Entry 1 What a spring line actually does
A spring line runs forward or aft at a shallow angle from a midship or bow cleat to a dock point well ahead or behind the boat. That angle resists the boat sliding forward or backward along the dock in a way a straight-across line cannot, because a straight line only resists sideways drift, not fore-and-aft motion.
Entry 2 Why over-tightened breast lines cause problems
A breast line pulled bar-tight holds the hull close to the dock but does nothing against surge, and in a boat with any rise and fall from wake or tide, a too-tight breast line becomes the line that snaps taut and takes the shock load instead of easing into it.
Entry 3 Setting up a basic four-line arrangement
A bow line, a stern line, and a forward and aft spring line cover most slips. The springs do the real holding; the bow and stern lines mostly stop the ends from swinging out. Leaving a little give in each line lets the boat ride small wakes without any single line taking the full load.
Entry 4 Questions people ask
Should dock lines be as tight as possible?
No. Some give allows the boat to rise, fall, and shift slightly with wakes or tide without shock-loading a cleat or line.
Do I need all four lines for a short stop?
For a quick stop in calm water, two lines at an angle often suffice. For an overnight stay, all four are worth the extra few minutes.