Mailport January 2014 Issue

Mailport: January 2014

Thoughts on PFD Design

I have been aghast at how ill-conceived the standard system is for the design of inflatable PFD-harnesses for sailors. Practical Sailor is uniquely positioned to research the issue and inform the sailing community of better-designed systems.

As a rock climber, I have some serious concerns about the design of inflatable PFD-harnesses, and actually about the entire jackline/tether/harness systems employed by most boats. A good system would keep crew aboard and minimize the forces required to do so. As a climber, I know that the farther you fall, the greater the forces generated, and the shorter the distance over which you come to a stop, the greater the force.

Typical jackline system designs allow sailors to fall way too far, and then jerk them to an abrupt stop. Reports of failed gear and internal injuries abound for this reason. Offshore racing regulations have been beefed up to require bigger and stronger gear to reduce failures, but this has resulted in more injuries. The problem is in system design, not the strength of the gear. Even extremely strong gear can fail easily, and even short falls can cause serious trauma.

Jacklines are typically at deck level. In addition to being a tripping hazard, this requires a long tether, therefore, enabling longer falls. If jacklines were strung at chest level, or close to it, tethers could be much shorter, and long falls prevented. This would have the added benefit of giving the crew something to hold on to and act as an extra high lifeline. An adjustable tether, like those used by arborists, would allow crews to extend their range as necessary, and minimize length when possible to reduce the potential fall distance.

Jacklines and tethers are almost always made of nylon webbing, which has very little elasticity. As result, a fall can generate high forces. A way to avoid this would be to design elasticity into the system. Jacklines should be made of webbing with some degree of elasticity, like the polyester webbing used for slacklines; it is stronger and more elastic than mil-spec webbing. Tethers should include a shock absorber like those used in rock climbing and industrial applications.

A well-designed harness should fit around the upper chest, under the armpits where humans have significant muscle mass and strength. Even a short fall in normal sailing harnesses, which sit way too low, will likely result in injury as the forces wind up on the lower rib cage, a relatively weak area. Climbing chest harnesses are designed to fit high on the chest.

I think a well-designed inflatable PFD with harness would actually be two separate systems. The chest-style harness with inflatable bladders distributed more broadly around the torso like a modern whitewater PFD, instead of bunched up around the neck like the crappy orange vests you had to wear as a kid that always float up over your head. The current designs try to integrate the PFD and harness into the same structure, which is a compromise to both systems.

Tripp Presnell
Piper, 1984 Najad 343
Beaufort, S.C.

Comments (2)

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Vic Lucas

Posted by: lvictorlucas | February 2, 2014 10:23 PM    Report this comment

Thank you Neal for your insightful comments.

We would totally endorse your ideas that the energy absorbing characteristics of the chain in a snubber/chain rode should not be ignored and we did make mention that the kinetic energy of a moving yacht would be 'shared' between chain and snubber. However we were trying to emphasize the elasticity of the snubber as our observations are that snubbers are simply not frequently used and when evident are both too short and too large in diameter to utilise their energy absorbing characteristics.

All chain rodes are frequently mentioned and illustrate the problem. Many who buy 'modern' anchors always comment that when they set their anchors the suddenness of deceleration when the anchor bites as the vessel drifts back on the wind is sufficient to knock over the unwary. Catenary never 'disappears' and even under benign conditions with still some 'sag' in the chain the suddenness of the 'stop' is sharp - basically the chain impact as if the vessel has hit a brick wall (or a very unforgiving rock on the seabed). Try the same experiment with the snubber attached - and it is possible to both stand upright and do so holding a generous glass of chilled chardonnay! Basically the nylon snubber absorbs the kinetic energy of the moving vessel better than does the catenary. But this needs to be checked - particularly as there is a move toward lighter (and stronger) chain.

Sadly with your comment, you have anticipated some of our further work. We intend, in the fullness of time to look at the effects of combining a rode of nylon and chain and comparing it with only nylon and only chain. We also want to look at different types (different manufacturers) of anchor plait - as we think there might be significant differences in performance.

'Anchor rodes' are a work in progress - we hope to quantify the answers to some of your questions in the future, in the meantime we are grateful we have provoked some insightful comment from you.

We might make one comment on bridles. If one arm of the bridle fails (usually with a sound like gunshot) then the other arm will take an increased load as the vessel with then lie at an angle to the wind and present a higher windage. We would strongly recommend that anyone using a snubber or bridle always has a spare, that snubbers are considered as consumables and that a snubber that is adequate upto, say, 35 knots will not be adequate at 45 knots and 'storm' snubbers should be part of the armoury.

With thanks

Posted by: Jonathan N | December 16, 2013 3:08 AM    Report this comment

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