# Rigging Tension



## casey1999 (Oct 18, 2010)

Been reading up on sailboat rigging and don't quite understand rigging tension.

Most documents I read state tension shrouds and stays to 15% of wire breaking strength. Why 15% and say not 5%?

Also, say a boat has shrouds and stays that are 316ss and 1/8 inch diameter with break strength of 1,780 lbs. I would tension to 15% or 267 lbs. 

Now say I want to get some safety factor and I up the size to 1/4 inch with a break strength of 6,900 lbs. Why can I not just tension the stays and shrouds to 267lbs (so I do not hog the keel), and then have a bunch of safety factor- all turnbuckels and other fittings would be sized for 1/4 cable.
Regards


----------



## smurphny (Feb 20, 2009)

The tension is important because wire does stretch. If the initial tension is too low, the leeward shrouds will be flapping around. One way of tensioning is to alternately tighten the leeward shrouds on different tacks while on a beam reach until they are both just slightly loose and the mast is straight at rest. There was recently a pretty good thread on this with links to Selden Mast. They have a page on the topic.


----------



## casey1999 (Oct 18, 2010)

smurphny said:


> The tension is important because wire does stretch. If the initial tension is too low, the leeward shrouds will be flapping around. One way of tensioning is to alternately tighten the leeward shrouds on different tacks while on a beam reach until they are both just slightly loose and the mast is straight at rest. There was recently a pretty good thread on this with links to Selden Mast. They have a page on the topic.


Thanks, I have a copy of the Selden instructions. I understand the stretch part, but if I up the size to say 1/4 inch, the stretch will probably be insignificant (stretch based on wire size and load applied), so then why not tension to only 267 lbs?


----------



## tommays (Sep 9, 2008)

You still need that 10 to 15 % on any size wire to get the sloppy out of it


----------



## dsmauney (Feb 29, 2000)

*Resist Metal Fatigue*

One of the reasons for tension in rigging is so the wire is always in tension on Windward or Leeward. When loads fluctuate on metal the higher the average load that the varying load fluctuates around the longer the fatigue life. This of course within limits that the hull can handle. The same reason you tension bicycle 
spokes so tight.

Good Winds
DaveM


----------



## mitiempo (Sep 19, 2008)

I agree, the rigging will be sloppy loose if 1/4" wire is tensioned to 267 lbs (3.8% of breaking actually). 

Also, if you are planning on upsizing the wire, remember to upsize everything else from chainplates to turnbuckles to mast tangs to match otherwise it is a waste - and regardless the load on the boat is greater.

Wire seldom has a load problem - mostly it is a corrosion issue. Larger wire will corrode as fast as smaller wire.


----------



## casey1999 (Oct 18, 2010)

mitiempo said:


> I agree, the rigging will be sloppy loose if 1/4" wire is tensioned to 267 lbs (3.8% of breaking actually).
> 
> Also, if you are planning on upsizing the wire, remember to upsize everything else from chainplates to turnbuckles to mast tangs to match otherwise it is a waste - and regardless the load on the boat is greater.
> 
> Wire seldom has a load problem - mostly it is a corrosion issue. Larger wire will corrode as fast as smaller wire.


I do not plan to increase my wire size (5/16inch) but just trying to get an understanding of the dynamics of the rig tension- Thanks.

One thing I find interesting is that it seems sailboat rigs seem so fickel, when all around us are power poles and guyed antennas that get almost no inspections and still stay up. Seems they are using galvanized wire. Do large antennas need to have their guys replaced evey so many years? Some of these antennas are huge and the wind load must cycle them just like a sailboat rig. The below tower is 1000 feet tall and has an elevator to get to the top- located in Baltimore MD.


----------



## GaryHLucas (Mar 24, 2008)

I'm not sure I'd agree that you need to increase the tension so much when going to a larger wire size. Lets say the new wire size is twice as strong. For a given load on the mast it will stretch about half as much, consequently the slack side won't have to take up nearly as much either, it equals out. Putting more load on the wire just sends more load into the hull and chain plates. However, it is possible that the hull and deck move so much that it is that motion that the rigging needs to absorb, not cable stretch. In that case you may need to stretch the larger cable by an even greater amount, because the hull and deck are stretching farther too! I'd love to see someone measure before replacing the rigging and then again afterwards.

