Monday, May 26, 2014

Jacklines, Tethers, and Why Monohulls and Catamarans are Different

Often I post just to help order my thoughts. Some comments are posted and some come by e-mail and are not posted. They all help.

I've posted on tethers many times, always with caveat that my views were catamaran-specific. Now I have begun with actual drop testing, with an eye towards a published article, and solutions must be more universal. Or at least we need solutions for all boats and all uses.

A few of my notes. Scary if you can read them.

The crux is figuring out where sailors actually fall.

The bow, catamaran. On a cat the bow is wide and you should be on the windward jackline, even if it is long. If very rough and I need to lean out to add something to a clew, I've been known to clip both the leeward jackline with the short tether and something else with the windward line (the windward line will not allow me to lean out). If you do happen to go over, there is no bow wave trying to suck you under and drown you; the hulls are too fine for that. Upwind, waves can come over and wash you around, but a tight line to the weather jackline manages that, and falling on a tramp doesn't hurt. Getting thrown forward is the greater risk, as the boat stuffs a wave and slows suddenly; the cure is to terminate the jacklines some distance aft of the bow, just as they terminate forward of the transom. A clipping point in the center of the tramp is often handy, as many cats lack a forward lifeline, and on many boats the lacings are strong enough and serve very well. Lacking that, adding several strong sewn points 3-5 feet aft of the forestay can be logical and simple. Because the tethers are long, stretch is needed to reduce impacts, but only when clipped to a hard point; jacklines provide cushion. Falling off the weather side is managed by running the jacklines well inboard--we've got wide decks.

Getting lifted off the deck is also a problem. Catamarans commonly experience negative Gs up front, and the best answer is to have a jackline you can pull up against, keeping the feet on the deck. Again, the jackline should be well inboard, since the lift can throw you to windward or leeward. Again, a hard point on the tramp near the forestay is handy; when kneeling near the forestay it is common to float upwards.  For the longer tether leg, shock absorption is nice; I've been thrown 12 feet before, airborne from the cabin roof, landing on the tramp. Between the give of the tramp, tether, and jacklines, not a bruise.

Catamaran, aft. The risk here is simply falling off the back, and stumbling cause by a cat's quick motion. Since the tether is clipped to a hard point, there is no shock absorption, and the tethers are long. Getting swept by away by a wave is a minor risk on most cats; forward, yes, but not in the cockpit, not ever. The boats move over and away from waves. However, tethers are needed when working outside and aft.

Monohull, bow. The scary fall is to leeward, under the bow wave. Folks have drown there. And thus, a quick release is required, something of much less interest to a cat sailor. The answer there is a short working tether to a fixed point to weather, and a secure grip. It gets skinny up there, something cat sailors don't face.

Monohull, aft. All documented tether failures have been in the cockpit. The helmsman was stuck by a wave, thrown the width of the cockpit, and lacking any shock absorption (clipped to hard point by a static tether), overloaded the system. Broken ribs and other injuries due to tether impact forces were involved. There are 2 answers to this problem: clip short and use a dynamic tether.

Climber's bolt hangers provide solid, simple, inexpensive hard points.
Short, working tethers are important, particularly for boats racing in extreme conditions. Very few falls are documented while people are moving; they are holding on. It is when they get to the work station, take their hands off the rails and focus on the task, that they get thrown. Thus, there should be a good anchor for every station, located such that the sailor cannot get thrown. On boats prone to stuffing the bow--some cats and some sport boats--a tether from behind may be prudent.The new AC cup boats have a tether requirement, specifically directed at stuffing a bow when bearing off. Keeping sailors on-station can prevent a capsize, since they can still do their job. The personal tether must have a short leg, or a fixed tether can be provided at each station.

Dynamic tethers are another part of the answer. ISO standards now required a drop test that should create nearly unbreakable tethers, but they may break the sailor in the process (unlike climbing drop tests, they did not specify a maximum impact force). I'm still using 8mm dynamic rope tethers, which are considerable lower in impact and tougher (can absorb more impact) than ISO  tethers, but they have one drawback; the rope can roll under foot. I would love to find a webbing with required stretch properties, but I'm not certain it exists; the weave may prevent that.


There are other issues.
  • The best quick release.
  • The quick releases must be accessible when the PFD is inflated (most are not).
  • Harness design. The ergonomics are terrible, but there is no practical test method, and short of a full body harness, no good way to transfer the load to a body without damage.
  • Where you you park the spare tether leg clip when not in use? Don't clip it to your harness, or you will have no release. But most tethers provide no alternative. My advice is to add something.
  • Folks don't like tethers, particularly if there is crew on board.  But is the crew skilled enough to get the chute down and get back to you? Hmmm.


So what are your MOB concerns? Where can you take a hard fall on your boat? What have you witnessed or read of? We need data to design solutions. 

1 comment:

  1. I went MOB in 2012 solo race to Hawaii. Boat = Olson 34 monohull. Even a short teather wouldn't have prevented me from landing 90% in the water. As it was I was in deep.