Well, that sounds pretty promising to me. Not all that many people got to this country as stowaways--in fact, I don't know of ANY, specifically, except for your great-grandfather and mine, both of whom just happen to have the relatively uncommon surname of Krell, so I'd say it's not such a long shot at all! The fact that he was 13 supports the story. since Fred was 12 and his brother was older. Also the fact that he named one of his sons "Fredrick." (The variant spelling doesn't mean anything, because the real name was "Friedrich," and both of them would have been translating that into English.) What would help to nail it down would be John Krell's date of birth. (Fred's was February 2, 1844.) Do you know that date, or have an estimate of it?
So I'd love to get more information on John Krell. I'll send this message to your email address also.