Woorabinda Lake - Stirling South Australia

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SAIL MAKING - Using Diagonal 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

Diagonal seams

The top seam in a sail often has a very short chord of 150 mm or less.  To get the broad seaming to work over this distance requires very accurate work as errors are much more significant than with a seam of 300 mm.  One way around this is to make the top seam diagonal sloping it up from the luff to the leech.  This also has the advantage of forcing that top curvature higher in the sail so supporting the top section of the leech.  In order to keep the calculations the same I ensure the point of maximum camber is still placed at the same location as the replaced horizontal seam.  The angle used is between 45 and 60 degrees.  This really powers up the top section of the sail and improves the light air performance significantly.  In fact I have been reducing the curve in that seam by a degree or two to produce a sail with better all-round performance.

A similar concept can apply to the bottom seam but here I do it a different way.  Instead of replacing the first horizontal seam with a large diagonal seam I add a diagonal seam from the clew to the luff just below the first seam.  The reason for not using a larger diagonal seam is that the length of the diagonal seam would become a little impractical without building a special board.  In addition what this does is to introduce the curvature from the foot to the first seam in a graded way rather than in a step as the horizontal seam does.  The required curvature is split between the two seams roughly 50/50.  Remember tho that as the distance to the seam is smaller in the diagonal seam it will need to be given a greater lift on the board.  An example might be to replace a horizontal seam requiring a 4 mm lift of the building board, I would use only 2 mm on the horizontal seam and about 3 mm on the diagonal seam.  Again experience suggests this produces a faster and more flexible sail.  I'm sure the diagonal seam spread along the direction of the stress in the sail material from the clew helps support the sail as well.

Remember that some classes such as IOM have special requirements which prevent seams being closer than 120-150 mm to a clew or head point.  Seams would have to be adjusted accordingly but sails can still be made with diagonal seams even here.


More on Luff curves

While this applies more to marblehead main sails than others, the principles apply equally well to all classes.  I have an issue the setting of the mainsail near the tack when the luff curve is just brought down the sail to the clew with little added curve.  It does not allow for much adjustment in or out from the mast to get the sail to set there.  I have been now putting a much more pronounced curve near the bottom of the luff so the tack is designed to sit a good 10-15 mm from the mast.  This allows much more space to adjust this distance to get a good set of the sail.  It has an added advantage in a marblehead sail of pushing the clew further outboard (the 10 - 15 mm lost at the front).  This means that the clew supplies the force up the sail now much closer to the leech than is the case with the normal marblehead sail.  This supports the leech better and reduces the tendency for the sail to develop a diagonal fold towards the upper middle of the luff.  Again experience suggest a faster and more flexible sail.