Shock Load Calculator for Yacht Jacklines

Every yacht should have jacklines for keeping sailor onboard while working on the foredeck. It is also a common sense that you have to be clipped on it while sailing. I’ve followed many discussions about the tightness of jacklines. Some say that it must be as tight as possible. Others leave half a metre slack for easier moving on the deck.

Recently Yachting Monthly magazine tested what happens if the tether is so long that one goes overboard despite of being clipped on. The results were devastating. Even if the doll was wearing a lifejacket, the boat speed dragged the unfortunate sailor face down and at typical sailing speed of 5 – 6 knots the water would have been rammed down the victim’s throat.

Thus the answer is that there can be slack in jackline but not as much as the clipped tether would allow one be dragged alongside the moving boat.

Jacklines should also be inspected regularly as the webbing is probably made from polyester and prone to deterioration in the Sun’s ultraviolet radiation. Webbing’s attachment points and fittings need also to be strong enough to hold the load and prevent an accident.

The load on jacklines is huge because of simple force multiplies in the jackline rigging. Here is a calculator for the strength of force of yacht’s jacklines. Please note that this calculator is the ideal case. In the real world the knots and splices are weakening the line. For example the bowline weakens the line about 30 % – 40 %. In addition you should use a safety factor. Do not use a line with a breaking load of 2000 kg if the calculation below gives a result of 2000 kg!

So how do I test that Suwena’s jacklines are strong enough? Simple. I hold from the middle of the jackline with both hands and use my whole body weight together with a body swing to give it a good pull. If I go overboard then it is much better to happen in the marina 🙂 If not then all is in working order.