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-   -   Jacklines (http://www.sailnet.com/forums/general-discussion-sailing-related/46137-jacklines.html)

xort 08-16-2008 10:27 PM

Jacklines
 
Time to rig jacklines. I have an inner forestay mount (no inner forestay) to attach the head ends to.

I have a center cockpit boat and there is a bracket for liferaft on the aft cabin top. I could run the jacklines under the lip of the bracket and tie the two ends together. It would keep the lines inboard as much as possible at the stern. The bracket has an overhang of about 3" to hold the lines down. The other attachment point would be the aft cleats and that puts the jacklines closer to the edge. Assuming I can tie the two lines together properly, any problems with this idea?

I'm going to use nylon web strap. Was thinking of visiting a climbing shop to buy the stuff. suggestions on what to look for?

Saildoggie 08-16-2008 10:39 PM

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sailingdog 08-16-2008 10:47 PM

Xort-

The best jacklines I've seen are the ones made up of 1/4" spectra cord with a dacron webbing chafe protector over them. They're a bit more expensive than regular webbing, but IMHO worth it.

SEMIJim 08-17-2008 08:53 AM

Xort,

Sailrite sells a variety of flat and tubular webbing material. I can't find it on their web site, but their catalogue lists a jackline kit consisting of their #1271 (white) or #4532 (royal blue) webbing (both 4700 lbs breaking strength, ordered to length) and their #2326 Wichard Snap Hook (5730 lbs breaking strength).

Jim

sailingdog 08-17-2008 09:13 AM

The recommendation for jacklines is usually a breaking load of 6000 lbs. The forces generated by a person falling across the deck of a boat can be substantial.

Whampoa 08-17-2008 10:53 AM

Morning SD,

Any idea what kind of G forces are involved in the scenarios we are talking about here?

I've seen estimates of the forces imposed on a hunters body involved in a fall from a tree stand but caught by his harness but I have not seen anything similar for these scenarios.

Curious,

John

Boasun 08-17-2008 11:16 AM

This looks like a project for a math major in a university.

sailingdog 08-17-2008 11:41 AM

Just to give you an idea of the G-forces that can be involved, I did a bit of research for a post on another forum:

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.

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.

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:

Quote:

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 practical 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!
If you figure that you can reach up to 30 G's in a broach, you might want to consider the breaking strength of the tether and jacklines.... If you weight 180 lbs.... and reach 30 G's... you're effectively loading the tether and jacklines with 5400 lbs. for a very short duration. If you've got jacklines with a breaking load of 4750 lbs... they might not hold. If you've got jacklines with a breaking load of 6000 lbs., they might not hold, but the chance that they do is far better, don't you think?

Whampoa 08-17-2008 11:51 AM

I figured you had already done some research into this. Thanks for sharing....The numbers speak for themselves and need to be factored into not only the type and strength of the materials used for the jackline but also the attach points and the harness, etc.

Thanks SD.

Best Regards, John

SailorMitch 08-17-2008 11:54 AM

The ORC recommendation for breaking strength is 4,500 lbs. I just bought jacklines for my boat from JSI. The highest breaking strength I could find was about the 5,700 lbs quoted above.

here's the ORC reference.

<TABLE border=0><TBODY><TR vAlign=top><TD>4.04.1</TD><TD>The following shall be provided: </TD><TD width=110></TD></TR></TR><TR vAlign=top><TD></TD><TD><TABLE border=0><TBODY><TR vAlign=top><TD>a)</TD><TD>Jackstays:</TD></TR></TBODY></TABLE></TD><TD width=110></TD></TR></TR><TR vAlign=top><TD></TD><TD><TABLE border=0><TBODY><TR vAlign=top><TD></TD><TD>i</TD><TD>attached to through-bolted or welded deck plates or other suitable and strong anchorage fitted on deck, port and starboard of the yacht's centre line to provide secure attachments for safety harness:-</TD></TR></TBODY></TABLE></TD><TD width=110>MoMu0,1,2,3 </TD></TR></TR><TR vAlign=top><TD></TD><TD><TABLE border=0><TBODY><TR vAlign=top><TD></TD><TD>ii</TD><TD>comprising stainless steel 1 x 19 wire of minimum diameter 5 mm (3/16 in), or webbing of equivalent strength; </TD></TR></TBODY></TABLE></TD><TD width=110>MoMu0,1,2,3 </TD></TR></TR><TR vAlign=top><TD></TD><TD><TABLE border=0><TBODY><TR vAlign=top><TD></TD><TD>iii</TD><TD>which, when made from stainless steel wire shall be uncoated and used without any sleeving;</TD></TR></TBODY></TABLE></TD><TD width=110>MoMu0,1,2,3 </TD></TR></TR><TR vAlign=top><TD></TD><TD><TABLE border=0><TBODY><TR vAlign=top><TD></TD><TD>iv</TD><TD>20kN (2,040 kgf or 4,500 lbf) min breaking strain webbing is recommended;</TD></TR></TBODY></TABLE></TD><TD width=110>MoMu0,1,2,3</TD></TR></TR><TR vAlign=top><TD></TD><TD><TABLE border=0><TBODY><TR vAlign=top><TD></TD><TD>v</TD><TD>at least two of which should be fitted on the underside of a multihull in case of inversion.</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE>


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