January 2014 Issue
Table of Contents
Where Credit Is Due
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.
Piper, 1984 Najad 343