In the anonymous "The Seafarer," the speaker writes much about the terror and hazards of seafaring during his time. When he writes that no man is so proud, etc., not to feel fear when "the sails unfurl" (42) he's simply telling about how treacherous sailing is during his day. It's not that he's afraid of the sails, he's afraid of what unfurling the sails means! I'm not a sailor, but I've always assumed this means the voyage is getting underway. That's what scares him.
Sailing the sea is treacherous for the narrator: it involves smashing surf (6) and sweating in the cold (6); feet cast in icy bands (9,10), hunger that tears at his sea-weary soul (12), a world blown clear of love and hung with icicles (16,17), and freezing waves (19).
He is afraid of seafaring, and when the sails unfurl it is time to shove off.
The speaker does not exactly say that he feels fear when the sails unfurl. He does say that there is no man who is so brave as not to feal fear when the sails unfurl.
What he is saying here is that any ocean voyage is so dangerous that any man will be afraid. He will not know if this is the time that he will get killed by some sort of accident, or bad weather, or whatever.
We can see this when the speaker says that when the sails unfurl, every man is
Wondering what Fate has willed and will do