Standing rigging strength needs?

[mastreb]

Are you sure that under load the HMPE is getting shorter, Matt? Under load, even cycling loads, the lines should be getting longer (stress-strain). The only explanation I can offer is that the line is being twisted (there might be a pattern on the line so that you can detect that) which would make it appear as though it is getting shorter (?)

Even the thick 1/2" dia. HMPE top-down furler line, which gets twisted in use, needs a "set-in" time during which the material creeps (like wire rope) and elongates.
-Brian.

Yes, it's counterintuitive. When the rigger made my stays, he pulled them out with a 4000 lb. winch. As the strands move and flex, they will draw tighter over time. They can always be pulled back out if they're in good shape. He recommended not making loops tighter than 2' in radius to avoid having them creep shorter faster.

This is why they're making artificial muscle out of the stuff--it also shortens under electrical impulse as it turns out.

I see, that is what sets it apart, it has been pre-stressed/tensioned and it is returning to (almost!)its original length. HMPE would normally stretch longer in length, it would be important to know whether or not that had been done prior to installation.

Yes. If they're not pretensioned, they're too elastic to work as stays.

[mastreb]

For the original poster, I too felt the original stays were insufficient when I bought the boat. I've since learned that that's not the case. These boats only require about 300 lbs. of tension max, and routinely go a little slack on the leeward side when sailing. The total force incumbent upon the stays is <1000 ft. lbs, and these stays are rated to 4X that amount, which is more than sufficient.

Putting on larger stays isn't better, especially if you trailer. They can subject the rest of the rig to greater shock loads and they're heavier and harder to deal with than the stock stays. Also avoid the temptation to go to turnbuckles--they don't stand up to trailering. The stock stay adjusters do not lose their set when they're intentioned the way that turnbuckles do.

What I _would_ upgrade is the jury rig going on at the top of the forestay. My boat came stock with the upper side stays on a tang and the forestay _outside_ the shackle on the pin end. Granted, it never came down, but it also wasn't on a swivel as it should have been. I replaced that with a swaged Torstan swivel when I re-did my rig (which was out of necessity due to a collision with a channel marker).

Another thing the factory could have done right is put Tefgel (or anything, really) between the aluminum mast and the stainless fittings. I had white aluminum oxide under every fitting when I rebuilt my mast.

[walt]

Putting on larger stays isn't better, especially if you trailer. They can subject the rest of the rig to greater shock loads

This is one I have a hard time "grasping".. Say you have a 1/8 forestay and tension it to 300 pounds. You replace this with a 5/32 wire and tension it to the same 300 pounds.

For a 27 foot length of wire (about what we probably have in the forestay), the difference in total weight between the 1/8 and the 5/32 wire is only 0.39 pounds. This weight would be distributed over the length of the forestay and I think would have negligible "swing weight" affects.. I know to get to the same percent of the breaking strength you could tension the 5/32 higher. But if both diameter wires were still tensioned to 300 pounds, it seems to me that shock loads to the rig would be nearly identical.

[kadet]

I know to get to the same percent of the breaking strength you could tension the 5/32 higher. But if both diameter wires were still tensioned to 300 pounds, it seems to me that shock loads to the rig would be nearly identical.

I agree weight is not a real issue in this instance as it is minimal the problem is that you don't tension the 5/32 to 300 pound you have to tension it to the same percentage to avoid fatigue failures. The wire needs to be tensioned to a reasonable proportion of yield. To achieve a similar value in a wire with higher cross sectional area the force required would be such that it will but addition forces on the boat and mast which could cause shock load failure in another area. Wire is cheap masts and decks are not .

Rigging guides I have seen recommend 20-25% of breaking load for tensioning so that is something like 600 pounds for 5/32 wire.

I think this is a case where 'bigger' sounds like 'safer' but the opposite coud be true if the rest of the boat is not upsized to match. There is no history of rigging failure so the question remains, why?