Gary H. Lucas


----------



## GaryHLucas (Mar 24, 2008)

casey1999 said:


> I do not plan to increase my wire size (5/16inch) but just trying to get an understanding of the dynamics of the rig tension- Thanks.
> 
> One thing I find interesting is that it seems sailboat rigs seem so fickel, when all around us are power poles and guyed antennas that get almost no inspections and still stay up. Seems they are using galvanized wire. Do large antennas need to have their guys replaced evey so many years? Some of these antennas are huge and the wind load must cycle them just like a sailboat rig. The below tower is 1000 feet tall and has an elevator to get to the top- located in Baltimore MD.


Yes the fatigue life of galvanized wire is better than stainless. However sailboat rigging also sees lots of other loads, not just wind, from people, other boats etc. You also have to be aware of conditions where the pull is not exactly fair. Anything that bends the wire rope during loading and unloading really shortens the life of the wire.

Gary H. Lucas


----------



## casey1999 (Oct 18, 2010)

GaryHLucas said:


> I'm not sure I'd agree that you need to increase the tension so much when going to a larger wire size. Lets say the new wire size is twice as strong. For a given load on the mast it will stretch about half as much, consequently the slack side won't have to take up nearly as much either, it equals out. Putting more load on the wire just sends more load into the hull and chain plates. However, it is possible that the hull and deck move so much that it is that motion that the rigging needs to absorb, not cable stretch. In that case you may need to stretch the larger cable by an even greater amount, because the hull and deck are stretching farther too! I'd love to see someone measure before replacing the rigging and then again afterwards.
> 
> Gary H. Lucas


That is why I was saying if the wire size is increase, maintain the original tension, that way the keel will not hog, which would deform the deck and boat. I am still trying to understand all of this.


----------



## RichH (Jul 10, 2000)

casey1999 said:


> Been reading up on sailboat rigging and don't quite understand rigging tension.
> 
> Most documents I read state tension shrouds and stays to 15% of wire breaking strength. Why 15% and say not 5%?
> 
> ...


VERY simple answer .... its because the sails are cut for operating on 15% tension loaded stays. 
Ultimately the general rule of thumb of 15% will wind up reacting with the forestay. A sailmaker EXPECTS that the forestay will be AT 15% tension for sailing in 12-15kts. and so CUTS the leading edge of luff in a smooth curve 'hollow' to be equal and MATCH the EXPECTED and very predictable sag in the forestay at 15% tension, not 5%, not 20%. The tension in the shrouds/stays must be at an operating tension so that the FORESTAY is at 15% ...... and the 'luff curve' (luff hollow) cut into the windloaded sail ***MATCHES*** the sag in that forestay. 
15% ..... This is the basic tension that the (plain vanilla, cruising) SAILS are designed and cut for. For ultimate simplicity, set all rig tension at 15%, go sailing on a hard beat and THEN 'tweak' the tension by minor tension adjustment so that the mast stays perfectly 'straight' and "in column" ... but remember that the sailmaker EXPECTED that the forestay will be operating at 15% tension when you are sailing in 12-15kts.


----------



## JimsCAL (May 23, 2007)

Actually, the larger shrouds will stay tighter if tensioned to the same load as the smaller ones. Reason is the shrouds on the windward side will stretch less at the same load.


----------



## SloopJonB (Jun 6, 2011)

smurphny said:


> The tension is important because wire does stretch. If the initial tension is too low, the leeward shrouds will be flapping around. One way of tensioning is to alternately tighten the leeward shrouds on different tacks while on a beam reach until they are both just slightly loose and the mast is straight at rest. There was recently a pretty good thread on this with links to Selden Mast. They have a page on the topic.


This method is, in my experience, near universal. I wouldn't have to take my socks off to count the number of times I have seen anyone setting up a rig with any kind of tension gauge, even one as simple as a Loos.


----------



## funjohnson (Aug 20, 2008)

SloopJonB said:


> This method is, in my experience, near universal. I wouldn't have to take my socks off to count the number of times I have seen anyone setting up a rig with any kind of tension gauge, even one as simple as a Loos.


