Becket ~ top or bottom?

[Julian]

Does it matter?

[Arghiro]

Sir Thomas or Samuel?

[little boy blue]

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

[Arghiro]

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.

[Julian]

On boom, 4:1 2x2 pully blocks, one with becket.

I have been looking about and checking photo's.

It seems to be there is no rule, some have the becket on the top block (boom) some on the bottom.

personally I can't see it makes a difference so I did it the way I had it before with the becket on the bottom.

I was just asking to see if people had an opinion on this.

[cpedw]

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

[Julian]

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

I am going to have to think about that one as my initial thoughts are telling me... huh?

The rope wouldn't go through any more pulleys if I turn the rig over, surely it only changes the anchor point, which is irrelevant, as irrelevant as if I was able to make the anchor half way between the two pulleys somehow? No hang on, I see the flaw in my statement above a bit, as the anchor is not the anchor is it? it's actually the lifting point, the becket is essentially the load.

ok, I need to draw this out on a bit of paper, back soon.

[Arghiro]

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

[cpedw]

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

[sahona]

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.

[cpedw]

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

[Gardenshed]

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.

[Julian]

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

I was thinking more on a theoretical level, if you draw out as a vector type diagram the becket (attachment point) will be the load, the pull on the sheet is trying to lift the becket but as it can't lift it it shortens the lines and hence pulls the boom downward.

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I am actually quite glad there is a little confusion and opinion on this as I thought I wasn't seeing something obvious when deciding the best method to set up.

As I have a relatively small boom and main I honestly probably don't need the nice new full bearing blocks I have added this season, although I did lift an engine into the dinghy using the rig (cheapo non bearing types) a couple of years back and it was essential. I am always aware that it may also one day be useful for a MoB so I will not be downgrading it any time soon.

[Arghiro]

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!