Mounting & use of Jacklines and tethers and sharks. Oh, my!

[BobW]

Jacklines and tethers are a major concern and a major topic of discussion.  Here's a link to a thread on the Singlehanded Sailing Society Forum about jacklines, including a vivid description of a fatal incident during the 1999 Doublehanded Farallones Race.

http://p4.forumforfree.com/safety-jacklines-vt151-sss.html

[AdriftAtSea]

Good article Bob.  I have the jacklines on the Pretty Gee run from the aft part of the bow pulpit along each side to the stern cleat.   The lines run outside the shrouds and let me go forward with little trouble, as I have the entire ama deck and netting to go forward on.  It does help that the boat is 18' wide. 

However, I don't generally clip in unless I'm single-handing or the weather/seas are really rough, since the boat is quite stable and doesn't heel much in even strong winds and seas.  A good example is when we were out last October, in 20 knots of wind with 4-6' seas to start with—no one was tethered.  Even at the end of that day, which was more like 28 knots of wind, with gusts to 35, and seas as large as the occassional nine-footer or so thrown in... it was fine. 

One thing that helps a lot, is that unlike many trimarans, the amas on the Pretty Gee have a foot high bulwark along the outboard edge, which keeps the boat very dry, and makes going overboard much more difficult.

[Godot]

I was going to make this part of my Prepping my Seafarer for the Scoot thread; but figure it would be better here.

I want to discuss jacklines.  Maybe we'll talk a little about tethers, too. 

On The Scoot, I have two big concerns regarding falling overboard.

#1 How to avoid it.
#2 How to get back onboard should #1 fail.

First, if we could, let's consider the strength requirements for the Jackline....

Marisafe also has good deals on other equipment.  I got the 20' jacklines and they are a good length for boats in the 23-25' range:
http://www.marisafe.com/store/viewItem.asp?ID=353510790&CID=35350000

BTW, I wouldn't get the jacklines there...since most recommendations for jacklines suggest a minimum breaking strength at 6,000 lbs. :) The ones at the site you mention have a BL of 4,400 lbs.

I believe I have seen two recommendations for jackline breaking strength, one around 4400 or 4500 lbs, and one at 6000 lbs (outside of the referenced thread).  Excuse me for throwing conventional wisdom, or generic recommendations, out the window.  But could someone give me an idea of what kind of forces a jackline is likely to experience.  Let's assume a crewman who weighs 260# goes overboard on the high side (so he has further to fall ... 6 or 7 feet maybe?), and then gets dragged along at around 6 knots.  The estimate I think I recall hearing (dammit, I'm working on flakey memory an awful lot lately) was that the jackline might have to absorb a ton of force when the crewman gets jerked up.  But there is stretch in nylon webbing, and the force would be coming from the side.  It seems to me there would be a fair amount of shock absorption.  But let's say that my recollection is correct and the strap has to handle 2000 lbs, that is still significantly less than the 4400 lbs provided by the inexpensive jacklines.  Is it really necessary or desireable to go up to 6000# (or even 9800# like you find at hathaways.com)?  I'm thinking the higher recommendations might be more valuable on larger, faster boats.  Please educate me!


Second, how should jacklines be installed?

The way I see it mentioned mostly is to attach the jacklines to a large, through bolted cleat or hard point at the bow, and have them run back on either side of the boat to  cleats near the stern.  This would have the jacklines near the sides of the boat which goes against the other common rule, which is to keep them as close to centerline as possible.

I'm thinking the best solution would be to have two halves of a jackline run from either side of the companionway hatch (or dodger) up to a point a little forward of the mast and shrouds.  This would keep them closer to the centerline of the boat.  From that same point forward of the mast, attach another line which would run to the large cleat/hardpoint at the bow.  I would say that two tethers would be mandatory with this system.

Disadvantage:

• You would need to unattach/reattach yourself on the way to/from the bow.  In the event of a time critical emergency, the sailor may neglect to hook on. (Bad, BAAAD sailor <slapping wrist vigorously>)
• Vendors don't appear to sell ready made straps in these smaller lengths.

Advantage:

• With the jacklines closer to center, if the sailor slips, he is more likely to be caught short before going swimming, and if he does get wet, probably will be hanging off the side of the boat a little higher up.
• With shorter lengths, the jacklines shouldn't stretch as much.

Third and finally, how the heck are you gonna pull yourself back onboard?  I did not sign on to be shark bait.

