On the subject of bridles.

I keep my boat on a mooring. Like almost everyone else, I use two pennants, one to each bow cleat. Many years ago I had the opportunity to be on the boat during a storm. My boat like everyone elses’ was swinging back and forth in the heavy wind. I observed the load shift back and forth from one pennant then to the other as the boat swung in the wind. Each pennant would alternately take the whole load while the other went slack. Being of nylon, there is a lot of stretch in the lines. They would alternately saw back and forth in the chock. I had heavy pennants with good chafe protection, but what I observed gave me something to think about. I decided that having two pennants of equal length was a bad choice. There is very little time when the load is shared between the pennants. Most of the time one or the other pennant will take the full load. Using two pennants doesn’t reduce the load on the chocks nor pennants. I still use dual-pennants, but I make them of different lengths. The short pennant takes the full load no matter which way the boat swings. The long pennant takes no load and is used only as a backup for the short pennant. I’ve been making my mooring pennants that way for years.

A few years later I read an article about nylon mooring pennants. If I recall correctly, the article was published by the Coast Guard. They had observed that there were a lot of nylon pennant failures. They found that the nylon core had melted. They concluded that nylon was not the best choice for a pennant and recommend polyester instead. It is my opinion that their conclusion was incorrect. Although the nylon surely melted, I believe the cause of the failure was alternate loading and unloading of the lines due to the use of equal-length pennants. The chafe material acts as an insulator and allows the line to heat up. If a single pennant were kept under a more constant load it would not build up anywhere near as much internal heat. Changing to polyester line reduces the spring constant of the pennant and therefore reduces the potential energy released with each swing of the boat. Since the polyester line has much less stretch, it transfers more of the dynamic load to the block.

Here are a couple of articles on the subject. I found the second article most interesting. I have some dock lines that are getting ragged. I plan to replace them after reading the article.

http://www.boatus.com/hurricanes/chafe.asp
http://www.practical-sailor.com/marine/used-nylon-rope-endurance-test.html

Rich

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