Plenty of poorly secured boats have floated away from the dock after a shift in weather or tide. It happens to everyone once, but after the first time, you'll want to learn the basics.
To understand how to tie a boat up securely, you need to understand basic boat architecture and terms. These are quite basic, so most of you may already know many of these terms. However, it never hurts to review them.
Let's start with the two basic lines that hold each boat to the tying fixtures on the dock. If you are tying to a buoy then you want to review mooring basics since our current discussion is about tying to docks.
The bow line runs from a cleat or chock and over the forward gunwale, where the line should be fitted with a chafe guard. The line is then finally secured to the tying fixture on the dockside which could be a cleat, bollard, post, or ring. The knot required will vary according to the tie point.
The stern line is attached to the stern tying fixture that is closest to the dock. Securing the stern from the outboard tying fixture or a central bit is not recommended since it will be more difficult to retain tension. A chafe guard can also be used here, but the stern line moves over the gunwale much less than at the bow.
In situations where the boat is in a slip or berth, then the second set of lines is attached to hold the boat in a central position.
Lines should be tied tightly unless a spring line is used.
A single spring line makes a boat much more secure, so it's highly recommended.
There are two types of spring lines: forward springs and aft springs. The name of a spring line refers to which direction it is traveling when leaving the boat. So a forward spring travels from the stern forward from one-half to one-third of the vessel length before being secured to the dock.
The forward spring brings the stern of the boat close to the face wall by pulling forward.
An aft spring line travels from the bow or forward gunwale back to the dock with a length about half the length of the vessel. An aft spring is best rigged after a forward spring to keep tension even throughout all lines.
Spring lines are useful to keep winds that are parallel to the dock from pulling the vessel away from its position.
Tides and Tying
There are plenty of devices of various qualities made to secure boats against tidal forces, but your regular dock lines will do the job if you know how to rig them correctly.
Bow and stern lines need to be long enough to slack and keep the vessel from pulling the tying fixtures free. In some cases, boats can sink from poor tidal planning, so be careful and check the tide charts for your area.
Spring lines will keep the vessel in position horizontally as long as they are set to the correct length, which should be barely tight at low tide.