All wave energy must pass


The weather is approaching, and you are checking the lines on your boat. The owner of the boat near you is attaching his boat to the dock with a spider web of lines. The lines are effectively absorbing huge pitching and rolling forces.  Stressing the docking lines unnecessarily is counterproductive to the mission of keeping the boat safely alongside the dock. Wherever a stressed line crosses a chock, heat builds up and strands part. Your sinking realization is that the real problem presented by the upcoming weather is that this neighboring boat is likely to visit your boat for an impromptu and expensive dance.

Independent dock lines 
Docking lines work independently to pass rolling and pitching stresses back and forth between them, positioning the boat alongside a dock so that it can breathe and let the waves pass under it without removing energy. The exercise is about minimizing unnecessary forces on the lines and tuning necessary holding forces to work together. When the boat can’t pitch freely, the chocks concentrate the forces of lifting the dock and become dull knives. 

In most situations, one only needs four well-placed dock lines (two spring lines, a bowline and a stern line) and three robust fenders to achieve a configuration that positions the boat alongside.

Spring lines: You’ll want two long spring lines, preferably at least half the length of the boat, leading fore and aft from the motion center of the boat. A spring line is the radius of a circle. The longer the radius, the closer a chord of a given length comes to the circumference. When the two radii come together at the center of the boat, the stretch in the tight line becomes the springing force.

The spring lines have four functions:

• To position the boat alongside the dock. 
• To be a hinge in conjunction with the fenders, allowing the boat to pitch and roll freely.
• To keep the boat close to the dock. 
• To damp the fore and aft motion due to wave surge relative to the dock. Surge is a force that can make a dock tie-up untenable. Force is equal to mass multiplied by acceleration; loose or stretchy spring lines allow the boat to accelerate with the surge and come to an abrupt stop when the stretch is used up. Stretchy spring lines accelerate the boat in the opposite direction after the surge becomes neutral and increase the distance the boat can move off the dock. This distance off the dock makes it harder to tune the bow and stern lines to act independently. A second lighter set of spring lines with snubbers can be tuned to reduce the shock on the tight, low-stretch spring lines.

Bowline: The bowline leads perpendicular to the axis of the boat. The longer the radius, the better — consider adding a snubber or a cleat across the dock to get more length. The purpose of this line is to allow the boat to move around the midship fender hinge toward and away from the dock as it pitches. The pitch of the bow is the chord of the circle defined by the radius (the length of the bowline). The bowline is NOT to keep the boat in position fore and aft on the dock. The bowline should be “drooping” loose and as long as possible, enabling the bow to rise freely. At the end of its scope, the real “work” of the bowline is to prevent the stern from hitting the dock. As a wave passes, it pulls the bow horizontally along the circumference — a distance defined by the chord (pitch) and radius of the bowline — toward the dock. It can do this freely because the stern line does not resist. The bowline eventually becomes slack, which allows the bow to move away from the dock on the fender hinge as the stern line pulls the stern in.

Stern line: The stern line is attached like the bowline. When the wave passes, the slack allows the bow to lift and move toward the dock. But when the stern line runs out of slack, it keeps the bow from hitting the dock.

Three robust fenders: The fenders should be large in diameter and placed at the midship pivot around which the boat moves. As the boat pitches and rolls, these fenders are the point of contact with the dock. The fore and aft fenders contact the dock first, before the bow or stern line scope stops the swing abruptly. The compression of the fender significantly absorbs the momentum and realigns the boat. The middle fender is the pivot point. In calm conditions, with the springs adjusted, they keep the boat aligned to the dock. Diameter matters!

Chafe gear at the chocks: Chafe gear should be placed at the chocks and anywhere the loaded lines might contact the boat. The chafe gear isn’t sacrificial; it reduces the point stress caused by the change in direction through the chock. 

Snubbers are a further refinement and are useful shock absorbers when the conditions are extreme.

Phineas Sprague is a circumnavigator who sails aboard the 72-foot Alden-designed schooner Lion’s Whelp and is the owner of Portland Yacht Services in Maine.

By Ocean Navigator