If you are on the low side it may be possible to scramble the short distance back on deck.  If well heeled over perhaps the high side would be horizontal enough to scramble onto.  Somehow, I seriously doubt it would be this easy.  And if relatively upright, I can't think of anything promising.

An aft boarding ladder might be useful in general; but I don't see how, using my planned jackline system, you would ever get there while remaining attached to the boat.  If you fall off forward of the shrouds, a rope ladder you can pull down from the shrouds might be useful.  If you go under the lifelines, you might not make it back that far (actually, on my Seafarer I would make it back as currently configured ... maybe I shouldn't add that extra stanchion after all).  If you fall overboard behind the shrouds, well, I guess it would be necessary to have another set of rope ladders (anyone know of a design that is reasonably easy to climb) that can be grabbed from near the cockpit (that would make it four emergency rope ladders ... I wonder if I can get bulk pricing).

Hmm.  I guess keeping the tether short so as to not fall overboard at all may be the best solution.  But then would I be able to do work?  Scary topic.

Related thread: Mounting hardware for tether

[Auspicious]

Simple rules:

Keep the boat in the water
Keep the water out of the boat
Stay in the boat

Okay - I have that off my chest. <grin>

Many people run jacklines from bow cleats to stern cleats. That works fine. I have separate attachment points for mine mostly because I don't like having to disconnect one thing to attach another -- the thought of wrapping a jackline around the prop while maneuvering around a dock is embarrassing at least.

You make good points about staying on the boat. Although my jacklines run outboard of the shrouds, I do wrap one leg of my tether around the mast when working there.

The combination of jackline and tether should let you get all the way to the stern.

I've given occasional thought to the problem of getting back on the boat. I keep coming back to the reality that not falling off is a real good idea. <grin> If we put too much safety stuff on our boats they will be safely on the bottom. Frankly if adrenaline doesn't get me back on I'm not sure I'll make it.

[OptiMystic]

There was a debate as a tangent on something else (single handing) on SailNet the other day and one poster, who makes custom jacklines and tethers, had a very strong opinion about tethers keeping you from going over even if it means having multiples of different lengths. He brought up a lot of the same issues you did. It's like there are 2 schools of thought - is a tether supposed to keep you on the boat or just with the boat? If the boat isn't absolutely self righting (multihull or whatever), a short tether would mean that you would have to unclip in order to surface if you capsized and risk getting separated from the boat. You would likely have to unclip anyway, but it would be good to be able to surface and get your bearings, and hopefully be holding on when you unclipped. On a traditional blue water monohull, I think what he says makes sense. But I am an armchair blue water sailor; haven't been there, haven't done that and anybody can buy the shirt.  8) I have thought about this more for making sure I stay with my boat under less perilous circumstances.

[AdriftAtSea]

The main reason for the need for such a high breaking load is that the shock loads of a person thrown across the boat by a broach or accidental gybe can generate much higher loads than their actual weight.  Rotational acceleration, like that caused by a broach or gybe can easily exceed gravitational acceleration by an order of magnitude. 

As for tethers, many are available that have a 1-meter and a 2-meter leg.  That gives you two points to anchor yourself with as you move around the boat, as well as a few different ways to attach to the boat.  :)

I have my jacklines setup to keep me on the boat for the most part... the only time they wouldn't really work to do is when the amas are retracted, but they would still allow me to get to the stern and get back aboard.

[Captain Smollett]

Rotational acceleration, like that caused by a broach or gybe can easily exceed gravitational acceleration by an order of magnitude. 

10 G's, really?  A SAILBOAT broach producing g-forces in the same regime as an F-16?

Got a creditable citation for that?

[s/v Faith]

FWIW,

Just a little more 'opinion' with a little experience mixed in.

  If we are talking about jack lines on small boats, I doubt a system can be made that would both allow mobility and keep you from going over the rail.  The distance from the sidedeck to the rail is just too short, your jacklines (even low stretch) will stretch enough if rigged fore/aft for ~20 -25'.  Even if you ran the jacklines through the 'D' ring on your harness you are going to have enough stretch to flip over the rail.

  The distances are simply too short... unless you are talking about multiple attachment points along the jackline.... which would be a real pain when going fwd.

