Rudder & Daggerboard Foils
 by Warren D. Messer - Seattle, Washington - USA

Designing NACA 0000 Foil Sections

I do not claim to be any sort of expert on the subject of foil design. I have only read up on the subject and experimented with various tools that I now own or have downloaded off the internet. With that said, I only claim to be an interested observer and tinkerer who likes to figure out how things work; and spread any information that I have gained to other tinkerers of like mind. Ducks of a feather so to speak.

When I was close to finishing the construction of my first 8ft Nuthatch Pram, I started thinking about what type of rudder and daggerboard I would need and how big each would have to be. That started a series of long searches on the internet for informational theory, and articles on foil design and construction. I found some good sites here and there, and gleaned as much as I could about the concept of foil design. I even read several books on sails, hulls, and foil design by C.J. Marchaj. The books are intended for more than the casual observer; and/or those looking for an aid in getting to sleep. It wasn't until after I had copied down a very complicated mathematical formula for foil design, to put in a series of cells in a spreadsheet; that I came across the NACA4gen program online in a zip file.

A quick download and install brought up a very basic program in DOS. Luckily, I'm old enough to have cut my computer teeth (still have most of them) in the dark days of DOS. Always wanted a Mac in those days, but couldn't afford one. Do you remember the blank screen and the blinking prompt? Or all the sticky notes with obscure symbols and letters stuck to the monitor to remember how to make DOS work on your \$5000 IBM XT?

The 4Gen program just asks a couple of basic questions with "mouse less" keyboard strokes. What's the NACA number? Enter your XXXX number and then the "cord" length (fore and aft width of foil, not thickness). The program then spits out a diagram of the foil and a text box to the right with the X distances on the cord starting at the leading edge; and the Y (+/–) offsets from the cord. There's not much to the program, but the information is very helpful. You will be generating several program inputs/outputs until you find the "thickness" you need for your foil. We will come back to this again later in the story.

A little regression here, as older people always seem to do. Stories always need a bit of filler to make a point. Think of it like fiber in your diet. Before I ever started looking for foil information, I had played around with several blanks of 5/4ths cedar. I knew what a foil should look like from sailing other boats, and also from spending large sums of money for the "go fast" fins I used on my sailboards. "Try this one out, everybody at the Gorge is using it!" I have some very nice fins in the loft of my barn that the spiders hide under now. So I would lay out some lines on the blanks and rasp away, saw away, plane away, and throw away. I eventually got one that looked ok, but the thickness was a little off from side to side. I did some more rasping to even it out; but what looked like a foil close to an NACA 0010 when I started, became an 0006. I had other things to do so I set it aside. When I came back to it again, the foil had warped to port. I read an article online about cutting the cedar blanks into 1x1's and gluing them back together after you change each one's location in the pile and swap ends on alternating sticks. The alternating method should reduce any chance of warping that may occur later. I have one untouched blank glued up, but I got strip foiled, Hallelujah, before I needed to use it.

I got inspired one day cleaning up some 1/8" scrap from making one of the boat models you see in other stories, hanging on the wall in the background of my shop. I stacked several of the pieces on top of one another and the light came on. Now I only needed to find a way to figure out how to do it. The 4Gen program gave me the numbers, but I needed to add the necessary layers of 1/8" plywood to the foil's cross section. Looking at the foil outline in the 4Gen program gave me an idea. I wondered if the program I used to design the hulls of my boats could also generate the bezier curves of the foil? No scab goes un-scratched here.

I copied down the numbers off the monitor screen (use the screen capture program linked at the end of this story and print it out) onto a scrap of paper, and used them to set up the "guides" (adjustable grid) on the drawing page of my program. The main information for us that 4Gen spits out, is the "thickness" of the foil and how far back from the leading edge that point is. On the bottom of page two of the accompanying PDF file for a NACA 0010 – 12 inch foil; the maximum Y(+/–) measurement is 0.600 inches, and the X distance is 3.60 inches from the leading edge. That gives us a foil that is 1.20 inches thick and a cord of 12 inches. The thickness will give us problems as that is only 9.6 strips of ply, but is close enough for this story. As I alluded to earlier, you will be generating several 4Gen lists as you figure out the foil that you want to make. You will want to see what the 4Gen program generated, and what was the overall thickness for each iteration; and how close that doubled Y number was to a full thickness stack. An odd number is ok, as the very center can be a single sheet with even numbers of layers on either side; as in the PDF example.

Note: I found the screen capture program Captura 1.0 on the internet and have been very happy with it's functionality. It's a very simple program to use with the 4Gen program; and any other uses you may have. After you have input your NACA number and cord length, the program generates the foil shape. Use the Control-C command to exit the program, or type QUIT. Open the Captura program; use the select area command and crop out the DOS window with the 4Gen information. Save the file, and remember to add the (dot)jpg file extension. I have added a screen capture folder in my Irfanview photo directory so I always know where they will be. You can then either print out the file, or run it minimized in the drawing program window as you do your figuring.