[walt]

I know there are some mechanical and structural engineers here (maybe Kadet is also??) but any more detail about how if you go to a 5/32 wire vs 1/8, and tension to the 1/8 wire spec you are more likely to have a wire failure. I still have to admit.. doesnt make sense to me. Also, it is never ever the wire in some lenght that fails (1/8 or 5/32), it is usually where the flexible wire interfaces with something ridged that fails from point loading such as where the cable/wire goes into a swaged connection and I think the slightly larger wire size will be better for this. But.. Im not an expert on this and also cant back that up with any sort of link..

[mastreb]

Kadet is correct. If wire isn't tensioned to its prescribed percentage of yield, it is not "elastic" when a shock load occurs: It is tight, and so when a shock occurs the entire shock force is transmitted to the connection points--eyes, swages, etc. rather than being absorbed along the entire length of the wire. This causes point-of-connection sudden failure. It's why you can't leave your shrouds loose.

Think of it like a muscle that hasn't been stretched before suddenly going into action: It's going to fail suddenly, where the same muscle correctly warmed up would be fine.

This effect is one of the reasons why I switched to dyneema--it becomes more elastic over time as it shortens, rather than less elastic as it lengthens. This means the failure I will see is it simply becoming more difficult to get the mast up over time until I have to lengthen the stay adjuster, rather than constantly checking tension and worrying about a shock load failure.

It is oftentimes difficult to imagine how things act under different regimes of force or time, but that's what engineers are trained to do. To our normal experience, steel wire strong and stiff, not like a rubber band. But at high tension, it is elastic like a rubber band, just not for much distance. We don't see it or feel it, so it's difficult to image that this is the reason why it works. In our normal experience, the air is not a thick fluid like water, but to a Jet traveling 600 miles per hour, it has the same viscosity as water and that plane is literally "planing" atop it like a boat over the surf. There's an entire hidden world of high forces and long time-scales that engineers learn to understand by analogy with the real world that they inherently feel.

[walt]

If wire isn't tensioned to its prescribed percentage of yield, it is not "elastic" when a shock load occurs: It is tight, and so when a shock occurs the entire shock force is transmitted to the connection points--eyes, swages, etc. rather than being absorbed along the entire length of the wire. This causes point-of-connection sudden failure. It's why you can't leave your shrouds loose.

A link would be good here. Im not a mechanical engineer but tried to verify this and what I found implies something different - or at least Im not looking for the correct thing.

What I found that seems to to relavent is "hooks law". What hooks law says is that for a material like steel, there is a linear relationship between tension and extension and the linear relationship (ie, slope or proportionality) is defined by youngs modulus.

A linear relationship means that a change in stress produces a given change in lenght and this relationship would be constant over a range of stress values. This relationship holds up to the limit of elasticity.. but we are of course always staying under that limit where the wire would have a constant youngs modulus.

In other words, Hooks law says that a wire tensioned at 300 pounds or at 500 pounds and that gets a transient stress of 50 pounds, the wire will see the same deformation lenght change for both 300 or 500 pounds. It apeared to me that you are implying that the 50 pounds of stress change would produce a larger defomation at 500 pounds. This would be a non linear relationship implying a different youngs modulus at 500 pounds vs 300 - and violate hooks law.

Maybe things behave differently when you put a bundle of wires together (ie, 1 x 19 or other sets)? A link would be appreciated.

A link below..
http://www.britannica.com/EBchecked/top ... elasticity

In a simple tension test, the elastic response of materials such as steel and bone is typified by a linear relationship between the tensile stress (tension or stretching force per unit area of cross section of the material), σ, and the extension ratio (difference between extended and initial lengths divided by the initial length), e. In other words, σ is proportional to e; this is expressed σ = Ee, where E, the constant of proportionality, is called Young’s modulus. The value of E depends on the material; the ratio of its values for steel and rubber is about 100,000. The equation σ = Ee is known as Hooke’s law and is an example of a constitutive law.

