THE CLAUDIO TOOL for making Seams
by
Ben Morris (last edited
08/06/2021)
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
-
1/4 and 3/4 of the beam length.
-
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.
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
