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Old 03-01-2011, 12:45   #1
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Boom Load Distribution - Reeving Ideas ?

Greetings,

I am about to rerig my mainsheet and am planning to increase purchase to make life easier for my wife.

For quick stats: boom length 14 ft, luff 42 ft.

At the moment, I have a 4:1 purchase using a fiddle block on the traveller side and two separate blocks on the boom - distributing the load over 2 points on the boom.

Going to a 6:1 purchase, I am planning on going for a triple block with cam (something like this):

Mauri Pro Sailing Lewmar 80mm Triple Ratchet w/Becket & Cam

..however on the boom, the only example of reeving this that I've seen is always the 90 degree funny offset per the "6:1 reeved offset" example here:

http://www.harken.com/rigtips/mainsheet.php

The issue with this is that I would like to distribute the mainsheet load on the boom across at least two points and preferably 3 points.

As far as I am aware however, using the triple block on the traveller end and trying to reeve the line to 3 separate blocks (in line) on the boom will cause twisting right?

Any ideas on making this work would be greatly appreciated.

Thanks muchly!
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Old 03-01-2011, 13:46   #2
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on your setup now do you have boom bails for the blocks? if so wouldn't that stop the twisting. i know my boom does only twist slightly, until i put the preventer on
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Old 03-01-2011, 16:04   #3
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Rotate the individual blocks on the boom 90 degrees so they're oriented like the triple would have been.

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Old 03-01-2011, 16:23   #4
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Or go KISS.... 3 swivel blocks=no twist...
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Old 03-01-2011, 20:02   #5
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I've tried to picture a double block on the back side of the pictured triple, and a single block on the front side (as viewed from the tail side),but there will still be some twist.

I changed from 4:1 to 6:1 differently. I have a triple on the traveler, but rather than tailing off the traveler block, the tail goes forward along boom to mast and back to cockpit.

This probably doesn't help you, but I have a single with becket on the aft boom bail, and a single block on each of the next two boom bails going forward.
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Old 03-01-2011, 22:49   #6
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Thank you for the replies - greatly appreciated.

As for bails, no I don't have them - I have a slot which runs along the bottom of the boom and tangs which slide into the slot and can be positioned anywhere. These tangs have holes in them to which blocks are attached.

On block rotation - I'm more than happy to go out and buy three swivel blocks if that will solve the problem - but in my mind, there's still a question of the triple on the traveller imparting a non-linear force on the blocks above since sheave #1 (stbd) on the triple will be offset to port 1.5 x sheave widths from the centre of effort on the triple itself, and then forward say a foot on the boom. The centre sheave will be ok with centre of effort on triple in the centre and block on boom directly over. Sheave #2 (port) will be offset 1.5 x sheave widths again to port and the corresponding boom block offset a foot aft.

So if reeved as above - i.e. the stbd block on the triple is reeved to the forward block on the boom, centre on triple to centre on boom and port on triple to aft block on boom - there will be an overall clockwise twist...

Clearly, going forward to the boom first then to the cabintop will solve the problem - but I'm looking for a solution that I can keep just between the end of the boom and the traveller.

As I haven't bought the triple yet, I'm certainly open to other suggestions as well to get my 6:1.

Thanks again!
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Old 04-01-2011, 00:15   #7
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Triple fiddle block!

OK, I've just found this:

Mauri Pro Sailing Garhauer - Series 40 Stainless Steel Fiddle block w/ becket & cam cleat (2 5/8 in. sheave)

..which looks like I could use this to keep all the boom blocks in-line - however how on earth do you reeve the lines through this thing?!?!?

It looks like the lines would chafe against each other?

Thoughts?
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Old 04-01-2011, 05:21   #8
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Quote:
Originally Posted by akio.kanemoto View Post
OK, I've just found this:

Mauri Pro Sailing Garhauer - Series 40 Stainless Steel Fiddle block w/ becket & cam cleat (2 5/8 in. sheave)

..which looks like I could use this to keep all the boom blocks in-line - however how on earth do you reeve the lines through this thing?!?!?

It looks like the lines would chafe against each other?

Thoughts?
FYI that's a double block that simply uses two smaller sheaves opposit one another to lead the line to the cam cleat.

Note that regardless of the number of falls you use, you do not increase the loading on the boom. If you need 1000 lbs of load on the boom to control the main that can be done with a single line, in whch case the line is loaded with 1000 lbs of tension; or you can sling a single block with a bail from the boom to advantage (3-falls) in which case the tension on the line is reduced to 333.34 lbs per fall (still 1000 lbs at the boom) or a double block to advantage (5-falls) in which case the tension on the line is reduced to 200 lbs. Blocks do not increase total load, they merely reduce the tension required in the line to achieve the total load, albeit with the addition of some friction.

FWIW...
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Old 04-01-2011, 13:02   #9
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Originally Posted by svHyLyte View Post
FYI that's a double block that simply uses two smaller sheaves opposit one another to lead the line to the cam cleat.

Note that regardless of the number of falls you use, you do not increase the loading on the boom. If you need 1000 lbs of load on the boom to control the main that can be done with a single line, in whch case the line is loaded with 1000 lbs of tension; or you can sling a single block with a bail from the boom to advantage (3-falls) in which case the tension on the line is reduced to 333.34 lbs per fall (still 1000 lbs at the boom) or a double block to advantage (5-falls) in which case the tension on the line is reduced to 200 lbs. Blocks do not increase total load, they merely reduce the tension required in the line to achieve the total load, albeit with the addition of some friction.

FWIW...
Shame about the "triple fiddle block" then.. I'm actually suprized it doesn't exist..

On the boom loading, yes I know that the load on the boom doesn't increase - what I'm trying to do is to spread the load over multiple points.

If you have an idea for how to get 6:1 purchase with the load spread out over multiple points on the boom without leading forward to the mast, I'm all ears! The standard solution seems to be a pair of triple blocks at right angles to each other, but that's just a single point on the boom.

If I use a triple and 3 separate blocks per my post above - I get twist..

Any ideas?

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