The ship and the sail is quite a good annalogy. The spirit of the Lord moves yet cannot be seen just as the wind blows and cannot be seen from where it comes or to where it goes. The awsomeness of our creator God in His wisdom has chosen to use the mouths of men to brings His truths to the ears of humankind. His words are powerful and when His servants are used to proclaim His words they do so in the power of God's Spirit. Thus the words of His servants are in fact thiose of the Lord God and thus should be listened too. In the same way that servant needs to be haering and listening to His Lord so that he is only saying and bringing to the hearers what God is actually saying and not using the opportunity to say what he wants to say , to get onto his or her hbby horse. The servant bringing the words of the Lord thus have a challenge to obey what God is saying.