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  #11  
Old 01-29-2011
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Basically, a simple way to tell what the leverage of a block and tackle setup is, is to count the number of times the line is attached to the moving part of the block and tackle.

On a basic mainsheet, with a triple block at the boom and a triple block with becket and camcleat at the traveler, you have a six-to-one purchase. There are SIX lines coming off the upper block.

On my new mainsheet, which is a double-speed mainsheet, it has a double fiddle block at the top and a double fiddle block and a single block at the top with a double camcleat setup...if you pull one of the two lines, you get an 8:1 purchase, and if you pull on both lines, you have a 4:1 purchase.

If you count the number of lines coming off the double fiddle block, you'd see that the line is attached to it EIGHT TIMES.

BTW, Nemier is wrong... I have yet to see any marine blocks that have the SWL or BL stamped or indicated on the block or block cheek plates.
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  #12  
Old 01-29-2011
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Normally the blocks are stronger than the combined SWL of the parts of the line roved at the moving block.
But if you find that the strap of the block, the becket of the block or the fittings on the block are distorted then replace that block immediately. Because it has be strained beyond its safe working load.
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  #13  
Old 01-29-2011
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Depends on the line... while this used to be true, with the new spectra/dyneema-based lines, this is probably no longer the case.
Quote:
Originally Posted by Boasun View Post
Normally the blocks are stronger than the combined SWL of the parts of the line roved at the moving block.
But if you find that the strap of the block, the becket of the block or the fittings on the block are distorted then replace that block immediately. Because it has be strained beyond its safe working load.
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  #14  
Old 01-30-2011
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Quote:
Originally Posted by sailingdog View Post
Basically, ...
I understand all that. What I want to know is, can I use the same method to calculate the breaking strength or SWL (I know they're not the same) of the entire system, given that I know the strength of the line, and assuming the blocks and adjoining hardware are infinitely strong?

In other words, if I take your basic 6:1 mainsheet and use it to hoist, I dunno, a small whale, and the line has a breaking strength of, say, 5,000 lbs, can I hoist a 29,999 lb whale without the rope breaking?
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Old 01-30-2011
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While in simple terms it is a 6:1 ratio there are many factors with friction and the load carrying ability of the sheave bearings

If you look at a cable crane then yes it could pick up a weight far greater then the same cables straight line breaking strength
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  #16  
Old 01-30-2011
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Dear Adam:
Hoist your whale, but make sure the block and tackle is attached to the boat with the same six wraps of the line (6x5000lbs), because if you use a shackle, it will probably break. The sheaves could all have the same 5000 pound strength, since the force is divided among them, but the attachment points get the whole effort refocused back on them. That's why boom vangs distort and fail when people "improve" them with stronger line.
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Old 01-30-2011
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Exclamation

Quote:
Originally Posted by paulk View Post
Dear Adam:
Hoist your whale, .
Is this not illegal? As for the original question and the answers given,my brain has blown the safety fuse.
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Old 01-30-2011
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Quote:
Originally Posted by sailingdog View Post
Depends on the line... while this used to be true, with the new spectra/dyneema-based lines, this is probably no longer the case.
Excellent point; many of these were designed for polyester. If threaded my dingy tackle (small blocks) with Amsteel and led it to a winch, I'm SURE I could break something!

Which brings another point; larger blocks are often designed with the basis that they will be led to a winch. Many smaller blocks were intended only for hand tensioning.

Another thing this thread has skipped is cascade tackles, common on vangs and out-hauls, where a simple purchase pulls on the end of another.

It's too bad every would be DIY sailor doesn't have quick access to a year of engineering statics and dynamics. Guessing is tough; fortunately there are many rules-of-thumb developed by hard expereince.
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  #19  
Old 01-30-2011
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I'm thinking at the point where the two sides meet the line will be pulled apart at the sum of both, anchor side =1000Lbs, wieght = 2000Lbs, pulling side =1000Lbs, load on the spot where anchor and pulling meet would = 2000Lbs - any friction from the sieve
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Quote:
Originally Posted by P35juniper View Post
I'm thinking at the point where the two sides meet the line will be pulled apart at the sum of both, anchor side =1000Lbs, wieght = 2000Lbs, pulling side =1000Lbs, load on the spot where anchor and pulling meet would = 2000Lbs - any friction from the sieve
A simple book on highschool physics or engineering statics might help. You are over thinking this.

The tension on every part of the rope MUST be the same. Otherwise there is no balance and the rope will move. There might be some very small friction effect when the tackle is moving, but none when stationary (even with no bearings--think deadeye). If the sheaves do not have good bearings, then the friction effect becomes huge after the first turn.
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