I have a Loos guage just because of the "slightly loose" statement... I have no idea how loose that really is. Can I flex that with my hand? Is that starting to flap in the breeze? I like hard and fast numbers... I wish they had those for all rigs. 

My Hall Spar doesn't seem to bend much at all. I kept loosening the shrouds, but it never seemed to bend either way.


----------



## mitiempo (Sep 19, 2008)

You create bend by tightening a shroud or stay, the backstay for example.


----------



## RichH (Jul 10, 2000)

casey1999 said:


> Now say I want to get some *safety factor* and I up the size to 1/4 inch with a break strength of 6,900 lbs. Why can I not just tension the stays and shrouds to 267lbs (so I do not hog the keel), and then have a bunch of safety factor- all turnbuckels and other fittings would be sized for 1/4 cable.
> Regards


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". 
;-)


----------



## Daydreamer22 (Oct 16, 2008)

Also, if you increase rig size it will also increase rig weight. Wouldn't that make the boat more tender, as in wanting to heel more?


----------



## celenoglu (Dec 13, 2008)

smurphny said:


> The tension is important because wire does stretch. If the initial tension is too low, the leeward shrouds will be flapping around. One way of tensioning is to alternately tighten the leeward shrouds on different tacks while on a beam reach until they are both just slightly loose and the mast is straight at rest. There was recently a pretty good thread on this with links to Selden Mast. They have a page on the topic.


This is the correct method of rig tensioning. After done take measurements and use these measurements for next rig tensioning. 15 % is just about right and might change. Considering the length of lowers one can easily decide that the lowers should be tensioned less than the others.


----------



## LandLocked66c (Dec 5, 2009)

So what kind of gauge do you use to measure said rig? Great thread!


----------



## RXBOT (Sep 7, 2007)

I agree that 15 to 20% of breaking strength is the right ballpark, a loos guage is not much money compared to a lot of boat expenses, probably a good investment. I also think going up 1/32 as in from 1/8 to 5/32 is ok. It won't add much weight and should be ok at the lesser tension. Maybe the boat was designed as a daysailer and is now used as coastal cruiser and island hopper. The slightly larger wire may provide some extra peace of mind.


----------



## casey1999 (Oct 18, 2010)

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.
Regards


----------



## celenoglu (Dec 13, 2008)

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.


----------



## casey1999 (Oct 18, 2010)

RichH said:


> 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'.
> ...


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


----------



## casey1999 (Oct 18, 2010)

celenoglu said:


> 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?


----------



## RichH (Jul 10, 2000)

casey1999 said:


> 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)
> 
> ...


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. 
;-)


----------



## mitiempo (Sep 19, 2008)

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:  <script language="javascript"> var openwindow = null; function showLoginForm() { openwindow = document.getElementById("loginForm"); openwindow.style.display = "block"; } funct


----------



## ccriders (Jul 8, 2006)

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


----------



## mitiempo (Sep 19, 2008)

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.


----------



## casey1999 (Oct 18, 2010)

ccriders said:


> 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?


----------



## LandLocked66c (Dec 5, 2009)

casey1999 said:


> 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.



> 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.


----------



## lancelot9898 (Dec 30, 2008)

RichH said:


> 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.


Hi Rich

I too have a T-37 and have used the selden folding rule method on the backstay to supposely set tension on the furling headstay. I'm still not certain if that tension gets transmitted to the headstay if the mast is beefy. For example undo the headstay and then look up the mast. I can detect no bending what so ever with the headstay undone and with 20% on that backstay. Not sure what spars you have, but mine are keel steped New Zealand spars. Best Regards.


----------



## RichH (Jul 10, 2000)

ccriders said:


> 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


Except for one very important consideration .... you dont have sails mounted to bicycle spokes. If you did you'd also have to consider the distributive loading on all the spokes by the sails.

Rigging does TWO functions: Holds the mast up AND provides a consistent 'catenary' platform from which to fly 'jibs/genoas, etc.'.


----------



## ccriders (Jul 8, 2006)

casey1999 said:


> 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?


