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Old 07-01-2012, 21:08   #16
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Re: Changing My Rigging One Stay at a Time Solo

Just because you run larger cable does not mean you tenson higher. The gross tension you need is a functin of the mast section, rigidity of the hull, sail area. The boat geometry doesn't change if you upsize the wire. Your tension needed remains the same and your margin of safety is increased. The chart shows the capacity by wire size, not the tension required. Notie it is titled Breaking Strength. To tune otherwise would be like saying you must increase the halyard tension if you use stronger halyards. All you would do is tear the head out of the sail. Unlike high end racing sleds designed for the ligthest weight aloft (which results in the highest loads) we are contemplating heavier rigging & therefore lower stresses at the same loading.
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Old 07-01-2012, 21:25   #17
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Re: Changing My Rigging One Stay at a Time Solo

Quote:
Masthead Rig: There is a simple criterion for shroud tension. The initial rigging tension should be high enough that the leeward shrouds do not go slack when sailing close-hauled in a reasonably brisk breeze. The proper value for your boat can be found by a few trial runs under sail. Once the correct tension is known, the gauge can be used to maintain the value. For many boat designs a shroud tension of 10% to 12% of the breaking strength of the cable is adequate. Thus, for 7/32", 302/304 1x19 stainless steel cable , the upper and lower shrouds would be set to 600 to 700 lbs. tension. On some rigs it may be desirable to carry more tension in the uppers than in the lowers.
OK lets use this guys boat for an experiment. After he put on the 1/4" wire we'll have him tighten it up according to the quote above.

But before that happens lets take 30' piece of 3/16" wire rope and hang it horizontally and put tension on it until it only sags 1/4", and measure the tension.

Then take a 1/4" piece 30' and hang it and put tension on it until it only sags 1/4", then measure that tension. Which do you think will take the most tension to stop the sag?
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Old 08-01-2012, 08:41   #18
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Re: Changing My Rigging One Stay at a Time Solo

Del, your experiment introduces a variable that is irrelevant, radial deflection of the wire, or "sag". The rig needs X pounds of tension dictated by the mast section, sail area, etc. The mast doesn't care if this tension is provided by 3/16" or 1/4" wire. The fact that the two wire sizes may show slightly different amounts of sag due to windage, etc, makes no difference as long as the tension is set to X.
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Old 08-01-2012, 09:05   #19
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Re: Changing My Rigging One Stay at a Time Solo

I keep hearing people talk about waiting until the leeward shroud goes slack, but I'm not sure what "slack" means... does mean flopping around?
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Old 08-01-2012, 09:07   #20
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Re: Changing My Rigging One Stay at a Time Solo

At the same absolute tension value the 1/4" lee side cable will sag less in use in the rigging. That is because your stays are very stiff springs in tension. Young's modulus is the same for the steel in 3/16 or 1/4" cable. The stronger cable will stretch less under identical loads in direct proportion to the cross sectional area ratio of the steel in the cables. The windward side cable will stretch less. What you are referring to is catenary sag which has nothing to do with shroud tension. The tension required is a function of rig geometry. You can use nearly any shroud that makes you comfortable including Deenama but the tension necessary is not dependant on the ultimate capacity of the shroud. If I tensioned my 3/4 and 7/8 inch shrouds according to the ultimate tension chart I would crush the mast or ram the mast step out of the bottom of the boat.
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Old 08-01-2012, 09:23   #21
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Re: Changing My Rigging One Stay at a Time Solo

FUNJOHNSON

I raced a 70s vintage Heritage One Ton for 18 years on Lake Erie. We were 3rd place boat of the year in class. Most of the crew and owner were engineers or otherwise technically trained. At rest, the shrouds always felt tight as heck. At about 10 to 15 degrees of heal close hauled the lee shrouds were at about neutral tension. They were decidedly floppy at 20 degrees or above 20 knots apparent close hauled. The Heritage had a keel stepped mast and the mast section was massive compared to today's racers. As an engineer, I always considered that tension higher than what we ran was simply excessive preload on the rig and prematurely shortened the life of everything involved. We also had hydralic backstay and lower forestay. Using the hydralics, we could induce an impressive rake in the mast. We used this to pull the main flatter to weather. Typically, we might pull 8 to 14 inches of forward bow at the lower forestay. We used to joke that after a race when we eased the hydraulics that we could hear the bow and stern slap the water as the hull straightened out.
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