  Rose and I simply used a pair of double braid dock lines running from the bow cleats to the stern cleats.  I like, and had looked into, flat strap jacklines, but they were going to cost too much.  Yes, the round dock lines can roll under your feet just like in the magazine articles, but going forward off shore your CG should be low enough you are not gonna be standing upright anyway.   Our inflatable vests (only worn off shore) have built in harnesses... our tethers were 3/4" 3 strand with seized bowlines to heavy SS snap hooks (rated at #3900 IIRC).

  We were clipped in whenever on deck, and were able to go forward with a minimum of tangling.

  Keep in mind, that going over the side if you have lifelines is going to be messy anyway... your tether is likely to go over the top of your lifelines and bend some stanchions.  The problem of getting back into the boat is a much better one to have then the problem of being left behind IMHO.

  If anyone is thinking about getting something complicated, keep in mind that if your 'system' is too much of a pain to use.. you probably won't. 

[Godot]

Rotational acceleration, like that caused by a broach or gybe can easily exceed gravitational acceleration by an order of magnitude. 

10 G's, really?  A SAILBOAT broach producing g-forces in the same regime as an F-16?

Got a creditable citation for that?

Here we go ... THIS is EXACTLY what I was thinking.  Even if the force is 10 G's, and even with my oversized butt, that would bring the force to 2,600 lbs.  1800 lbs less than the breaking strength of the cheaper jack lines.  That would hurt, but it seems like I should remain attached to the boat.  A 6000 lb breaking strength jack line would hold me through 23 G's.  If that happened, I'm not sure there would be enough of me left to drag back onboard.

[Auspicious]

I just *knew* you guys were going to make me do math.

Remember the load is perpendicular to the original line of the jackline, and all the load has to be taken in tension (two ends pinned with an elastic structural connection).

So over-simplifying a bit for example:

If the initial deformation of the jackline results in a 20 degree angle and the load is 500# (you plus drag of you in the water) the tension in the line is

tension = 500# / sin (20 deg) = 1432#

That's just the static case. If you add dynamics (you hurtling across the deck before you take up on the tether) and if you have a jackline that doesn't stretch much, and you cinch it up real tight, the numbers get real big real fast.

Ignore drag, assume 3G deceleration when you hit the tether, and assume stretch and tightness limit the angle to 12 deg (seems a decent number <grin>) and you get

tension = 3 * 250 / sin (12 deg) = 4300#

If you use Excel, remember that it does trig functions in radians and you will have to convert.

[okawbow]

I seem to remember reading that it is easier to break a line by pulling on the center of it, when the ends are secured, as with a jackline, than it is with a straight line pull. I have also read about sailors in roll overs, breaking their tethers. I believe I would err on the high side when it comes to break strength.

On my bristol 24, I run a single 6000# jackline from the portside of the bow, to the stern. I always move along the starboard side of the boat when possible. That, at least, prevents me from going over the starboard life line, and If I go over the port side; I can reach the boarding strap or ladder. I hope with enough adrenaline, I can get back aboard. I can also reach the autopilot control from the port side. i also have strong, hard points for the cockpit and bow.

[Zen]

Keep the boat in the water
Keep the water out of the boat
Stay in the boat

revised boating rules:

Keep the water out of the boat
Keep the boat in the water
Keep the people in the boat
Keep the slimey side down
Look good

[Godot]

Ah, ha!  The math!  Good!

Those numbers just seem hard to fathom.   

[Captain Smollett]

On my bristol 24, I run a single 6000# jackline from the portside of the bow, to the stern. I always move along the starboard side of the boat when possible.

How do you get past the mast?

[Shipscarver]

I wish I could remember the name of the solo multiple circumnavigator who always kept a trailing line off the stern in case he went over. He did, in a storm. Said he would never forget that you can't climb back on-board while being towed behind the boat. Kind'da like trying to climb back on-board while being keelhauled. Exhusted and in poor shape, still attached, he was fished out 4 days latter off NZ by a passing boat

[Captain Smollett]

Someone here (Auspicious Dave was that you, by chance?) I think posted a link a while back to some MOB rescue 'data' that outlined a BUNCH of at-sea MOB cases.

Many resulted in drowning, with the MOB AT the boat...being drug through the water...but unable to board.  A crewperson tired BEFORE the stress of finding himself overboard, cold water and being trolled at 6 kts means that you have very little time ( about 2 minutes?? does anyone remember the 'stat' on this) to get back aboard.  Total and complete exhaustion happens about that fast.

It's truly scary stuff.  Offshore, MOB with a shorthanded crew is NOT a good situation.  STAY ON THE BOAT is the way to approach it, imo, and you have to believe that if you DO go over the side, you have a very, very small chance of surviving.