Time for another side trip on computer programs. The one I use everyday is no longer available, and I can't find anything out there that comes close to the ease of use and functionality that it has. I was still using Win3.1 on a 386-25DX when I first got it, and it still works on my P4 XP machine. Except that Bill can't leave well enough alone, and now I have trouble putting text in my drawings; and only with M's and W's. Something to do with OpenType fonts, the bastard child of TrueType. I was Beta testing a 2.0 version of the program, but Win95 came out before it was bug proofed; and they shelved it, even though it worked fine on 95. Both Adobe Illustrator and PageMaker would have been shelfware years ago if they had finished ProDraw 2.0 on time.

You are in luck, in that the drawing program in OpenOffice.org and the vector drawing program Inkscape can do the tasks for laying out the foil cross sections. Both are FREE, but neither touches ProDraw in power or ease of use. Look for the links to both programs at the end of this story.

Note: The rest of the story will assume that you have one of the vector drawing programs I have talked about or familiarity with the one you already own.

With your drawing program running and on a blank page, go to where you change the paper size and orientation. You will be using 8 1/2 x 14 inch paper in the landscape format. Also find and set the rulers to inches. We will start by bringing in the first two "guides". Usually dragged on to the drawing area from the top and left hand borders. One will be a vertical, and one will be a horizontal guide. Place the vertical guide close to the left hand side of the page so you have enough room to draw a 12 inch foil.

Find the "reset 0/0" button on the drawing page and move it to the point where the guides cross. The button is usually in the upper left corner of the drawing boundary. Drag it to the intersection of the guides. Look in the menus for the "snap to guides" entry and select it. Look for the "Zoom" function of the program and zoom in on your new 0/0 point to see that it is actually on the mark. Or find the "edit guides" dialog box and make the adjustments there. Both should be 0/0.

In drawing #1 of the PDF file that comes with the story, I have added vertical guides at the maxim thickness point (3.60) and at 6.0, 9.0, and 12.0 inches; and a horizontal guide set at (0.600). Once you do this a few times and prove to yourself that this method works; you will only need the leading edge, maximum thickness, and trailing edge vertical guides lined up on the horizontal centerline guide.

Now comes the "fun" part. Don't you hate it when people say that, cause you know it ain't true. You need to find the "Bezier Curve" tool to draw the "three point" line in drawing #2, to form one half of the foil's outline. Good time to look in "Help", as if it ever does. In OpenDraw in OpenOffice.org, it's in the tool bar on the left side of the drawing page. Grouped with several other tools that slide out to the right. Hold the mouse curser over the lower left button in the group and it should say "curve" or something like that. Click it and move the "cross" back to the drawing and click the (0,0) spot on the guides. Keep the left button down as you drag to where the (0.600, 3.60) guides cross. Click there and go on to the end point where the guides cross at (0, 12.0). Double click to end the line. You are done with the curve drawing tool, and can go back to the "pointer", aka the "selection tool". Who's on first, what's up next?

OpenOffice.org can be a bear when dealing with the handle points. Inkscape is easier, but as you have found out, setting the 0/0 point doesn't exist. So you have to add 1 to all the measurements on the X-axis. 0 is 1, 3.60 is 4.60, 12 is now 13. If you have another program, or know how to use any of them; just make sure the end points at 0/0 and 0/12 don't have any "handles" sticking out that are attached to the end "points". You want them to be just "corner" points without any handles. You will know something is wrong by the funky way the line curves into the end points.

Double click the .600/3.60 handle point to select it and then go looking for where the turn into "symmetrical" point button is located. Make the .600/3.60 point a symmetrical point and drag out the handle points. You may have to put the mouse over the point, click and drag out the handles. They may be looped in the wrong direction, but you just need to twirl them around to the correct orientation and drag them out. If the end points are not corner points and have handles attached; the curve you get will not look like the one shown in drawing #3.

Drawing #3 on page 1 of the PDF file, shows a horizontal guide centered on the .600/3.60 point. Keep the point handle lined up with the guide, and drag it out to the vertical guide at 0/0 and release. That's it, you have just created one half of the foil section. Now select your new curve and go looking for the copy and flip commands. Line up the leading edge point of the new flipped curve (drag the line and not the end point!) with the parent curve and go looking for the "join" or "combine" command. Make it so #1.

You can use a fill color to show that the joining worked and can now save your file. You do save your files right? You can move/set some guides to check for yourself that the Y(+/-) values at the X distances; do fall on (or too close to actually measure in the real world) the arcs.

