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How a 2:1 halyard works


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Sounds like a job for Myth Busters!

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Hi Wheels,

the issue is that a single block is not 1:1. If the sheave could move, it would move 1/2 the distance of the halyard recovered. That is 2:1. Like this..

 

 

Single Block 2 to 1.jpg

 

This better be right, or I've done some really crap physics over the years :lol:

 

It is NOT THE HALYARD LOAD that we are talking about here - of course that is still 100Kg at the tail. It is the compression load applied from the sheave, gooseneck fitting, and halyard termination fitting. That compression load may or may not be transmitted into the hull, depending on the positioning of the fittings. It will be taken by the mast.

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It's not 1 + 1. That is where you guys are all going wrong. It is 1 x 1. 1:1 is that same as saying 1X1. 1:2 is 1X2. 2:1 is the same as saying 2 divide by 1. But whatever the mechanical advantage, the reduction is in what a person has to pull on the halyard tail ONLY. The actual mechanical pull is not seen as tail to mast head sheave, but block terminal to masthead sheave.

Guys, we are pulling lines on the test bed almost daily. When we are working in tons and have to proof load lines, we need to be correct. It ain't a game for us, as much as we like playing with the Test Bed. We have to be accurate or someone ends up getting hurt.

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THAT Wheels, is the crux of the issue. The Halyard certainly still has 100kg on it. No argument. The discussion was about the compressive loads taken by the rig, which absolutely can be reduced by running a 2:1 halyard... Looks like we were coming at that from two different perspectives!!

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Because you do not have two 100Kg weights. The Halyard is creating 100Kg of tension on the sail, it is not an extra 100Kg of weight. If you placed a weight of 100Kg on the halyard, then yes you would have a sum of 200Kg. All the Halyard is doing is "transfering" the load of the Sail to the Halyard cleat. It is not a weight in itself.

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Yep I can show you really easily. One load cell on the "mast head" so to speak (top of the bed) and one cell on the "load" and scales on the halyard tail. Pull the Scale to read what ever and you see the same load displayed on both load cell readouts.

 

I think the important point to understand is that the Halyard is only transferring. It is not adding anything apart from it's own weight in rope of course. It simply transfers the Sail load to where ever you wish to terminate it. If the halyard was straight and terminated at the Mast head, the mast head is taking the 100Kg. The sheave is simply a point which you bend that halyard around 180degree's.

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