He traced a circle on the deck and then placed his finger over about a quarter of the circumference of it.
"She won't go there."
"But supposing you want to?"
"Then, if the wind's ahead, you have to beat." He drew two lines across the circle at right angles to each other and laid his finger at the end of one. "Say we're here at north and the cove we're going to lies about south. Well, you get your sheets in flat—same as we have them now—and you sail up this way, at this angle to the wind." He ran a slanting line across the circle until it touched the rim. "That brings you here; then you come round, and go off at the same angle on the opposite tack, which brings you right up to the cove. You can do it in two long tacks, or—and it's the same thing—in a lot of little ones, each at the same angle to the wind; but how many degrees there are in that angle and when you get there depends on how your sails are cut and how smart you are at steering her."
Frank understood the gist of it, but there were one or two difficulties, and he was not ashamed to ask a question:
"What makes her go slantways against the wind? Why doesn't it blow her back, or sideways?"
"It does," Jake broke in dryly, "if you don't sail her right, or it blows hard enough."
"What makes a kite go up slantways against, or on, the wind, which is the same thing in sailing?" continued Harry. "Because with the wind and the string both pulling her, that's the line of least resistance." He paused, and added deprecatingly, "I was at school at Tacoma and as I'd a notion I might take up surveying, they pounded some facts into me that made this kind of thing easier to get hold of. A boat goes ahead on the wind because, considering the shape of her, it's the easiest way; and this is what stops her going off sideways to lee." He kicked a high narrow box which ran along the middle of the boat. "It holds the centerboard—a big plate that's down deep in the water now. Before the wind could shove her off sideways—and it does a little—it would have to press that flat plate sideways through the water."
Frank made a sign of comprehension.
"That's about the size of it," said Jake. "Now I guess it would be more useful if you got some of the water out of her."