC fabrication at your disposal. Please let us know what you come up with…

JSS

4

I’ve been designing and cutting fin molds using NACA foils for a few years on MasterCam. Last year I worked with Nick, Graham & Dave at Swansea University on developing a new oftware program that will make designing fin foils much easier and it will incorperate NACA foils as an option. You can check it here:

http://cetic.swan.ac.uk/surfs/

The Clark Foil is very similar to the Next Generation X-2 base foil. But, there is slight more inside surface curvature.

5

Hey tom,

Professor Selig from the University of Illinois has dedicated a lot of his career to low reynolds number foils, I’m surprised more surf folks don’t tap into his vast knowledge bank. Here’s a link:

http://www.ae.uiuc.edu/m-selig/uiuc_lsat.html

And here’s the foil database:

http://www.ae.uiuc.edu/m-selig/ads.html

JSS

6

Some great links there Max, this one is great for the offsets and coding

http://isoar.ca/~andrewm/rc/airfoils/1.html

7

Great site, Rikds. Now I can show pics of some of the foils I mentioned above:

SD7037:

NACA 0012: The thinner ones, like 0009 or so, offer less parasitic drag but less AOA range as well.

JSS

8

INterestin’!

Separation (not “cavitation”!):

Thing is, air is compressible, where water isn’t, so I always wonder how that factors in – it’s not that there’s no translation, it’s just that it seems like it’s gotta be compromised in some way, don’t it

And then what about saltwater versus fresh water? Salt water is less dense, isn’t it?

This might be a fun thread to start talking about the Coanda effect and what constitutes lift, if you think so, JSS

9
Quote:
Hey tom,

Professor Selig from the University of Illinois has dedicated a lot of his career to low reynolds number foils, I’m surprised more surf folks don’t tap into his vast knowledge bank. Here’s a link:

http://www.ae.uiuc.edu/m-selig/uiuc_lsat.html

And here’s the foil database:

http://www.ae.uiuc.edu/m-selig/ads.html

JSS

What do you estimate the Reynolds number to be for surfboard fins travelling in the 13-18 MPH range?

10
Quote:
INterestin'!

Separation (not “cavitation”!):

Thing is, air is compressible, where water isn’t, so I always wonder how that factors in – it’s not that there’s no translation, it’s just that it seems like it’s gotta be compromised in some way, don’t it

And then what about saltwater versus fresh water? Salt water is less dense, isn’t it?

This might be a fun thread to start talking about the Coanda effect and what constitutes lift, if you think so, JSS

Salt water is more dense (see how well you float in the Dead Sea).

Cold water is more viscous.

Water flow typically forms a vortex on the low pressure side of the low aspect-ratio raked fins used in surfing, but this does not imply flow separation off the trailing edge.

11

Hey janklow,

Separation can happen in air or water, it’s just the separation of the boundary layer away from the foil. The separation is something that can happen especially at low reynolds numbers, and is a huge drag. Different from cavitation, much more subtle in effect (cavitation is violent). To avoid separation, turbulators/vortex generators are used to ‘trip’ the boundary layer into turbulence before the separation point, so the boundary layer stays attached to the foil surface, making for less drag. Turbulators are usually zig-zag pieces of tape:

Cavitation, by definition, can only really happen in a liquid. It’s when such a low pressure is generated that a small part of the liquid actually changes phase into gas, and then collapses again back to liquid. The shockwave caused by the formation and collapse of the ‘bubble’ can be really damaging, or really beneficial (like the non-invasive breaking up of kidney stones with shockwave-lithotripsy). Cavitation is also how ultrasonic cleaning baths work.

I’m pretty sure that given the above, it’s can be seen that cavitation cannot happen on a surfboard, I’m pretty sure the pressures generated will never be that low (I could be wrong). I think what people call ‘cavitation’ is actually ‘ventilation’, where the low pressure side of a fin is low enough to ‘suck’ air down into the water from the water/air interface. Another misnomer is the ‘cavitation’ plate on an outboard motor. It should actually be called a ‘ventilation’ plate, as you are just trying to prevent the motor from sucking down air from the surface down to the prop…

I’m not sure what is more dense, fresh or salt water, but my guess would be saltwater is more dense, that would explain why we float better in saltwater, it has more solutes in solution than fresh, but I don’t have hard facts, just a guess…

About the Coanda effect (aka Newtonian Lift), I think it is a perfectly valid theory. So is the Bernoulli effect mediated conventional theory, the explanation of lift as pressure differences. I think a good analogy to this is when I was back in undergrad, I was puzzled by the ‘dual nature’ of light, how one theory explains light as particles, and another as waves. So I asked one of my physics professors which one light was. His answer was: “light is light, and it is satisfactorily explained by both theories, depending on how you are looking at the light. In the macroscopic world, it looks like a wave, microscopically, it looks like a particle. It is just light; humans gave it its ‘dual nature’.”

So, in my view, lift is lift, and both ways of explaining it (differential pressures above and below the foil [bernoulli], and redirection of flow and the subsequent reaction force [coanda/newton]) are valid, depending on how you approach the problem.