You know, this question has never been satisfactorily answered and there is no single simple answer. There are times the hub is suspended from the rim's arc at the top of the wheel (static mode) and there are times the top spokes and opposite side down spokes are supporting the rim (going around a corner at speed). The rim goes a little flat at the contact patch and those spokes go out of tension when on a straight line. That reduced tension allows the opposite spokes to also go out of tension, therefore none of the vertical spokes are supporting the bike for a split second.
For sure it is not like a wagon wheel but a dynamic entity of several parts. Which is why I think of a bicycle wheel when I think about tuning the rig of a sailboat. An unstayed mast would be like a wagon wheel. Modern rigs are like bicycle wheels.
John


----------



## ccriders (Jul 8, 2006)

RichH said:


> Except for one very important consideration .... you dont have sails mounted to bicycle spokes.


Except when you put fairings over the wheels to reduce air turbulance and improve aerodynamics. The sail effect can be huge and that is why UCI mass start events don't allow disc wheels.

Also, when you say hold the "mast up" you are simplifing considerably the dynamics of a modern sail rig.

John


----------



## casey1999 (Oct 18, 2010)

ccriders said:


> You know, this question has never been satisfactorily answered and there is no single simple answer. There are times the hub is suspended from the rim's arc at the top of the wheel (static mode) and there are times the top spokes and opposite side down spokes are supporting the rim (going around a corner at speed). The rim goes a little flat at the contact patch and those spokes go out of tension when on a straight line. That reduced tension allows the opposite spokes to also go out of tension, therefore none of the vertical spokes are supporting the bike for a split second.
> For sure it is not like a wagon wheel but a dynamic entity of several parts. Which is why I think of a bicycle wheel when I think about tuning the rig of a sailboat. An unstayed mast would be like a wagon wheel. Modern rigs are like bicycle wheels.
> John


Yea that bicycle wheel is pretty complex for somthing that on the surface looks so simple. I once saw a bike wheel where the spokes had been replaced by long rubber bands (wheel not on a bike) to see how the thing works when forces are applied to the axel.


----------



## smurphny (Feb 20, 2009)

casey1999 said:


> 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)
> 
> ...


 I've found that the forestay tension is critical to both sail shape and operation of the furler. If your furler starts to get cranky, the first thing to check is sag in the forestay. The right tension here is the first step in getting the rig balanced. On deck-stepped masts like mine, it's important to get it just right to avoid too much down-pressure on the deck. (A note: I've completely done away with any balsa core here and have made the entire area around the mast step solid glass. I don't know why these builders even thought about cored decks under mast steps because it seems like none of them ever hold up over time.) As far as the 40%, I bet it's to eliminate as much metal fatigue as possible when the headsail is moving about. The amount of abuse the forestay takes what with the furler foils rotating around and the genoa movement always scares me. My headstay is 9/32, wish it were even larger but there's no way to change to 5/16 without changing the furler.


----------



## RichH (Jul 10, 2000)

lancelot9898 said:


> Hi Rich
> I too have a T-37 and have used the selden folding rule method on the backstay to supposely set tension on the furling headstay. I'm still not certain if that tension gets transmitted to the headstay if the mast is beefy. For example undo the headstay and then look up the mast. I can detect no bending what so ever with the headstay undone and with 20% on that backstay. Not sure what spars you have, but mine are keel steped New Zealand spars. Best Regards.


_*Caution*, the following discussion is 'mostly' for CUTTER RIGS, not for 'sloops', although the 'link' described at the end is applicable to 'both'. _

Cutter rigs with TWO stays in front of the mast are an ENTIRELY different 'animal' from a simple rigged sloop with one stay in front of the mast. The problem is with a cutter you have TWO stays reacting to a SINGLE backstay, Intermediate stays to react with the forestay (that stay thats immediately in front of the mast) .... and worse the sails in the foretriangle are of different areas and differing aerodynamic functions and reactions TO the stays on which they are mounted.

The FUNCTIONAL problem with a cutter is: if the larger headsail becomes windloaded it reacts by 'transferring' part of it load to the smaller forestay (in proportion to the aerodynamic forces generated by each sail ... and the staysail 'under' a 'topsail' isnt going to have the same aerodynamic forces being generated as if it were flying 'alone') .... and the headstay becomes 'more slack' as a result. With a slack headstay a cutter rig will not 'point', will heel over aggressively and will be slow as hell and start 'slipping off to leeward' when going upwind. The net result is an extremely complicated rig tension problem ... and with MANY 'variables' acting at the same time.

