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Here you say "apparently copied", in the other kayak-sailing thread it's "allegedly copied". Are you yourself saying the design was copied, or are you repeating someone else's accusation? I have respect for your blog and your writing, so am confused by your wording, which you have to admit seems a little weaselly. Hey, you may be right, but you should justify the statement or remove it - right now it sounds like so much casual internet stone-throwing.
Comparing the two designs, I see that the rigging is similar (not identical) and the sail shape, batten layout and boom arrangements differ. The two designs are about as similar to each other as they are to your own DIY sail design.
From looking at detail photos, the Falcon sail looks to be better engineered and of higher quality than Flat Earth. Plus it's available in North America, while the Flat Earth sail doesn't appear to be - how do you get yours?
No, I am a fully unbiased observer - I have neither sail, nor have I ever kayak sailed. Your comment struck me as a little disingenuous, particularly when I saw your video linked by Flat Earth, and the extensive reviews of the product on your blog. I really dislike baseless sniping on discussion forums - pretending that a coy remark about copying does not discredit a company is not really logical.
I read the thread and checked out the Falcon sail and like the looks of it. Based on your comment, I checked out Flat Earth and was not as impressed. This is based on how the rigs are engineered (I am a mechanical engineer).
I have no way to judge their relative performance, but it sounds like people really like the Flat Earth, including you. People also appear to like the Falcon sail. Maybe you should try the Falcon, compare the two, and see what's up.