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Go Back   SailNet Community > On Board > Gear & Maintenance > Designing block and tackle systems
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Thread: Designing block and tackle systems Reply to Thread
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Topic Review (Newest First)
06-06-2011 11:19 PM
Hudsonian If you were to rig the system 2:1 x 2:1 x 2:1 x 3:1 you'd get 24:1 and the length of the control line would only be half as long as it would be for the 48:1 system. The control line has to be big enough to handle; 5/16" or bigger.
06-06-2011 10:13 PM
allene222 Faster has some good stuff in his post. Hopefully I can add something to the discussion.

I put a lot of fundamentals in this page:
Block Systems and that might help.

This system is 48:1:
http://www.harken.com/uk/img/Beneteau-lg.jpg

I think it is exactly what you are looking for. (click the link for a larger image)

You can see that the little blue bulb would jam in the cleat if the control line was released which would prevent the mast from going free. You can also see that the control lines run port and starboard.
The system is 2:1 x 2:1 x 2:1 x 6:1 = 48:1 The write up is here Optimise your cruiser / Racer - Part 2: Backstay and vang

Allen
06-06-2011 09:41 PM
JimsCAL Why not just a mechanical (Wichard) or hydraulic (Navtec) unit? They're designed for that.
06-06-2011 09:19 PM
SlowButSteady
Quote:
Originally Posted by sandycohen View Post
PS - Sorry for not replying to your concern re: failure - I have a keel stepped mast so this is less of a danger
I don't think we need to actually crunch the numbers to realize that the lever-arm on the mast as it passes through the deck (mast partners?) is tremendous. If the backstay tensioner fails with boat on a run (just when the backstay is really needed to keep the mast up), and there is nothing left to keep the mast from bending forward, I seriously doubt that the it's is going to be able to resist that sort of bending moment.
06-06-2011 08:52 PM
Faster
Quote:
Originally Posted by tommays View Post
If you have straight spreaders on and older style mast then a back-stay failure is really BAD
... keel stepped or not.....

To figure the purchase advantage on a tackle you look at the number of moving parts... your pic looks like 8:1...cascade that with a two part and you have 16:1... another two part after than makes it 32:1 and so on.

You can get away with very small line and blocks as they share the load.. making the whole installation a little less bulky. But you should have a fixed strop parallel to the whole setup with its length set for the 'released' setting as a backup in case your tackle fails or is accidentally let run through.

The other thing to keep in mind is that you'll be pulling a lot of line through to make a small adjustment.. with the backstay on hard you'll have to figure a way to safely store the tail of the adjusting tackle.
06-06-2011 08:37 PM
tommays If you have straight spreaders on and older style mast then a back-stay failure is really BAD

VS more modern sweptback spreaders that give the mast some support
06-06-2011 08:19 PM
Liquorice Try this for a bit of basic backstay rigging:
Harken Tech Corner: Backstay Adjusters
They're not 22/1 but it shows a few basic setups.

Your diagram has 2 attachment points on the stern!
Your not going to avoid that if you stick with a cascading adjuster.
06-06-2011 07:38 PM
sandycohen PS - Sorry for not replying to your concern re: failure - I have a keel stepped mast so this is less of a danger
06-06-2011 07:33 PM
sandycohen For instance what purchase would this provide? I just don't understand how the calculation is done, but this one has an extra block between the layers that seems like a 'multiplier'

06-06-2011 07:31 PM
sandycohen No - I really don't want one of those - but I was referring to one like this



I don't see why it's necessary to have the 2nd anchor point, if my mainsheet tackle doesn't need one, in concept.
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