January 9, 2016

Breaking stuff - for fun

I noticed some chafe on one of our spectra (dyneema) lifelines. This spot is at the forward most stanchion, where I was in the habit of clipping a spinnaker halyard. Its also where the lifeline makes a sharp angle down to the deck. This photo shows the new lifeline place. The chafe was very obvious on the old one.

Here's the chafe point - clearly a lot of the strands are broken:

Once I replaced the lifeline with a new piece of spectra, I decided to do a bit of semi-scientific testing of the old lifeline. I broke it. I attached the eye splice to a deck padeye with a spectra lashing and then led the lifeline to our primary winch.

It took about 90% of my maximum effort to get the lifeline to break. In other words I really had to crank very hard. Our primary winch is a #46 power ratio. Typical maximum handle load using a single winch handle is around 50 lbs for a "strong" user. Using a double handle as I did, the maximum load might rise to 65 lbs or so. So the most force I can exert on the line is about 46 x 65 = 2990 lbs. The chafed portion broke at what felt like 90% of that or around 2700 lbs. That's low compared to a new wire lifeline.

Here it is after breaking:

But what about the non-chafed portion of the old lifeline? I tied a bowline in the end, and loaded it as much as I could with the winch. I could hit a pretty good musical note with it. But I couldn't break it. So the load on the line was around 3000 lbs. The bowline is supposed to reduce the breaking strength of single braid spectra around 50%*. So the unknotted portion of this used rope probably has a strength of >6000 lbs. Approximately. Anybody with a small load cell that they want to send us?

I think these old lifelines were Samson Amsteel 60 grade. They are 1/4" diameter with an average breaking strength of 7400 lbs and minimum breaking strength of 6500 lbs. So I think that these lifelines, after 7.5 years, mostly in tropical UV conditions, were still close to the original minimum breaking strength, and still quite stronger than typical vinyl covered stainless steel wire lifelines. New vinyl covered stainless steel wire lifelines with a 3/16" core have a breaking strength of about 3700 lbs.

In summary, older spectra lifelines are still showing adequate strength, after several years of use. Here is the lower lifeline, aged 7.5 years. I adjusted the lashing to check for chafe. The left side of the lifeline is the portion that used to be inside the stanchion. It shows a small amount of chafe.

- Evan

*Evans Starzinger found that bowlines in regular polyester double braid only reduce the strength of the line by ~25%  http://www.bethandevans.com/load.htm. He found that bowlines would slip in single braid spectra at around 50% of breaking strength but I was using well worn rope and the rope didn't slip. Other bowline like knots such as the water bowline had a strength loss >50%. So assuming a non-slipping bowline reduces strength by 50% seems to be a reasonable and conservative assumption.
He also has some data that 8mm spectra can suffer uv damage resulting in strength loss of between 30 and 60% after 5 years and suggests that typical strength loss is somewhere in between these extremes. 

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