In "Stolen Ingots" in Dr. Thorndyke's Case-Book by R. Austin Freeman, Mr. Badger was standing on a bridge watching a barge and brawley
“Yes. There’s a little ramshackle bawley from Leigh—but her crew of two ragamuffins are not Leigh men. And they’ve made a mess of their visit—got their craft on the mud on the top of the spring tide. There she is, on that spit; and there she’ll be till next spring tide. But I’ve been over her carefully and I’ll swear the stuff isn’t aboard her. I had all the ballast out and emptied the lazarette and the chain locker.”
“And what about the barge?”
“She’s a regular trader here. Her crew—the skipper and his son—are quite respectable men and they belong here. There they go in that boat; I expect they are off on this tide. But they seem to be making for the bawley.”
Actually these bolded parts are a bit obscure for me, but that's my guessing about them
on the top of = drived by
As for "there they go", I don't know why didn't he say "they do" directly, or does he mean "they usually go"?
off on = not sailing during
Is that correct?