Amount of Backstay Adjustment?

[Firehoser75]

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.
Thanks in advance for your advise and help in "demystifying" this control,
Tom

[GordMay]

Selden has an excellent Rig Tuning Manual, which includes the Folding Rule method, and Tuning Under Sail instructions, and more:
➥ http://www.riggingandsails.com/pdf/selden-tuning.pdf

See also, a few earlier CF discussions:

➥ http://www.cruisersforum.com/forums/...ing-13711.html

➥ http://www.cruisersforum.com/forums/...sion-9844.html

➥ http://www.cruisersforum.com/forums/...ning-1290.html

[Joli]

The folding rule method measurers stretch, it indicates how much tension you have on the caps. Do the same with the backstay but instead of turning a turnbuckle tension the backstay with the Y purchase. Mark the max tension on the purchase.

As the breeze comes on tension the backstay to flatten the sails and keep the boat on it's feet.

Offwind throw the rig forward by releasing the backstay tension completely.

In light air keep the tension light, you are trying to add depth to the sails.

Remember: Depth is power, flat is speed. Draft forward is more forgiving, draft aft is less forgiving but you gain point.

[Cheechako]

It sounds like your boat is set up with a "bendy" rig. Tensioning your backstay should put a curve in your mast. This takes some of the belly or draft out of the mainsail, making it flatter. When the wind pipes up, try experimenting with adjustments, go to the base of the mast and look up at the mainsail shape. If you develop weather helm, put a little more tension in etc....

[Bash]

upwind, you want enough backstay tension so that the forestay doesn't bow to weather. downwind, you want to ease the backstay almost to the point where the forestay appears loose, which means ease it to that point and then tighten it back about six inches on the line.