"Or take ships as an example. Although they are so large and are driven by strong winds, they are steered by a very small rudder wherever the pilot wants to go." NIV translation
The second example (remember that two is the number of witnessing) is of a ship. With a small part under the water, the pilot can determine the direction of the ship no matter which way the wind tries to send it. The pilot must make a choice of which way to turn the rudder and take the whole ship.
"Likewise the tongue is a small part of the body, but it makes great boasts. Consider what a great forest is set on fire by a small spark. The tongue also is a fire, a world of evil among the parts of the body. It corrupts the whole person, sets the whole course of his life on fire, and is itself set on fire by hell." NIV translation
This passage reminds me of a wildfire which can be easily started when the conditions are right. When it is dry and windy, a small spark can start a massive roaring fire that consumes everything in its path for thousands of miles.
Satan knows that the tongue can do the same thing in the lives of people and he is more than willing to help to create that little spark. That is why James is urging us to control our tongues.
"All kinds of animals, birds, reptiles and creatures of the sea are being tamed and have been tamed by man, but no man can tame the tongue. It is a restless evil, full of deadly poison." NIV translation
We have a contrast here to let us see how hard it is to tame the tongue. We all have seen various pets but all of the pets we see came from a wild animal at some time in history. People have all sorts of exotic pets even lions these days but rare is the man who has control of his words. This passage may seem like there is no hope to do this but we can remember that our hope is in Jesus and, through the Holy Spirit, we can control our words most of the time.
"With the tongue we praise our Lord and Father, and with it we curse men, who have been made in God's likeness. Out of the same mouth come praise and cursing. My brothers, this should not be." NIV translation
When we praise, we are led by the Holy Spirit but, when we curse, we are not. We are called to walk with Jesus every second of our lives and that is done by letting the Spirit have control at all times.