Okay, let me explain how it works.

But first lets derive the tension in each line because we'll need that later.

Black line is held in hand and so all the black lines are under 1 unit tension; the pull of the skipper.

The red line is attached to the black line by an 1:2 purchase system so the tension in the red line is 2 units.

It is impossible for any line to have different tensions at different points. Think about this.

When a line makes a full 180 wrap around a block then it pulls 2 times the line tension downward, that is obvious. If a line makes a 90 wrap then the block is pulled down by 1 times the line tension. A termination point equal 1 times the line tension downward. All still pretty straight forward.

Now lets count the loads when looking only at the boom blocks, the others are unimportant.

Black line : about 1 partial wrap so roughly 1 unit down pull and nothing else

red line : 1 full wrap, 1 partial wrap and a termination point ; total = 4 times the line tension of 2 units = 8 units of downward pull

Add them all up and you get (by approximation) 9 units of downward pull for each single unit of pull by the skipper, ergo it is a 1:9 system.


And this is one of the few ways to do it right.

Wouter