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sandycohen 06-06-2011 05:10 PM

Designing block and tackle systems
 
Hi All -

I am trying to design a cascading tackle to use as a backstay adjuster. I have a single backstay, and don't want to create a 2nd attachment point on the stern as many systems I've seen do.

In my mind, there's no basic difference between 'sheeting in' my backstay and sheeting in my main, except needing alot more purchase.

I figure I need somewhere around 22:1.

My understanding of these systems is pretty rudimentary, and I did quite a bit of googling around and couldn't find a basic explanation of cascading setups, or prepackaged mainsheet sheeting systems. I also looked at the sailnet article section.

Can anyone point my in the right direction?

Thanks

SlowButSteady 06-06-2011 06:19 PM

When you say, " I have a single backstay, and don't want to create a 2nd attachment point on the stern as many systems I've seen do", I assume you mean that you don't want to go to a split backstay system, like this (or similar):

http://www.defender.com/images/612333.jpg


Having a split backstay with a tensioner gives one a more fail-safe system. If any one component below the plate that actually splits the backstay fails there is a pretty good chance that the mast won't fall down. If you just had a set of blocks (or a cascading system) the failure of a block or a bit of rope could cause your mast to have little or no aft support.

sandycohen 06-06-2011 06:31 PM

No - I really don't want one of those - but I was referring to one like this

http://www.ronstan.com/images/diagra...Backstay-2.gif

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

sandycohen 06-06-2011 06:33 PM

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'

http://lyc.catsailor.org/photos/mainsheet1.jpg

sandycohen 06-06-2011 06:38 PM

PS - Sorry for not replying to your concern re: failure - I have a keel stepped mast so this is less of a danger

Liquorice 06-06-2011 07:19 PM

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.

tommays 06-06-2011 07:37 PM

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

Faster 06-06-2011 07:52 PM

Quote:

Originally Posted by tommays (Post 737893)
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.

SlowButSteady 06-06-2011 08:19 PM

Quote:

Originally Posted by sandycohen (Post 737867)
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.

JimsCAL 06-06-2011 08:41 PM

Why not just a mechanical (Wichard) or hydraulic (Navtec) unit? They're designed for that.


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