The meaning of the word curtail'd is

answer; /// the above statement is (true);

first of all the mood needs to be defined: it is obviously a mood of deceitful relief and serenity. why? because the cape horn is one of the most violent maritime regions of earth and although it has been left behind, it is a personification of ahab’s delirium and the fact that the end of the passage speaks about it about being “hidden” but “raving” immediately conveys the feeling the calm is not permanent but fleeting.

furthermore, the oxymoron “mild stun'sails” is the first instance of such feeling as the adjective “mild” is associated with “stunned” underscoring the fleeting nature of this “tranquil” moment. and the contrasts keep piling up, with the opposition between “dark den” and “blessed light and air” to end with the final conclusion that leaves the reader with the uneasy knowledge that this is just a reprieve.