Some years ago, a rather long article on birdsmouth's spars (written by me) was published on this Web site. Buried deep down in that article were calculators for birdsmouth's spars size calculation. Some people have expressed the desire to have a quicker access to these calculators. So here they are, on a separate page. This should work much faster if you have an old computer.
I've added two bonus features: first, two new calculators for modified bird's mouth spars. Second there's also a downloadable version at the bottom of the page. That way, once installed, you could use it on your computer even if your Internet connection is down. Enjoy.
The Calculators will work with most browsers out there (Internet Explorer, Firefox, Google Chrome, Opera, Netscape). Internet Explorer users may get a message that some scripts were blocked for security reasons when they open the page. If so, you will have to click on that message to authorize the scripts, otherwise the calculators won't work. These Calculators are written in a language called Java Script. Java Script is a limited programming language that cannot touch your files. It is safe to authorize these scripts. All the other browsers mentioned will work without any hassle. (Take that, Microsoft!)
Choose the Calculator that suits your needs. Enter the values of your choice on the left, then click on the "Calculate!" button. Results will appear on the right. An error message will appear in the Status box if your inputs don't work out. You can clear all entries by clicking on the Reload or Refresh button of your Web browser. Or you can just enter new inputs and click on "Calculate!" again. Clicking on the pictogram in the middle will lead you to Figure 1, in case you need a reminder for all those definitions. Dimensions must all be in the same units. Ex.: Inputs in inches give results in inches.
Please note that the value of O.D. is for a rounded exterior. That detail is especially important if you ever consider doing a modified birdsmouth spar with an odd number of sides.
Hints to avoid error messages (in the Status box)
Input values must be numbers higher than zero (no negatives, no letters).
The number of sides must be 5 or more.
I.D. must be smaller than O.D. (obviously).
H or K must not be too small or too large (less obvious).
H must be smaller than the radius or half the diameter.
Figure 1- Bird's Mouth Spar : Definitions
O.D. is the outside diameter I.D. is the largest inside diameter R1 is the shortest inner radius R2 is the largest inner radius or half I.D. N is the number of sides (eight in this case) a (alpha) is the angle between adjacent staves or 360° / N L is the width of material H is the thickness of material
K is the thickness to width ratio of staves or (H / L) M is the "conversion factor" or (O.D. / L) A is the inside to outside diameter ratio or (I.D. / O.D.)
Modified bird's mouth spars have their staves cut differently so that their outside edges fit flush together, creating a clean polygon. The size formula is not the same as the "regular" or traditional birdsmouth. Hence these new calculators.
O.D. is the outside diameter I.D. is the largest inside diameter 1 is the first angle of the stave 2 is the second angle of the stave L is the width of material measured on the outside H is the thickness of material
Download these Calculators for free
If you like, you can install a copy of this Web page content on your computer. What you get is pretty much the same page you see here, but without ads and other links. It is a standalone version: it will work in your Web browser, even without an Internet connection. Click on the link below to download a zip file named "bird-calc.zip". This is a zip archive whose content has to be extracted to a folder or directory of your choice. There's even a distinctive icon included for a desktop shortcut. The included “read-me.txt” file will tell you how to set things up.