A properly outfitted boat should have at least six dock lines on hand. Two (each) bow and stern lines, and two spring lines. The length of the lines depends upon how and where you moor your boat. Some boaters have custom made lines that are measured specifically for their berthing area, however, you should have designated bow and stern lines that are approximately two-thirds the length of your boat, and spring lines that are the full length for when you moor at unfamiliar places.
When choosing line for your fenders, anchor and mooring lines, consider using natural fiber, three strand line. The reason is that natural fiber line will hold better on your deck and dock fittings, and will also hold your fenders better when they are tied off to the side of your boat. If you choose natural fiber line for these applications, you will have to inspect and replace it more often because it is not as resistant to the elements, however the trade off is that natural line is less expensive than synthetic. Many boaters use synthetic fiber line for their mooring and anchor lines too.
Another type of line to have your vessel is polypropylene line. This line is great to attach a life ring or other floatation device because it usually comes in bright colors and it floats. This line is also used for skiing, wake boarding and tubing.
If you have the storage, you can never have too much line. Besides designated dock lines, consider varying lengths and sizes of lines to have aboard for practical or emergency use, or to just lend to someone in need. You may want to mix synthetic with natural fiber line as well.
