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Re: Jackline system
The problem with tubular or flat double or triple ply webbing is the undue stretch that occurs under (shock) load when wetted. For 'climbing' this is good as it adds 'shock resistance' during impact (increases the 'time' of the impact); but, on a boat may cause the geometry to cause one to 'rebound' outside the limits of the lifelines.
In mountain rescue we hardly ever use 'webbing' for situations where the 'tether' attachments are at near horizontal and where the impact loads are perpendicular to the restraint line (ie. Tyrolean Traverse, etc.), we use 'rope' for all 'horizontals' (a similar config. to the jacklines set on boats); plus, all such 'traverse' lines are always set up 'slack' and allow a LOT of stretch so that the trigonometry of configuration does not develop infinite force in the traverse lines. If such a line is 'taught' and you push/pull at 90į to the run of the line and the line does not 'stretch', the force can be multiplied many times (can approach infinite force) that of the applied perpendicular force. In trigonometry, such a situation would be equal to "dividing by the trigonometric SINE of the deflection angle" of the line from its attachment point ... Inotherwords, apply force at 90 degrees to a very taught line and the attachment connections of that very tight line can possibly develop forces that approach INFINITY, which can break the rope, pull the connections out, etc.
The good thing with webbing used as jacklines is that the stretch upon impact, etc. allows the attachment angles to increase to comparatively very large angles with a corresponding lessening of the induced tension developed in the 'rope'/webbing. The downside is when webbing becomes wetted and unduly and increasingly stretches and you 'can' have a functional failure .... your body at full impact load can now be beyond the boundary of the deck.
Simple speak Rx: when setting up webbing jacklines on a boat, run them across the cabin top instead of the deck and leave them slightly 'slack', NEVER bar-tight!!!!!! Wet them thoroughly and TEST them with your body weight (impact) to be sure that youre getting large angles of deflection from the axis between the connection (to the boat) points when fully loaded by impact of your body weight perpendicular to the 'run' of the jackline .... but test them and adjust them so that the 'stretch' that occurs, keeps you onboard. When testing, If little 'stretch' occurs in the jackline and the connection 'angles' remain less than ~10į when you (test) impact, you risk pulling the connection points out, due to the possibility of the generation of infinite force along the axis of the webbing. Rope/wire cable doesnt stretch like webbing, so you have to set up with more 'slackness'.
I use a tether with three connections: one to the harness, a connection at the terminal end of the tether (in the cockpit usage), and one close (about midway) to harness for moving along the deck.
Last edited by RichH; 04-30-2013 at 11:08 AM.