You can't. You can only repair the boat that is already there that Ned told you about.
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If you mean just a normal one, you can get it from Duke Horacio during the quest Dragon Slayer or you can buy one off of another player or from the Grand Exchange.
You just simply use an Enchanted Gem, you can buy them off a slayer master.
you can buy a dragon blade at valencias shop. you have to go to dragon slayer lair then quest for the dragon blade or get lucky when you hunt for monsters
world 2 is the best world to buy everythings
There are a number of ways to get a Whip in Runescape. The first is to buy it from the Grand Exchange, price is currently 1M. Second is to get it as a drop from another player in PvP, or buy it from someone. Third is to achieve 85 slayer, once you have this go to the slayer tower in the Canifis(check runescape world map for directions) They aren't that rare of a drop, although you definitely wont get one every kill. It could take 100-1000 kills to get one, its all chance.