This step was more interesting for me because I knew what I had to do, but was not sure how I was going to do it.  Problems:
1) I don’t have any stock sail waving footage
2) I couldn’t find any good ones on stock sites
3) time… need I say more

1) compositors tool kit
2) made it myself
3) didn’t take near as long as expected but still a few hours

So I started with some great stock from Digital Juice, compositors tool kit 1
I used 0014 fabric for the sail.  In motion I used the multiply blend mode with this graphic that our awesome graphic designer made me:

That go me a good look but the lines did not move with the fabric so I applied a displacement filter and in image well I used the 0014 fabric mov.  That got me this result:

Which looked great the multiply blend mode gave it good color and details and the displacement filter made the lines wave as the fabric moves, but still doesn’t loop, and since this is going to be playing for long periods of time it HAD to loop.

I know I don’t have to stress the importance of a seamless loop but I will….. 20 seconds of video is small, 20 minutes of video is big.  20 seconds of video looping for 20 minutes is small and could go on indefinitely … 20 minutes of video is big and will stop at 20 minutes.  20 seconds of video renders in no time, 20 minutes renders over-night. 

Now that that is out of the way
I sent it to final cut for the looping part, and discovered this transition filter I have never used before called non-additive dissolve.  THIS ROCKS!!!! It does exactly what it says, which is why I started with it… and boy am I glad I did.

I cut the video in the middle and swapped the order of the two parts. That makes the end-to-start seam seamless because the first frame actually comes next after the last frame, but the middle is all messy.  I made the two parts over lap eachother by putting them in diffrent tracks in FCP and added the non-addative dissolve filter to the top track and it just worked.  I dragged the transition out over 2 seconds and poof instant loop.  That easy!
Here is what it looks like on the FCP time line:

Remember track 2 was the original start of the clip by swapping the order I move the loop point to the middle and the non-additive dissolve took care of that.  After some color correction it was back to motion.  If you watched the final output closely you could probably find where the transition is but if you don’t look for it you would never just notice it by watching. 

At this point I actually stopped and took the sail down to test it on the fabric and to test the loop points.  It looked better then I hoped, but after putting it in the mask I saw that the top sail looked like a continuation of the bottom sail it made them look like 1 piece not 2… I couldn’t have that!!  Details like that eat my lunch.   So I needed to overcome THAT obstacle so NOW back to motion for Step 3 making the sails independent.