One of the first steps to prepare your boat for heavy winds, is to double up on your lines. A dock line has three points of potential failure.
- The line can chafe and fail.
- The boat cleat can fail.
- The dock cleat can fail.
Adding a second line on top of your primary dock line does NOT double your security. If you have your back up line attached to the same strong points on the boat and the dock, then the only one of the three failure points that you have covered is the potential chafe failure, you have not improved the risks at each end of the line.
While it is very unlikely that you could pull a cleat out of your deck, if both lines are attached to the same cleat, the loads are transmitted to one point on the deck. When you double up your line, secure the boat end on a different strong point from the primary line. It might be a cleat on the other side of the boat or a winch. This lessens the shock loads on any one point.
If two dock lines go to the same dock cleat all the loads, and the shock loads can be significant if there are big waves, are carried by one fitting. The dock cleat is often the weakest point in the whole dock line system. The cleat should be bolted through a dock plank(s) with backing plate, which is much better that screwing it into a plank, but the plank is only held to the dock stringers by nails. Your through bolted cleat is only held in place by 6-12 galvanized (rusted?) nails which fasten the planking to the dock. Dock cleats must be bolted to the dock structure or else you are relying on the dock nails to hold you boat in a storm. Take your second line to a different strong point on the dock. Take it to a piling or tie it around the dock stringer; with adequate chafe protection.
Chafe protection is critical. Most often your dock lines run from the cleat through a chock and out to the dock. The chock becomes a point of chafe and a frequent point of failure. The more acute the angle change at the chock, the more load and chafe the line will experience. Boat builders cannot place a chock in every place you might want one or every place available for every dock line in every marina. Often in storm conditions, particularly for your second line, it is much better to avoid the chocks completely. Add chafe protection where you line crosses a wooden toe rail or other hard chafe point, but the more directly the line goes from dock to boat, the less chafe it will experience.
In any event, do not ever use one line to serve two purposes. Your dock lines should be adjustable, and from each end if possible. For instance, if you double a bow line by putting the loop over a deck cleat, then tying it off on the dock cleat and then bring it back to your boat as a second bow line, that line cannot be adjusted during the storm as needed. Each spring line should be a separate line and not be used as a forward spring and an aft spring too.
Double up! Add chafe protection! Good Luck!
Two lines to the same dock cleat will both fail, if the cleat fails.
In this case one line should be secured to the piling. Note the cleat is bolted to the dock. The dock planks are nailed to the stringers with steel nails!
Dock lines secured to a cleat and to a windlass drum. Wonder why the cover was left on the drum?
Two sternlines one to a cleat lag bolted into the dock stringer and one to a piling.
One line serving two purposes is not adjustable.