You’ve been tracking the forecast of an upcoming storm. It might have passed offshore but now the track is shifting and you worry that your boat might be in the path. You take a ride to the marina and a number of boat owners are there discussing how best to prepare. Some decide to have their boats pulled from the water, others decide to ride it out at the dock. In either case, there’s a chance of wind gusts that might reach 100 miles per hour. You decide that the storm still has a chance of moving offshore and the cost and trouble of hauling the boat aren’t worth it to you. You side with the “ride it out” owners but decide to add some lines. You’re on a floating dock and you figure that your best bet is to double up on stern lines and shorten the spring line to move the stern about three feet off the dock. The next day the storm hits while you’re safe at home but the marina takes it on the nose. The storm didn’t veer and the wind gusts frequently topped 100 mph.
It only takes a day for the storm to pass and the sun to come out. You head over to the marina and as you walk down the dock you see your boat ahead of you bobbing in the slip. Dodged a bullet you think, but as you get closer you start to notice the damage. Your swim platform was crushed against the dock yet your spring still has it three feet off the dock. To make things worse, your gunnel is crushed at a spot that looks to be about even with the breast piling yet your lines are holding you well away as you survey the damage. What the heck happened? Unfortunately, you’re about to get an education from the insurance adjuster about how your nylon dock lines let you down, or more accurately, how you failed to select and use them properly.
Safe boating requires lots of judgment calls. The safest boaters are often those who have “seen it before” and their judgment is often based on experience but there are always factors to weigh and lessons to learn.
The thing that makes your nylon dock lines so nice is that little bit of “give” or stretch that keeps the line from abruptly coming taught every time you get rocked while in your slip or anchored out. Nylon makes for comfortable dock lines and a protective anchor rode but your setup and choice of lines was a wrong choice for the storm.
Typical dock lines are made from what most of us call twisted three strand nylon and under stress these lines will stretch by as much as 30 percent before they break. Your 25 foot spring line can become a 30 foot spring line with a 20 percent stretch and that can be more than the difference between bobbing in the water and slamming into the dock or a breast piling. On the other hand, a line with no stretch has more potential to rip out cleats or snap.
If you’re curious about how likely your lines are to stretch or snap, there’s some math and a little research involved. There’s a thing called “wind load” which is the strength of the force that wind applies to an object in its path. A 100 mph wind creates a pressure of approximately 25 pounds per square foot of exposed surface. A boat that is 35 feet long and 10 feet above the water line exposes a surface of 350 square feet. Multiply that 350 square feet times 25 pounds of wind load and you get 8,750 pounds of pressure.
So let’s consider your dock lines. A typical half inch nylon three strand rope has a breaking strength of about 5,600 pounds. That’s well below the load a 100 mph wind will create on our boat example. Also, remember that with 30 percent stretch before breaking, that 25 foot spring line will grow by more than 5’ and your boat is 3’ from the dock even if the line holds.
Companies selling dock lines know that many boaters will look for a line that does the job at the best price. I typically see companies recommend half inch lines for boats up to 36’ so let’s consider the specs.
First of all, nylon has some excellent properties. It can be incredibly strong, have incredible stretch, is ultraviolet light resistant and relatively inexpensive.
Moving from half inch to three quarter inch diameter three strand jumps you up from 5,600 to 12,780 pounds of breaking strength. The half inch line was just not enough for our storm scenario so clearly having gone one quarter inch beyond the “typical” recommendation would have made a big difference in the size of your insurance claim. And, as mentioned above, one of the benefits of nylon is the attractive price point. Had you given thought to storm preparation, you might have upgraded your three strand to what is known as “double braid”. The smooth outer sleeve on double braid makes it easy to coil, easy on your hands and about fifteen percent stronger for its diameter than three strand.
So back to those judgment calls. How much are you willing to spend? If a storm is headed your way your insurance company may offer to pick up a portion of your haul out to limit their potential for a claim. Call them and ask. The cost to you for a quick haul may be far less than the financial, emotional and structural issues you’ll experience from damage. If you decide to stay wet, give some thought to upgrade your lines. My recommendation is to go one quarter inch higher in diameter than “suggested” use, double braid to limit stretch and to double up every line when a storm is coming. You’ll look smarter the day after the storm passes and you’ll probably have spent less than a partial tank of fuel to protect your investment.
There was a guy in old England named Robert Burton. He’s credited with saying “don’t be penny wise and pound foolish”. I think he was a boater.