If I am mistaken on the differences between the lift theories let me know, but that’s how I currently undertand them. I am not an expert in aero/hydrodynamics, by far, I know just enough to be dangerous…

JSS

12

Hey blakestah,

I made a spreadsheet to calculate stuff like that, it is attached. Enter your parameters for a trapezoidal fin in the green cells, and it will spit out some calculated numbers, a planform drawing and a chart of Reynolds Numbers vs MPH. So, Re numbers vary with the chord you choose. I used a water temp of 70 deg for the Re number calculation, but changes 10 degrees either way didn’t change the numbers much.

It wasn’t until I made the spreadsheet that I realized there must be a ‘sweet spot’ for a fin’s aspect ratio where drag is minimized, because for a given fin area and speed, if you increase aspect ratio, you can potentially get more lift for less drag. But as you increase aspect ratio, chord lengths decrease, and Reynolds numbers drop accordingly, and you get an increase in drag with the reynolds number drop. So, there has to be a small range of aspect ratios that work well for the speed you plan on traveling (big vs small waves) that will give you the least amount of drag for a given lift force.

Play with it, it’s a handy tool…measurements are in inches on the sheet.

This sheet also made me realize that fin sweep/rake is mainly used for adjusting a fin’s stall angle, but I think it is a very inefficient way to do so, as the penalties of sweep are large increases in induced drag, and also decreased lift from the same fin area. The ‘spitfire fin’ thread made a lot of this make sense in my head, but I could be wrong…

JSS

13

Hey, what’s y’all’s understanding of Coanda? (Since I know y’all both speak southern)

HOW IS LIFT GENERATED?

There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments. To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail.

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton’s Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface’s part in turning the flow leads to an incorrect theory of lift.

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
14

Hey janklow,

My understanding of Coanda is that the redirection of airflow is what causes the equal and opposite force of lift. The bottom of a wing essentially shoves air downward, and the top of a wing does the same, although in a much more subtle manner, explained by boundary layers. I know that is a very crude way of explaining it, but I think I’m on the right track…

Which brings up separation, and the awesome pic you used above. When that boundary layer separates, the flow over the top of the wing isn’t being redirected as strongly downwards as it was before when the layer followed the wings upper surface closely; so much less lift, much more drag… The turbulators ‘make’ the air follow the upper surface to avoid separation.

JSS

15

Yeah, that’s what those do.

I think canards (quads, Twinzers, the Saab Viggen fighter and numerous others) in the right configuration use the downwash off the flat side of the ahead wing onto the foiled side of the rear wing to augment that Coanda attachment and associated downwash off the trailing edge and integrate the boundary layer over the foil to prevent drastic separations—I think also multi-element wings like those on F1 cars, slotted fins, I think Herb’s Superchargers

16

Absolutely, could not agree more. They all use mutiple elements working in concert to get more lift than a single element could. The ones on F1 cars are the most drastic case, their goal is maximum ‘downward’ lift, almost regardless of drag penalty, and the curvature/camber in each element is extreme. The rearmost elements on those wings can have 50+ degrees angle of attack! Same thing on a 747, look at the lowest element of the trailing edge flaps, pretty extreme AOA…

OK, I have to get to class now…

JSS

17

Hey Guys,

Sorry this is sorta off topic but I need some help. About 2 months ago I ordered a custom old school fish, modified just a bit with added wings and quad fin set up. When I went to pick it up the other day, he had only put in a twin fin set up. To “fix” the screw up, I think he’s saying he’s just going to put the other 2 fins below the twin fin set up. I was wondering if that will be the same result as a regular quad (lokbox speed dialer) or if I should just say that the board is not what I ordered and paid for. There are some other things about the board that are not what I expected. It really seems like just a thicker version of his “fish” shortboard shape than a slightly modernized retro fish, which is what I asked for.

I guess I just need confirmation in my gut reaction to not accept the board, because I’m not one to be picky. But this is my gift to myself from the money I made teaching surf lessons this summer.

Thanks,

Katie

18

Hello Katie, You’ve got a good question that many of the participants here can help with but seems you’ll get more replys by starting a fresh thread just for your question.

19

I like these threads and I’ve learnt a lot from this one already.

Two things bother me, though:

  1. Surfboard fins are usually raked

  2. Foiling is often changed between base and tip.

In my view foiling in fin tip has been largely ignored (one just has to look at popular fin ranges)

Do you just carry the foil from the base to the tip, following the rake?

Is there an aeronautics or hydrodynamics model that incorporates rake?

20
Quote:
I like these threads and I've learnt a lot from this one already.

Two things bother me, though:

  1. Surfboard fins are usually raked

  2. Foiling is often changed between base and tip.

In my view foiling in fin tip has been largely ignored (one just has to look at popular fin ranges)

Do you just carry the foil from the base to the tip, following the rake?

Is there an aeronautics or hydrodynamics model that incorporates rake?

The general trend is…

Thin the fin near the tip as the chord length decreases.

Make a line of max camber points at the same fractional distance from the leading to the trailing edge ie: in one case you may mark points that are 25% from the leading to the trailing edge at all depths.

Carry the same foil all the way up.

You can see this is how most fins are done if you follow the foil lines on unfinished fins.

This fin was foiled by Rainbow using such a generic foil (which is the same foil they use by default unless the customer specifies otherwise).