Woorabinda Lake - Stirling South Australia

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THE CLAUDIO TOOL for making Seams  
by Ben Morris (last edited 21/07/2013)

Shape in Sails Building Board Making Seams Set the Seam Curvature Making a Sail Sail Material Diagonal Seams etc Back to Intro page Setting the Sails The Claudio Tool Measuring Procedures


At first glance the Claudio Tools seems to be a novel and easy way to create the curved overlap for broad seaming sails.  I was concerned about the nature of the curve generated by bending a bar by supporting at its ends and supplying a force in the middle (or supporting it at it's middle point and lifting both ends - same result!).  I had the same issue when using a piece of timber to create a curve for the luff on a sail.  Supporting it at the two ends and pushing the middle out does NOT create a regular circular curve but one that is much too curved in the middle and not curved enough at the ends.  A little research on the Internet revealed some tools to calculate the displacement of a beam when bent in this manner.  See  Web Site

A little manipulation using excel showed exactly this issue.  In the graph below the vertical axis is exaggerated to reveal the differences between the bent beam, a circle and parabolic curves.  See the spreadsheet for a full outline of the maths.

Clearly the beam is too sharp around the centre section and not curved enough near the ends.  It differs by up to 9% from a circular path and a similar amount to a parabola.  What about the middle section?  By adjusting the curves so they became coincident at the -30 and +30 points the three curves were quite close between those points

The maximum difference in this section is now about 5%, probable getting close to the limit of accuracy when making the seams anyway. 

What does this mean?  If the Claudio Tool is to produce a curve approximating a circle then only the centre half section of the beam should be used.  For a 350mm seam this means the beam will have to be ~700 mm;  If a section with the maximum draft place at other than the centre of the chord, then the beam would have to be wider again to allow only the middle to be used.

Is there a way around this?  Perhaps using two supports separated by some distance in the middle could cause the curve on the outer sections of the beam to be forced closer to that of the circular  one. 

Two additional arrangements have been tried where the beam is supported at two points located at

  1. 1/4 and 3/4 of the beam length.

  2. 1/ 6 and 5/6 of the beam length

Selecting the best section of these curves (-40 to +40 for the first and -50 to +50 for the second) gives a much closer approximation of beam bending to a circle where maximum errors of 1% can be acheived.


The accuracy of these last two ways of bending the beam suggest these would produce a much fairer curve to the seam overlap and hence the curve built into the sail.  The second would be the easiest with two screws to support the beam at the 1/4 and 3/4 position and only use the beam from the -1/6 to the 5/6 position.  A beam of length 600mm then would be supported at the 150 and 450 mm positions and used between the 100 and 500 mmm positions.




Like the original tool, the two points at the 150mm and 450mm mark could be screwed to a wider board and the ends lifted with inserts of specific thickness as per original.  The difference would be that as the centre of the bar will bend inward the board will need to be curved away from the beam to allow it to bend.