Monday, November 14, 1768 We arrived at Rio de Janeiro, Brazil. The weather was pleasant with moderate waves and low wind. At five in the evening, we anchored in five fathoms of water just above the Isle of Cobras.

Tuesday, November 15, 1768 Fine weather. Obtained fresh greens and beef for the ship s crew.
Saturday, November 19, 1768 The weather was cloudy. We were busy getting water and other necessities on board, as well as caulking and repairing the ship. I had to punish Seaman John Thurman for refusing to help the sailmaker repair the sails.
Adapted from James Cook, Captain Cook's Journal During the First Voyage Round the World
What is the text's main purpose?
to teach investors how a ship is caulked and repaired
to prove to investors that the crew was well fed
to document necessary information about the voyage for its investors
to prompt investors to visit Rio de Janeiro

The text's main purpose is to document necessary information about the voyage for its investors.