Jacklines - Impact Force Calculations

[Hiracer]

And one other point. I don't think you get anywhere near the dynamic stretch from webbing, tubing, or rope, if your fall takes place near the anchor point for the jackline, which is highly probably if you are addressing an ill-timed jib furling issue: an archetypical jackline situation.

[thinwater]

Originally Posted by Hiracer

My understanding is that nylon tubing and webbing loses strength when wet. Anybody know by how much? I thought it was on the order of 20%.

Good point. I missed that, and dry strength doesn' matter much, does it?

The fiber looses about 10%, rope about 20%, and webbing about 12% (less because of diffences in weave).

So, ORC jacklines actually hold 5,200 pounds. Additionally, the elasticity is probably somewhat greater; I walked dry webbing tight-rope and wet ones, and the wet ones are signifigantly spongier, perhaps 20% more so based upon my observation. This increase in sponginess, though, probably lowers the impact force and does not change the general conclutions at all.

Be ware the lack of stretch in steel cable. However, I accept your logic. I built a large guard to keep ropes away from my stack.

[MarkJ]

Originally Posted by thinwater

1-inch tubular webbing is only ~ $0.17/foot, so you are talking about only $16.00! Cheap.

bring some over when you come vist!
Price I get from home is:
Yates 1 inch Tubular Webbing - per metre

RRP $3.75 per metre 4,000 lbf. (17.79kN)

So more than $100 for 2/3 the strength!

The USA does have very low priced goods.

[thinwater]

Originally Posted by Hiracer

And one other point. I don't think you get anywhere near the dynamic stretch from webbing, tubing, or rope, if your fall takes place near the anchor point for the jackline, which is highly probably if you are addressing an ill-timed jib furling issue: an archetypical jackline situation.

True, and I thought of that but reasoned that if you were near the bow you are on a short tether (or you are going over - different math) and will take a shorter flight. I think the math will be similar. Many variables.

My cat has a wide bow and so that is a lesser issue for me.

[Hiracer]

Originally Posted by thinwater

Be ware the lack of stretch in steel cable. . . . I built a large guard to keep ropes away from my stack.

I agree about lack of stretch. Broken ribs, etc. I can't see a guard on my set up, at least not yet.

Mast jackline will remain nylon, so I'm OK there and that's the one that would get the most use. OTOH, the deck jackline if called into action would most likely be on account of a furling issue, so it's back to the lack of stretch anyhow on account of nearing the anchor point. At least that's my thinking at this point. Experience is always the better teacher.

[Hiracer]

Originally Posted by thinwater

True, and I thought of that but reasoned that if you were near the bow you are on a short tether (or you are going over - different math) and will take a shorter flight. I think the math will be similar. Many variables.

Didn't notice you were sailing a cat.

My fear about being on a mono bow is, rather than getting thrown, catching a wave and in effect getting punched by solid water. As you say, many variables.

[nautical62]

Some very good points raised here. I actually run a climbing wall as a part of my job, so these things are on my mind quite a bit. (I use climbing gear for all sorts of things on my boat)

I think one point mentioned worth focusing on is the deck fitting. It could easily be the weak link. One also needs to understand that certain forces can cause the pull on these fittings to be much greater than the fall force of the person going over. Pull a rope straight between two people and have someone grab the middle and pull. The middle person can easily pull the two end people off their feet. This is exactly what one needs to be aware of with jackline use.

[thinwater]

Originally Posted by nautical62

Some very good points raised here. I actually run a climbing wall as a part of my job, so these things are on my mind quite a bit. (I use climbing gear for all sorts of things on my boat)

I think one point mentioned worth focusing on is the deck fitting. It could easily be the weak link. One also needs to understand that certain forces can cause the pull on these fittings to be much greater than the fall force of the person going over. Pull a rope straight between two people and have someone grab the middle and pull. The middle person can easily pull the two end people off their feet. This is exactly what one needs to be aware of with jackline use.

I have used rock climbing SS bolt hangers a number of places on boats, where I needed a fitting to take a strong shear load. However, a FRP deck is not granite and getting a single 3/8" bolt to bear it's full load potencial is a question mark.

[Hiracer]

Originally Posted by nautical62

Pull a rope straight between two people and have someone grab the middle and pull. The middle person can easily pull the two end people off their feet. This is exactly what one needs to be aware of with jackline use.

This is why all the SS cable jacklines I have seen have been rather loose. Doesn't eliminate the problem, only reduces it a bit.

On a fiberglass boat you would have to have very big backing plates for the anchor points if using SS cable.

[thinwater]

An excellent summary of jacklines and tethers by US Sailing:
US SAILING - Safety At Sea - Safety Studies - Safety Tips
It includes both their rules, explaination that other groups have different rules, and some of the reasoning. More than one view, which is good.

I have also made some small corrections (wet strength and some others) and added some information to the original blog post: Sail Delmarva: Sample Calculations for Jackline Stress and Energy Absorption