Paul Thompson wrote:

I'm using V69 (T-90) Anefil Poly Bonded thread from American & Efird Inc with mostly an  18 needle switching to a 20 when the going gets heavy. zig-zag size is 4 to 5 mm.

Thanks Paul.  Very helpful.  I'll play with the thinner thread on the next sail.  Certainly would increase what is on the bobbin :)  

  I've been told that V69 is plenty strong out of the box, esp if double row stitching.  Thicker thread is suppose to have a longer UV life and thus strength with age.  The sail maker was recommending 138, which is too thick for my machine and would give me very little thread on the bobbin.  

Here is the "puckering" I am getting https://photos.app.goo.gl/0aZxnTwmmH6OVikD2.  It is basically a light waviness around the seam where the cloth does not lay totally flat.  I'm about a week away from hoisting the silly thing (the sail that is) and seeing what it looks like and how offended I'm by it.  Sure I'll find other problems, too. 

When I have a problem with puckering (usually on thin stuff like spinnaker nylon), I sew with one hand ahead of the needle and one hand behind it, tensioning out the seam as I sew. This is slow, and not convenient when sewing a long seam, but does reduce the longitudinal contraction. It does nothing for puckering across the seam, though. A better answer is to get the thread tension and stitch length right.

Arne and Annie - thanks for your advice.  I've also  been told that too much thread tension can cause puckering.  I've balanced upper and lower and made them about as loose as I feel comfortable.  If out of balance, I can see the "loop" where the upper wraps around the lower on one or the other side.  If both are too loose there isn't a real "straight" stitch,  i.e. the thread is slightly curved between each zig at the top or bottom.  I'll upload a pic to show the pucker...

Erik

Unless you are using a very light fabric, puckering is almost always caused by the tension top and bottom being out of sync.  Another possibility is that you need more foot pressure.  I've been surprised what a difference that can make.

Sebastian has now made a number of sails from Weathermax.  He could be your 'go to' man for advice.  (Sorry, Sebastian!)

Paul Thompson wrote:

Hi Erik & Evi, I'm currently making 5 sails from Weathermax 80 ranging from 100 Sq Ft to over 600 Sq Ft. I'm using a Sailrite LZ1 with zig zag and a walking foot and also a Pfaff 130 (zig zag & straight) without any puckering... I wonder if your feed dogs could be worn or the feed timing is slightly out.

Paul - thanks for your thoughts and feedback.  Both of those machines are great. I had a Pfaff 130 and because of the wider stitch on the Bernina of 8mm elected to keep that one when Evi shamed me about the number of machines I have.  In some ways the Paff was nicer though.  Both have a small bobbin and using 92 thread makes for many bobbin changes.

What size needle and thread do you use?  And zig zag size?

I do my own machine adjustments so no guarantees I got it right.   The feed dogs seem plenty sharp and timing wise they are well under the needle plate when the needle reaches and exits the plate.  The dog may have had a slight bias with one side being incrementally higher than the other, but I took it apart this morning and carefully cleaned everything and now the teeth height seems even.  I did try out a presser foot with rollers to minimize drag and this didn't seem to make much difference.

I like WeatherMax. My one observation is that when the needle penetrates the cloth there is a slight popping of thecloth  surface, more so than with other types of material. Not sure how much this has to do with the weave which seems finer than the Top Notch.

Erik