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post #1 of 8 Old 06-12-2009 Thread Starter
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Help with Actual Use of Backstay Adjustor

The backstay on my boat is the "inverted Y" type of backstay with a 6 to 1 block and tackle on one leg of the Y (giving what I think is an approximate 12 to 1 mechanical advantage overall). The rig is a 9/10 fractional rig with only one lower shroud (each side), with swept back spreaders and a roller furling genoa. The wire sizes are 7/16" for the backstay, and 1/2" for the shrouds (lower and cap) and 5/16" for the intermediates (total of 2 spreaders). It is a production cruiser type of boat (sloop rig). I can't seem to find a loos guage for determining tension that will accept wire this thick?

The information from Selden on this rig, is that the backstay should not be tensioned more than 20% of it's breaking strength (at any time), and that it should never be allowed to completely slacken.

My questions are:
How do I tell what level of adjustment to use? When do I use the full 20% tension and how do I tell when I am there? For downwind, how much do I "let it off"? In light wind, how much do I let it off?
For example to loosen for downwind, do I let 3" of line through the 6 to 1, or ??? from the fully tensioned position?
Up to now, I have avoided using the backstay adjustor, probably because I am unsure of how to set it properly. I am not even sure how to tell when it is tensioned to the 20% of breaking strength point.
Thanks in advance for your advise and help in "demystifying" this control,
Tom

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Point Roberts, Wash. Marina

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post #2 of 8 Old 06-12-2009
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First, I would highly recommend you watch the following video regarding the use of a backstay adjuster.



To figure out when the backstay is at 20% you'll need a tension gauge. You'll also want a ruler or some other type of marked scale attached to the backstay adjuster in some manner, or marking the line that is used by it. Once you've used the tension gauge and marked the backstay adjuster for the 20% tension level, you'll have to experiment with the backstay to find what settings work the best for what combination of wind speed and point of sail.

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post #3 of 8 Old 06-12-2009
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You have the basic idea. The backstay tension will affect the genoa and main. One thing is does is allow the main to "twist off" at the top opening up the leech of the main. The more tension on the backstay the more the main will be twist off and be de-powered. This can be helpful in a blow. The other thing is does is affect headstay sag and therefore genoa shape. So again if you are trying to go upwind (maximizing your pointing ability) in a blow, that is when you would see maximum backstay tension, and therefore headstay tension. On a racing boat in gusty wind the backstay tension and the traveler position may be under constant adjustment. Every time a gust comes along the backstay tension will be increased to "spill" some air and the traveler may be dropped as well. After the gust the backstay would be eased to power up again. The correct settings for your boat can only be determined by experience. In a medium steady breeze go from min to max tension and back a number of times while close hauled. Carefully watch the shape of the main including the leech shape and draft position, and pointing ability.

As far as near zero to 20% goes, on a racing boat you will often see a stick or some sort (perhaps piece of batten material a few feet long) attached to something stationary at the backstay adjustment point. You will have a line drawn on the batten and on a movable part of the backstay that line up when the tension is near zero. The backstay will be very close to completely slack and "floppy" at this point. I've never seen a rig fall down by releasing all tension when the boat is at the dock. However, if you're too worried to do that ask a rigger for help. Then draw another line at max. tension. You can pull hard on the adjustment but don't run the line to a winch or something like that. It was designed with a 6 to 1 and I'm sure the assumption was that a typical human would be pulling on the line. Again, your going to have to ask a rigger if you want a more exact calculation, or perhaps install a tension gauge but I don't think this is needed. Anyway, with the stick and a min and max line, now you have a reference while sailing.
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post #4 of 8 Old 06-12-2009
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While sailing upwind stand where you can sight up the mast and watch how it changes shape when going from min to max settings. Do the same at the headstay. You'll get the idea.

Backstay tension does affect headstay tension on a fractional rig boat, but not to the degree it does on a masthead boat. The amount of effect depends on the spreader configuration and other elements of your rig. See it for yourself.

Last edited by SteveInMD; 06-12-2009 at 07:24 AM.
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post #5 of 8 Old 06-12-2009
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With that particular tackle it's unlikely you can actually load it by hand to beyond your limits... if you don't put it on a winch I don't think you can ever get too much load on it.

So I think you can play with your adjustment to your heart's content. As Steve says above, observe the effects and figure out what actually works.

Ron

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post #6 of 8 Old 06-12-2009
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When I can't open the door to the head I know my backstay is too tight! Ha!
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Summarizing and Thanks

OK. So if I understand the general consensus so far, I need to mark the backstay tensioning line at a location just before the system is slack (and maybe even tie a stopper knot in the line to ensure it cannot go totally slack), and also mark the line where the tension is "fully on" (at 20% breaking strength), and then experiment under various points of sail and wind conditions to see what works best (and when). The best method I have found so far to determine the 20% tension point is using the folding rule method described in the Selden mast book, attaching a 2 meter stick to the stay using tape and measuring the distance between the stay end and the stick. When the wire has stretched 1mm (the slack distance has increased ) that would equal 5% of breaking strength.
Sailing Dog, I like Alex's videos alot. They do give a good idea of what to do and why.
Thanks for the responses, this site is great! If anyone disagrees with my summary or wants to add some additional advise, please do.
Thanks again,
Tom

Firehoser75
Point Roberts, Wash. Marina

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Quote:
Originally Posted by firehoser75 View Post
OK. So if I understand the general consensus so far, I need to mark the backstay tensioning line at a location just before the system is slack (and maybe even tie a stopper knot in the line to ensure it cannot go totally slack), and also mark the line where the tension is "fully on" (at 20% breaking strength), and then experiment under various points of sail and wind conditions to see what works best (and when). The best method I have found so far to determine the 20% tension point is using the folding rule method described in the Selden mast book, attaching a 2 meter stick to the stay using tape and measuring the distance between the stay end and the stick. When the wire has stretched 1mm (the slack distance has increased ) that would equal 5% of breaking strength.
Sailing Dog, I like Alex's videos alot. They do give a good idea of what to do and why.
Yup...you got it in one.
Quote:
Thanks for the responses, this site is great! If anyone disagrees with my summary or wants to add some additional advise, please do.
Thanks again,
Tom
Glad to help.

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a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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