Soooo ... I first set up the rig like it were a sloop 15% in caps, 4 lowers and backstay/headstay ..... intermediates and forestay at 0%. then readjust forward and aft lowers to get at least 1/2" mast prebend. 
THEN 'work-up' in increments the forestay and intermediates to ~10-12%. You DO NOT want more than 10-12% on an intermediate or you risk pulling the chainplate off of its knee attachment. Final 'tune' of my 'basic' setting is to put the backstay at ~20% and 'leave it' (If I were circumnavigating, that backstay would be at ~15%).

With the 'basic' cutter rig tune as above ........
When needing to "point"/beat, I watch how the headstay sag to leeward is matching the luff hollow curve (where the luff hollow curve 'should' be . see 'article' listed below) and either slack off the forestay and/or when in 'normal' ≤15kts. apply running-backstay tension!!!!! .... alternatively its easier just to run forward and slack the forestay down to 3-5% (a good guess) !!!!!!! with such 'adjustments' the boat will 'stand up' with less heel, take off like a rocket, and point like a banshee. 
With the 'normally unloading' slack headstay you will be tacking through 95-105°, with a tight headstay / loose forestay you can easily sail to well over 35-40° apparent ... (and sheet the yankee 'inside' the cap shrouds if you want and without 'pinching' or 'barberhaul' the headsail/yankee !!!!!). Slacking off the forestay will 'transfer' most of the backstay (15-20%) to the headstay.
When going downwind, I keep a taught forestay ... as downwind it makes NO difference (relatively) how much the headstay goes slack. 
When sailing in BLAMMO conditions ... with no headsail, a staysail + deep reefed main ... I bring the forestay full TIGHT, upwind or down.

Cutters really need to be able to continuously adjust FORESTAY tension ... according to the present wind and seastate condition .... but of course while watching the 'rest of the rig' so that nothing goes much over 30% tension for accelerating 'fatigue' considerations.

Bob Perry, I think, has addressed all the 'tension', etc. complexity issues on cutters with: "Simply sail them like a sloop" .... which means to me get rid of the forestay/staysail/intermediates when sailing in normal conditions (slacking/removing) ... and only use the 'excess' when in Blammo or survival conditions.
I still occasionally race my Ty37, am a 'fanatic' even when 'cruising' and therefore dont mind doing all the on-the-fly rigging and sail changes.

Here's the link that refers to 'matching the 'cut' of the luff to the sag of the headstay .... applies to ALL sails that are attached to 'stays' ... cutter or sloop, make no difference. If anyone cant read this 'article' give me a PM and Ill reply with an email copy. http://i1086.photobucket.com/albums/j449/svAquila/MatchingLuffHollow.gif

If you want to get a cutter rig to 'point' that headstay HAS TO BE taught .... and I think that the forestay has to be SLACK as the 'easiest' way to auto-tighten the headstay. Headstay + forestay does NOT equal one backstay (in tension-speak) but enough load for TWO bacskstays. 15% + 15% = 30% ..... and thats at or near the 'fatigue endurance limit' for 300 series stainless.
MOST times you dont 'need' 15% in a forestay so why do it as it only automatically slacks off the headstay?

hope this helps. ;-)


----------



## casey1999 (Oct 18, 2010)

smurphny said:


> I've found that the forestay tension is critical to both sail shape and operation of the furler. If your furler starts to get cranky, the first thing to check is sag in the forestay. The right tension here is the first step in getting the rig balanced. On deck-stepped masts like mine, it's important to get it just right to avoid too much down-pressure on the deck. (A note: I've completely done away with any balsa core here and have made the entire area around the mast step solid glass. I don't know why these builders even thought about cored decks under mast steps because it seems like none of them ever hold up over time.) As far as the 40%, I bet it's to eliminate as much metal fatigue as possible when the headsail is moving about. The amount of abuse the forestay takes what with the furler foils rotating around and the genoa movement always scares me. My headstay is 9/32, wish it were even larger but there's no way to change to 5/16 without changing the furler.


