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post #21 of 46 Old 10-19-2011 Thread Starter
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I just got the Loos PT-3 guage I ordered last night. Has a manual with it (I tried to find a copy on internet that I could post here but no luck so far). Manual states to use the gauge then under sail make final adjustments making sure leeward shrouds do not go slack. The thing I like about a gage is that I can go to the boat and measure what the tension is now before I start to play with things, and I can make sure I am not overtension (or under) when I have the final set up (I like facts). I was also going to use the gage under sail just to get an idea as to what the tension is on all shrouds and stays- hey I'm an engineer, what can I say. For $200 not a bad deal.

One question, how do I measure forestay tension if I have a rolling furler? At the base not enough cable and at the top (where attaches to mast) I don't think there is enough cable.
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post #22 of 46 Old 10-19-2011
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You cannot measure forestay tension under these conditions. The best is to measure the backstay tension and calculate forestay tension with the help of forestay and backstay angles with the mast.
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post #23 of 46 Old 10-19-2011 Thread Starter
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Originally Posted by RichH View Post
In sailboat rigging the FACTOR OF SAFETY is already built-in in the design and the OEM selection of the wire. For you to add safety factor on top of the OEM safety factor would cause you select larger diameter wire and larger diameter connection terminals etc. Since the OEM wire at ~15% loading will produce a very predictable 'stretch' and 'sag' when windloaded by sails, the increased size to produce the OEM designed/defined strain / elasticity will have to operate at MUCH LESS applied load/tension .... hence the mast will be 'loose' and subject to increaesd impact values as the mast 'rocks' back and forth sideways, the jib, etc. will now be operating on a very slack wire and will no longer take its designed shape that the sailmaker cut into the sail AND the mast will no longer be 'dynamically' as strong because it will no longer be set with the amount of proper 'pre-bend' (far-aft bowing).
If you want your boat NOT to be able to 'point' well and want it to heel over aggressively while being exceptionally SLOW, have a LOT of 'helm pressure' (boat 'skidding off' to leeward when attempting to 'point').... be at less than 15% static forestay tension.

Normal fore/aft 'prebend' is defined as ~3/4" forward bow for a single spreader rig, and ~1/2" forward bow per each spreader set on a multi-spreader rig ... and the sailmaker expects that the mast will be set up for that designed pre-bend and the forestay to be ~15%. Without normal expected prebend a mainsail will set up in a powered-up (increased draft) shape because in 'good' mainsails the sailmaker ALWAYS adds a smooth curve to the front of the luff to accommodate the expected 'prebend'. Prebend mathematically makes a spar MUCH stronger (by increasing the geometric 'moment of inertia' or "I" to the third power to prevent/retard the mast from flexing or oscilating due to 'induced harmonics' ... called 'mast pumping'.

Rigging size, mast stiffness, etc. are not a 'black art'. Typically the boat designer selects the rigging/mast based on typical 'scantlings' that include normal SAFETY FACTORS that historically 'work' ... for safety and long service life. An inshore design will be at 1.5, A coastal design will usually have an inbuilt Safety factor or 2, an offshore design 3 ... or more. The wire load bearing capacity is selected so that when the rig is set at 15% tension ..... and then later when the boat is 'pulled over' and heeled, the rig tension doesnt (much) go over 30% rig tension, 30% being the limiting load factor that unduly promotes 'fatigue' in stainless components. It is important to realize the all 300 series stainless quickly fatigues when loaded beyond 30% stress (normal 'endurance limit' of 300 series SS is 30,000 psi, although normal 300 series has an ultimate load value of 90,000psi) .
So, the 'typical' method by a designer to arrive at 'correct scantling' wire size, etc. is to mathematically/theoretically pull the mast horizontally from the top until the boat is at a ~45° angle of heel, calculate the resultant rig tension that is needed to get the boat to that 45 degree heel angle ... then multiply by the applicable safety factor to arrive at the proper scantling sized wire. ..... YOUR need is to keep the rig at near the 'design' static (boat upright) loading is YOU must set the rigging to a basic 15% of tension so that the mast remains 'straight' (side to side), mast has a proper amount of 'pre-bend' ... and the forestay IS operating at 15% static tension for sailing in 12-15kts.
Guessing and By-Goshing the proper rig loading using eyeballs, wire pushing, 'What John does', .... will get you NOWHERE. Its all in the numbers .... basic ~15% tension for normal wind and seastate conditions. ........ All the rest is 'myths & mysticisms".
;-)
Thanks, there is a lot of good info here, I will need to read many times to understand all of this.

