Reeving a Tackle to Advantage -- What am I Missing?

[TeddyDiver]

Retinkering this once more .. The load in the tail would start at 1/3 of the load and grow to 1/2 at max when the load is up (block to block)

[Seaworthy Lass]

Originally Posted by monte

Usually you can still pull in line with a preventer or boom vang. You lose the purchase a bit when you cleat it in the jammer after trimming. I've seen pics of some salty sailors holding the mainsheet exiting from the boom block, I guess they knew a thing or two...

Not only is the purchase greater (as long as that standing block is still being utilised), but gravity is helping.

[conachair]

Originally Posted by 2Hulls

To keep it simple, let's neglect friction and assume a large weight that will not "swing" significantly when pulling it sideways.

It I swing. As the hauling line approaches 90deg the horizontal force approaches infinity and the vertical force approaches zero.

[jkishel]

No. With the haul at 90 degrees, the blocks will be 30 degrees off vertical and the tension will be 5.8 kg.

With T kg haul horizontal to the right, the horizontal reaction at the upper block will be T horizontal to the left. The 2T reaction at the upper block will therefore be 30 degrees off vertical. 2Tcos30 = 10 solving for T = 5.8kg.

The forces are:

Haul 5.8 to the right
Horizontal Reaction at upper block 5.8 to the left
Vertical reaction at upper block 10 up
Weight 10 down

Sum of the forces zero.

[conachair]

Originally Posted by jkishel

No. With the haul at 90 degrees, the blocks will be 30 degrees off vertical and the tension will be 5.8 kg.

With T kg haul horizontal to the right, the horizontal reaction at the upper block will be T horizontal to the left. The 2T reaction at the upper block will therefore be 30 degrees off vertical. 2Tcos30 = 10 solving for T = 5.8kg.

The forces are:

Haul 5.8 to the right
Horizontal Reaction at upper block 5.8 to the left
Vertical reaction at upper block 10 up
Weight 10 down

Sum of the forces zero.

Had to go back to see where this came from - 2 parts of rope on the left, doh! concede - you are right. With a single part each side the tension would approach infinity.

[jkishel]

As the haul angle increases beyond 90 degrees the tension increases until the angle reaches 180 degrees at which point there is no longer any mechanical advantage and the haul tension is equal to the weight.