[AdriftAtSea]

s/v Godot-

You also have to account for loss of strength due to chafe, uv-deterioration, and other such things.   The best jacklines I've seen were made of 6mm Spectra line inside a polyester tubular webbing jacket.  It had a breaking load of over 10,000 lbs, with the core spectra line providing 8,500 lbs. of the strength.

Capn Smollett-

As for numbers... here are some for you:

a (g)    event
2.9    sneeze
3.5    cough
3.6    crowd jostle
4.1    slap on back
8.1    hop off step
10.1    plop down in chair
60    chest acceleration limit during car crash at 48 km/h with airbag
70 - 100    crash that killed Diana, Princess of Wales, 1997
150 - 200    head acceleration limit during bicycle crash with helmet

So, I do believe that falling across 8' of boat rotated through 70˚ of heel from port to starboard might well leave you with g-forces of well over 10 G's.  In fact, you can probably break 20-30 G's in a spinnaker broach.

[Tim]

Gee Dan, thanks for putting it so...... graphically  ::)  ;D

[AdriftAtSea]

Tim-

Glad to help. :)   After all, Capn Mullet seems to doubt the G-forces that can be generated on a sailboat.  If plopping your butt down in a chair can generate 10 G's of force... getting thrown across a boat by a spinnaker broach or accidental gybe is probably at least 10 G's of force IMHO. 

He doesn't seem to understand that G-forces don't require great speed...since they don't measure speed but rates of acceleration and deceleration.  For instance, dropping a computer 60 CM onto the floor can generate up to 500 G's of force... since the deceleration cause by the computer hitting the desk and stopping suddenly is very high.

Here's a explanation using a Steel ball and actual numbers:

For simplicity, suppose that you drop a steel ball, which has a diameter of 10 cm and a mass of 1.0 kg, onto a thick steel plate from a height of 60 cm. The ball will, for all pracical purposes, bounce elastically from the plate.

Suppose that during the collision process the steel ball compresses one millimeter. [While this figure is a bit large, it makes things simple to calculate]

The speed of the ball when it reaches the floor can be found using energy conservation: GPE=KE therefore m*g*h=1/2*m*v^2 Solve for v = sqrt(2*g*h)=-3.43 m/s.

The time for the ball to stop can be determined from D=Vave*t, therefore the time can be calculated by dividing the distance traveled during the collision by the average velocity during the collision. t=0.001m/3.43/2=0.00058sec.
Finally, the acceleration can be determined from Vf=a*t+Vo

where Vo=3.43m/s,Vf=0m/s and t=0.00058sec

t=3.43/.00058=5914m/s^2.

Since the acceleration of gravity is 9.8m/s^2 this will give an acceleration in terms of the gravitational acceleration of 5914/9.8=603 g's!

[Captain Smollett]

(Adam, sorry for the hijack).

a (g)    event
2.9    sneeze
3.5    cough
3.6    crowd jostle
4.1    slap on back
8.1    hop off step
10.1    plop down in chair
60    chest acceleration limit during car crash at 48 km/h with airbag
70 - 100    crash that killed Diana, Princess of Wales, 1997
150 - 200    head acceleration limit during bicycle crash with helmet

So, I do believe that falling across 8' of boat rotated through 70˚ of heel from port to starboard might well leave you with g-forces of well over 10 G's.  In fact, you can probably break 20-30 G's in a spinnaker broach.

Uh, this is all well and good, but where does it say anything about a sail boat broaching on that web page?  Beliefs are not facts, nor are they citations of actual measurements.  Sorry.

The key misleading thing about that list is that those are very quick decelerations...not the same thing as centripetal acceleration due to a turning boat.

Let's see...as I recall from my days teaching college physics, centripetal acceleration of a body rotating at a constant rate was pretty straightforward to calculate.

a = rw^2

where r is the radius of the turn and w is the rotational velocity.

Let's say the boat makes a 90 degree broach in 3 seconds, and turns within one boat length...say 30 ft for a sailfar boat. 

This gives

r = 30 ft
w = 90 degrees / 3 seconds = (pi/2 radians) / 3 seconds = 0.52 radians per second

Thus a = 8.11 ft/s^2, or about 0.25 g, since 1 g = 32 ft/s^2

Even if we assume only one second to go through a 90 degree broach, that gives a = 2.3 g.