How much spaced required to hold 3/8" Acco G-4 chain

[groundtackle]

"a. G4 is lighter than G3 stainless, for equal weight."
Didn't intend to start something Thin, sorry if I come across that way, honestly!
My understanding in weights of materials for our purpose would put s.s. in the lighter category.
Can we aggree that both alloys in rod form are approx the same weight?
Stainless gets polished within nominal specs and the other gets galvanized. One gets material removed and the other added, am I wrong?
Regardless John has a great question with volume, think you guys answered it.
ce

[captstu]

We have strayed from the issue - safe anchoring with some reserve.

5/`16" G4 shot peened/galvanized chain has a MWL - also called safe working load = 3900 pounds with a breaking strength of 11,600 pounds.

5/16 316 G4 Stainless has a MWL of 2400 pounds and a breaking strength of 9600.

for a given size/weight, SS is weaker than Galvanized by 1/3 or more - so, for a given need, you can downsize one size from stainless to galvanized and still have the same published safety margins.

But, at the breaking strength limit, stainless -- breaks -- where galvanized stretches plastically to the point that it will not fit the wildcat any longer - but it will still hold your boat.

Shiney boats that sit at the dock use stainless - cruising boats use galvanized and either replace it or regalvanize it when required.

Stainless steel shackles and swivels have a significant benefit over galvanized - you always know where your ground tackle will break.!!

[groundtackle]

The issue was how much space is required.

[captstu]

We have strayed - but the original post referred to 3/8" chain and it is possible for the boat being anchored that 5/16" chain would be sufficient - and take up 1/2 the space.

I think the topic has expanded, but not really strayed.

[thinwater]

Originally Posted by Jim Cate

???? Don't quite get this...

Jim

Strength.

[thinwater]

Originally Posted by captstu

John,

Anchoring is more of a religious issue than an absolute engineering question - although I am, by trade, an engineer, most of what I learned was gained by asking the skippers in the anchorage what they did - if they were among those that did NOT drag during a blow....

Interesting story. I love a case study with good details.

It seems to me this came down to energy absorption. A wind shift so dramatic (I sat through an eye one--very weird) is a rare case, but it happens.

I don't know how much chain you had out. For the sake of math I'm going to use 150'. I don't think the amount changes the math too much (more chain means the more stretch but also that the boat might have been going faster--an offset).

I'm going to make a lot of wild guesses. Not enough info.

How much energy could the bridles absorb? Assuming the bridles were nylon and not too much affected by UV and chafe, and assuming it really was an abrupt impact load, testing of used climbing ropes suggests they could have absorbed about 1/3 of the theoretical max. I don't know the exact rope, but I'm guessing that might be about 20*2*.333*.4*29000*.5=75000 ft-pounds.

How much energy for the chain? About 45,000 ft-pounds at the WLL, but we don't know how far beyond that it went.

How much did the anchor absorb during the reset? Hard to guess. I'll say 1000 pounds * 12'=12000 ft-pounds. A hopeless WAG. I would love to know what the anchors did. Did the Bruce surface and re-bury, or did it track around. I'm inclined to think it somehow tracked, because if the Fortress was holding the load and did not shift, the Bruce would not have been able to set, it would have been up in the air. Or did the Fortress fail and drag? I am not sure.

How much energy did the boat have? Hard to guess. if 20000 pounds at 8 knots, E=(.5*20000*13.4^2)/32 = 56,000 ft-pounds.

There would also be wind load during the breaking phase, hard to guess, but perhaps as much as a ton. The wind was crazy strong. Say, 45*2000=90000 ft-pounds. This is a big number, the time period while the boat was sideways, turning.

So we have ~ 140,000 pounds of energy applied to the boat and 162,000 in breaking. Rough numbers that sorta balance, based on some horrible guesses.

What could be done differently? The bridles could have been longer, at least as long as the boat, probably a bit more. That would have added about about 25,000-45,000 ft-pounds of breaking energy. Most likely the attachments would not have broken and the chain would not have stretched. You can't stop a fall with strength, it has to be done with stretch. Mountain climbers have learned this. Other than that, everything worked reasonably well.

That said, you did very well. Boats aren't designed for that!

[groundtackle]

onavegador post #7
"And to Stuart what you wrote is against all I learned in Europe about anchoring. 1st) I decided to use galvanized chain instead of stainless steel because stainless steel chain is lighter than galvanized chain."

I guess you didn't understand my question regarding this comment..i'll try again. "Why if he has a weight sensitive vessel did he decide to use the heavier chain?" Strength component aside, we all know the galv is stronger.

[captstu]

I'm not sure what you don't understand:

Size by size, stainless weighs less per unit length than equivalent galvenized chain.

But, strength by strength, equivalent strength is almost twice the weight of stainless - look at the breaking strengt, not the safe working load.

/Stu

[groundtackle]

He used the term "lighter" when I belive he intended to say "weaker"
I think it was an english translation issue, to answer my own question.
Carry on, its all good stuff.
ce