You've actually got closer to a 3:1 purchase, rather than just a 2:1. If the lines came straight down to a block with cam cleat at the transom rather than further forward, it'd clearly be a 3:1 purchase, but since it comes forward a bit, you're losing some leverage, as the lever arm is shorter compared to the other lines.
Basically, to figure out the purchase, you need to look at how many lines are attached to the moving part of the tackle, which is the boom in this case. There are three lines... so the purchase is almost 3:1.
See the diagram here:
Of course, the diagram in question is actually upside down for our purposes... since the boom is pulling away from the cockpit. Of course, another way to measure it is to see how much the boom moves when you haul in 3' of line. If it moves about a foot, it is 3:1, if it moves about 18", it is a 2:1.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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