BLUEMOMENT

Sir Thomas or Samuel?

bottom, if the block is up top.
top, if block is down below.
hth

Ok, you want me to be sensible then? Doesn't it depend on the direction of pull relative to the fixed point? The pull could be horizontal.

It alters the gearing. If the becket is above, it gives 5:1. With the becket fixed you get 4:1.
Derek

You're trying to pull the boom downwards, against the sail tension, so the boom is the load. The deck is the fixed point.

A plain 2 sheave block has 4 lines coming from it (the same line 4 times). The 2 sheave block and becket has 5 lines coming from it. Assuming the same tension in all the lines gives either 4 times or 5 times the force on the block.
Derek

2 off 2-sheave blocks... Where is the far end of the sheet attached - has to be a becket on the upper side of the lower block or somewhere on the horse surely.

sahona wrote:Where is the far end of the sheet attached - has to be a becket on the upper side of the lower block or somewhere on the horse surely.

Not necessarily. The becket can be on the boom if the free end of the sheet comes from there too (not always very practical though).
Derek

http://www.harken.com/rigtips/Reeving_Diagrams_4-1.php

becket at the bottom or either you won't get the full benefit of using 2 x double blocks or you'll need another block/lead on the traveller/deck. Harken and Selden's sites usually provide a good info of this sort, including guidelines on loads and sizes to use.

In theory then, the last block is only a turning block for the sheet lead so that your pull is easier. The effort you apply to the sheet is actually trying to lift the deck block.

The other way up, the pull on the sheet adds to the downward force on the boom, but the sheet lead is inconvenient for sailing.

Does that make sense to you? It does to me. But I know what I'm trying to say!