Bird's Mouth Spar Size Calculator

Instructions

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.

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.

Birdsmouth Calculator: Given N, O.D. and H,
calculate L, I.D. and K
Inputs
 
N :
O.D.:
H :

 
Results
 
L :
I.D.:
A :
K :
M :

 Status :
 

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

Important Ratios

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.)