Friday, September 14, 2012

Armpit Engineering

A rather unflattering phrase, one engineers use when they believe a design was developed without the application of math. We see this a lot on boats; today's topic is lifelines.

In the September issue of Practical Sailor regular contributor Ralph Naranjo recounts experiments done at the Naval Academy to illustrate simple engineering truths to midshipmen. It was a companion piece to a longer article reviewing the application of high-tech lines for life line application.

The bottom line is that the wire never fails, nor does any well-founded cable hardware. It is the stainless railing that gives way, in this case on Navy 44s, which are very sturdy boats. The stainless crumpled at 1,200 pounds, which is a load very plausibly imparted to the lifeline by a flying sailor. This is why jacklines are such a vital compliment to lifelines; together, you stay on-board and in one piece.

Imagine a 4,200-pound load applied longitudinally to either the pulpit or the pushpit--and both must pass, weak link and all that. The simplest way to visualize this is to reorient the boat vertically, to where it is standing on its nose and hang the family car--a real car, not a Nissan Leaf. Honestly, there is no chance at all that it will take the load. It will bend. It will tear out of the deck. The through-bolts (typically 1/4-inch bolts which will shear at about 2,500 pounds) will fail. And yet folks worry the line to death.

The reality is that the loads on the cable will not approach 4,200 pounds, not after some initial bending. It's just as well perhaps; if it didn't flex in a extreme fall it would leave one heck of a groove and quite likely break bones, like a metal edge. The 4,200-pound figure is only to insure some basic ruggedness, and as such it is a good figure. But it has the effect of focusing our attention in the wrong place, as there is not a matching requirement for the pulpit and pushpit. Odd.


Practical engineering is often no more than simplifying the system and looking at each element in terms that can be understood. Simple levers and moment arms. I still am undecided as to whether I will go with Amsteel or SS cable when I replace the lifelines, though I'm leaning strongly to 1/4-inch Amsteel with Dyneema chafe guards as over strength enough to last many years even when UV and chafe are considered; I think it will be more hand-friendly. An it will look over-engineered and some dock-walker will mutter something about my armpit engineering....

You may enjoy this rant about Captain Safety and over-engineering.

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