Redsky49 - yes true. Load paths in a sail run between the corners. The highest loads run from clew to head
- paralleling the leech, then bending further and further into the sail.
So the idea of radial sails is that panels are oriented to align with the load paths. The long edge of the panels are parallel to load. Of course load paths are not static and change constantly, but still 1 row of stiching on the long edges is plenty strong for the relatively low loads. Horizontal seams cross the load paths so must be wide and joined with multiple rows.
Crosscut sail seams run purpendicular to an imaginary straight line between the clew/head. They cross the highest loads so require multiple rows.
The biggest sail I ever designed/built was from kevlar with a luff length of 160' and weighed 900 pounds - Radial panels were joined with 1 rows of stitching; and the horizontal seams had 9 rows.
I can think of only 1 reason to have 2 rows on the radial seams - and that is if going cheap
on the construction by using very wide radial corner panels, there is a point where the seam bias loads may need a 2nd row. This creates a big problem because the panels carry to much bias load and will quickly distort.
Hope that makes sense - I'm still groggy after just finishing a boisterous 1000 mile passage