One question, what do you mean by "Normal fore/aft 'prebend' is defined as ~3/4" forward bow for a single spreader rig ( I have single spreader). Also, how would I measure the forestay tension if the forestay is covered by roller furler? Also, the Selden manual (down load hints and advice from below link)

Seldén Mast AB

states tension forestay up to 40% breaking strength. Based on you statement that anything above 30% could lead to fatigue breaking, why would Selden say 40%.
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post #24 of 46 Old 10-19-2011 Thread Starter
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Quote:
Originally Posted by celenoglu View Post
You cannot measure forestay tension under these conditions. The best is to measure the backstay tension and calculate forestay tension with the help of forestay and backstay angles with the mast.
Hey that is a good idea. However I do have both forward and aft lower shrouds and a baby forestay. I would need to measure tension on all of these and then use some geometry to calculate the forestay tension correct?
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post #25 of 46 Old 10-19-2011
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Quote:
Originally Posted by casey1999 View Post
Thanks, there is a lot of good info here, I will need to read many times to understand all of this.

One question, what do you mean by "Normal fore/aft 'prebend' is defined as ~3/4" forward bow for a single spreader rig ( I have single spreader). Also, how would I measure the forestay tension if the forestay is covered by roller furler? Also, the Selden manual (down load hints and advice from below link)

Seldén Mast AB

states tension forestay up to 40% breaking strength. Based on you statement that anything above 30% could lead to fatigue breaking, why would Selden say 40%.
Regards
prebend is when you look up the mast from near the bottom, the front edge of the mast somewhere near the middle is 'more forward' than bottom or top of the mast ... slightly 'bent' by the rig tensions.

To set forestay tension .... forestay reacts, principally, with the backstay and if the angles that forestay and backstay attach to the masthead are 'approximately the same, then the forestay tension will be approximately the same as the backstay .... you adjust backstay tension to get the correct forestay tension.

Fatigue endurance limit also includes NUMBER of loadings applied. At above 30% tension or greater repetitive or 'cyclical' loads you will only get approx. 1 million load cycles before fatigue FRACTURE. 40% tension will result in the probability of failure at before approximately 1 million cycles ...
Think of the the 'bouncing' that the rig endures during a very long passage or circumnavigation.
At loads BELOW the 'endurance limit', fatigue fracture is not 'usually' a consideration.
;-)
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post #26 of 46 Old 10-19-2011
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Page 29 of the Selden rigging hints and advice manual explains how to adjust rigging without a Loos gauge. It is actually very easy.
The manual is can be downloaded here: Share with Facebook
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For what its worth, a bicycle wheel with thin gauge spokes is tensioned higher that the same wheel with thicker spokes and a longer spoke would be at greater tension than a shorter one (though that would make a strange wheel, like the one I made for a clown's bicycle)
When tensioned correctly (and given no damage has been done to the rim) the rim will be round and true and the tension on the spokes will be equal. Since the wheel receives dynamic rotational (radially and laterally) loading any unequal tension will walk around the wheel until the spokes loosen to the same tension.
While we don't sail bicycle wheels, our sailboat rigs share some engineering factors. So thinner gauge wire will be at a higher tension that heavier ones and opposing wires should be of equal tension and gauge. Also stress risers like odd bends at fittings must be relieved or the wire/fitting will fail from fatigue prematurely.
Keep on pedaling
John
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Quote "So thinner gauge wire will be at a higher tension than heavier ones..."


According to the experts, like Selden, rigging wire should be tensioned to between 15% and 20% of the wires breaking load. This will be a higher tension on larger wire, not on smaller wire.

Brian
Living aboard in Victoria Harbour
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Quote:
Originally Posted by ccriders View Post
For what its worth, a bicycle wheel with thin gauge spokes is tensioned higher that the same wheel with thicker spokes and a longer spoke would be at greater tension than a shorter one (though that would make a strange wheel, like the one I made for a clown's bicycle)
When tensioned correctly (and given no damage has been done to the rim) the rim will be round and true and the tension on the spokes will be equal. Since the wheel receives dynamic rotational (radially and laterally) loading any unequal tension will walk around the wheel until the spokes loosen to the same tension.
While we don't sail bicycle wheels, our sailboat rigs share some engineering factors. So thinner gauge wire will be at a higher tension that heavier ones and opposing wires should be of equal tension and gauge. Also stress risers like odd bends at fittings must be relieved or the wire/fitting will fail from fatigue prematurely.
Keep on pedaling
John
John,
Little of topic, but how does a bicycle wheel work? With the weight on the axel of the wheel, which spokes are holding this weight?
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post #30 of 46 Old 10-19-2011
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Quote:
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John,
Little of topic, but how does a bicycle wheel work? With the weight on the axel of the wheel, which spokes are holding this weight?
All of them.

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When you look at weight distribution, too, even under a very heavy load many spokes help spread out the weight so that it is more evenly carried and doesn't put too much stress on any single spoke.

1971 23' Oday Pop Top
S/V Frida

You can't steer a boat that isn't moving? Just like a life - P. Lutus
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