Yes, I also have a deck stepped mast. The mast and step are very strong. Reason for deck step is that boat (S&S34) originally from Fremantle Austrailia where they need to lower the mast to go under some bridges to return to port. I will check my tension on the fore stay. The thing that surprises me with the deck step is that I figure I will have about 13,000 lbs down ward force on a 5inch x 5 inch wood copression post.- That seems like a lot- I surprised these compresion post do not fail.


----------



## smurphny (Feb 20, 2009)

casey1999 said:


> Yes, I also have a deck stepped mast. The mast and step are very strong. Reason for deck step is that boat (S&S34) originally from Fremantle Austrailia where they need to lower the mast to go under some bridges to return to port. I will check my tension on the fore stay. The thing that surprises me with the deck step is that I figure I will have about 13,000 lbs down ward force on a 5inch x 5 inch wood copression post.- That seems like a lot- I surprised these compresion post do not fail.


Yes, indeed, a lot of force being set on the deck. It amazes me that the deck takes this kind of weight. It MUST deflect at times. On my old A35, there is a large header which unloads the weight to the bulkhead/forward cabin doorway below. I have thought about making some kind of removable post to go right under where the mast sits and down right to the keel...a lolly column of sorts...for long, rough passages.


----------



## casey1999 (Oct 18, 2010)

smurphny said:


> Yes, indeed, a lot of force being set on the deck. It amazes me that the deck takes this kind of weight. It MUST deflect at times. On my old A35, there is a large header which unloads the weight to the bulkhead/forward cabin doorway below. I have thought about making some kind of removable post to go right under where the mast sits and down right to the keel...a lolly column of sorts...for long, rough passages.


I think that is a good idea. I hear of a lot of deck stepped mast boats with no compression post do have problems with the deck defect and eventually bending in permanently. Once you get some deflection, then all your shroud and stay tension goes very low, as discussed in this thread- not good.


----------



## smurphny (Feb 20, 2009)

There is probably 1 1/4" of solid glass on top of the header now, so I'm really not worried about any failure there BUT, as you say, any deflection would have an effect on rigging tension. I'm just thinking about when I go offshore and run into a real extended pounding if a post would offer any additional measure of safety. It would sit right in the forward V-berth doorway but I use this mostly for storage anyway. If anyone wanted to sleep up there they'd have to be skinny


----------



## GaryHLucas (Mar 24, 2008)

casey1999 said:


> 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?


The hub is hanging from the top spokes, and the rest are keeping the rim from going out of round. A while back I needed a large pulley for a 5-1/2" diameter hose, it was 10 feet in diameter. I designed and built the pulley just like a bicycle wheel, with 1/2" thick spokes that were tensioned by T-nuts in the rim. It worked very well.

Gary H. Lucas


----------



## casey1999 (Oct 18, 2010)

Used the Loos PT-3 guage and found fore and aft stay at 20% breaking strength and shrouds at 5%. Gauge works great, took 5 minutes to do this. Also measured tenstions easily while sailing.

Now I need to work on bringing tensions up in the shrouds.

One question for those that have used a Loos gauge. The gauge is calibrated for 304 stainless. If I have 316 wire, do I need to use a correction factor?


----------



## tommays (Sep 9, 2008)

Diameter correction factor is the issue and that the guage is in range for that Diameter as there pretty limited in size range

Cable size 1/4", 9/32", 5/16", 3/8"


----------



## casey1999 (Oct 18, 2010)

tommays said:


> Diameter correction factor is the issue and that the guage is in range for that Diameter as there pretty limited in size range
> 
> Cable size 1/4", 9/32", 5/16", 3/8"


For the Loos gauge, you read the number then go to the chart (sticker on gauge) that gives you % breaking strength and also the actual tension (in pounds) on the cable. The instruction manual says the gauge is for 304 wire. So when unsing with 316 wire do you read the % breaking then calculate tension based on that (since 316 has lower breaking strength than 304 stainless)?


----------



## tommays (Sep 9, 2008)

15 on 1/4 is 1400 # tension no matter the material


----------