To figure out how many of the 1/8" layers you will need; divide the total thickness (1.2) by the thickness of one layer of ply (.1250). We get 9.6; which works out to an odd number of ply's. So I "copy" and "paste" a new cloned foil, and drag it to an area below our first. Line up the leading edge point on the vertical 0 guide. Drag the foil and not the point. Drag down a new horizontal guide to intersect with the leading edge point. Drag down a a new 0/0 point at that new intersection and to reset the 0/0 grid for a new task.

Add new horizontal guides above and below the 0/0 point. Go to the edit guides area and set these new guides to +/- 0.0625. This will give you the thickness of one sheet, split/centered over the new 0/0 horizontal guide. Now take a pencil and paper and add 0.1250 to the 0.0625 numbers. This will give you the numerical values (+/-) for the next pair of guides. Do this four more times for a series of guide lines above and below the 0/0 centerline of the foil.

Now fine the "rectangle" drawing tool and drag one out starting at the location of the upper corner of the "center" strip of plywood. Drag this box out to the end and down at the - 0.0625/12 location. You have the first layer of plywood. If it is still selected, click a color and fill it. You will be using different "matched" colors for each side of the foil. As I have done on page 2 of the PDF file.

You can see where one of the upper and lower horizontal guides for the next layer of plywood crosses the foil arc and where it also ends on the trailing edge. Place vertical guides at those locations and drag out a rectangle box using them as the end points for that layer of plywood. Repeat the process with the remaining layers. You now have the locations of the starting and ending points of the "strip widths". Click on the vertical guides to check the values given in the editing dialog box. Adjust to a normal measurement as needed, but still staying in the cross section boundaries. On this cross section, the starting and ending points fall close enough to standard measurements; and we will use them as is, for constructing our NACA 0010 – 12 foil.

The following screen capture jpg's show completed foil drawings done with the Inkscape (left) and Open Office (right) programs.

The only thing left to discuss is what shaping needs to be done at the tips of the foils. As I stated at the beginning of this story; I'm not an expert on any of the fine points of foil design. I can only go by the information in the articles that I have read, the shapes of the foils I have used with other small boats; and in my collection of sailboard fins and daggerboards. All of them have some shaping done to the tips. Part of it is reducing wetted surfaces; and part is increased performance. Water will prefer to go around a "shaped" tip than across a square one. This induces less turbulence off the foil's tip and trailing edge. Less turbulence off the daggerboard will help the rudder maintain it's grip in the water when it gets there. I like to have straight leading edges and taper off the trailing edges of my daggerboard foils. This also gives me a longer section of the thickest part of the foil's cross section, for strength at the ends. This helps when you are standing on the daggerboard during a self rescue. The rudders, I leave a little boxy to maintain working surface area.

I still haven't worked up any standard (researched) numerical information about how big a foil should be in relation to the size of the hull. I've been leaning towards {db-cord in inches, is equal to hull length in feet as inches, plus two inches}; so a 10ft boat would have a 12 inch daggerboard cord (10"+2"=12"). The exposed length of the daggerboard is twice the length of the cord. So a NACA 0010-12 would have 24" exposed below the keel of the boat. The overall length would be determined by the daggerboard trunk height, plus room for the hand grip and depth stop. I try to size the rudder to be 1/3 of the exposed surface area of a fully lowered daggerboard. So far my sailing boats have had no bad habits, and the estimated CLR's (with fully extended daggerboard, no rudder) from my scaled half model hulls, have been right on the money to their full sized sisters. Push at that point on the hull while it is sitting next to the dock, and the boat moves away with no induced turning.

That's it, with the programs we have just used, you can now design your own NACA 0000 foils. I hope all this made some sense and I haven't confused you about the process. If you don't want to download, purchase, or learn a new computer program; everything can be done with pencil and paper after you have generated the numbers with the 4Gen program. You can layout the X and Y values on graph pager, and outline the foil's arc with a french curve and be close enough to make a template. You can then figure out how many plywood sheets are required and how wide each needs to be from this method too.

The following are some links to sites for free programs that I have used to help with this story. For ease of use; the Inkscape vector program is less hair pulling, but still in early development. I will be updating the Inkscape program on my computer as it gains in features and usability. It looks like a winner. OpenOffice.org 2.02 is a truly great, full featured "Office Suite" program now, and can stomp "clippy" into his much needed oblivion.

 Screen Capture Software: Captura 1.0 from www.hernansoft.com/downloads For Macs, Linux, and Win95-XP Vector Drawing Software: www.OpenOffice.org www.InkScape.org for Linux and Win95-XP NACA 4Gen Software:

Thanks again for reading my stories. I hope you enjoy reading them as much as I do writing them. My next story will be on how and why I design boats the way I do. Until then, stitch and glue and stylish too.

Warren Messer
Red Barn Boats

Other Articles by Warren Messer

 SAILS PLANS EPOXY HARDWARE CANOE/KAYAK SAILMAKING MATERIALS MEDIA TOOLS GEAR
 Build the Nuthatch Pram QuickFair