Wow, you would trust forty feet of materials and work to those metal spikes??
I thought they were only good for putting up mailboxes.
You didn't say what the fencing was made of. An open fence like a picket fence might not be too bad at catching the wind, but a privacy fence will have to take a lot of force. And you know the wind will shift around every once in a while. Even so, 40X6 makes a pretty big sail!