[walt]

Some numbers..

FYI, this page also gives recommended cable tightness

http://loosnaples.com/how-to-use-90-91

I’ve actually been using a 5/32 inch forestay on my 26S for the whole time I’ve owned it (about 8 years) and probably am way under the 15% breaking strenght guideline. The forestay is properly toggled with a hank on and I have had not a bit of issue with it. I already have the new 5/32 forestay and am replacing it mainly because the old one won’t fit in the new furler drum and also.. just not a bad idea to replace.

I’m not sure what came stock on the old Macs (mine is 1990) but I’m assuming it was 1/8 inch since I understand the new 26M has a 1/8. I think the side stays have always been 5/32.

I’m guessing that I have been running the forestay at well under 10% for the whole time I have owned the boat. It is tight enough that I don’t get any "shock loads" from loose shrouds going tight.

When I set this up again, I want a tighter forestay than I had in the past because you need this for best pointing ability. Looking at some numbers,

320 pounds for a 1/8 inch forestay is 15% of breaking strength.

320 pounds for a 5/32 inch forestay is 9.6%

I’ve have never measured my current 5/32 forestay but would guess I have been running it at closer to 6% breaking strength.

I had a furler in the past with 1/8 cable on a 16 foot sailboat and almost lost the mast from the forestay at the top not having a toggle. This forestay was tensioned to 15% breaking strength.

So.. the main correlation I have seen with cable problems is not having a proper toggle. I don’t expect to have any problems at all running the 5/32 inch forestay at 9.6% rather than 15%. I won’t have any mast pumping with this tightness.

This 9.6% is about what Sumner is running now (about 320 pounds with a 5/32 forestay) on his Bahama trip. I guess we will both find out if running a 5/32 forestay at 9.6% breaking strength will have issues.

And.. I also completely agree with everyone that there is no reason at all to worry about the original 1/8 inch forestay. Just make sure its property toggled.

[kadet]

http://www.riggingandsails.com/pdf/selden-tuning.pdf

Looks like 15% for fraction rigs and 20% for masthead according to selden.

In the end the difference between the 2 is small enough to not worry about it, go with what makes you happy. 1/8 or 5/32.

The point is if you are going to 5/32 because you think it is stronger and safer than 1/8, the belief maybe misplaced.

[walt]

The point is if you are going to 5/32 because you think it is stronger and safer than 1/8, the belief maybe misplaced.

Yep.. if I were ordering this again right now, I would consider going back to 1/8 inch cable.

[mastreb]

If wire isn't tensioned to its prescribed percentage of yield, it is not "elastic" when a shock load occurs: It is tight, and so when a shock occurs the entire shock force is transmitted to the connection points--eyes, swages, etc. rather than being absorbed along the entire length of the wire. This causes point-of-connection sudden failure. It's why you can't leave your shrouds loose.

A link would be good here. Im not a mechanical engineer but tried to verify this and what I found implies something different - or at least Im not looking for the correct thing.

What I found that seems to to relavent is "hooks law". What hooks law says is that for a material like steel, there is a linear relationship between tension and extension and the linear relationship (ie, slope or proportionality) is defined by youngs modulus.

A linear relationship means that a change in stress produces a given change in lenght and this relationship would be constant over a range of stress values. This relationship holds up to the limit of elasticity.. but we are of course always staying under that limit where the wire would have a constant youngs modulus.

In other words, Hooks law says that a wire tensioned at 300 pounds or at 500 pounds and that gets a transient stress of 50 pounds, the wire will see the same deformation lenght change for both 300 or 500 pounds. It apeared to me that you are implying that the 50 pounds of stress change would produce a larger defomation at 500 pounds. This would be a non linear relationship implying a different youngs modulus at 500 pounds vs 300 - and violate hooks law.

Maybe things behave differently when you put a bundle of wires together (ie, 1 x 19 or other sets)? A link would be appreciated.

A link below..
http://www.britannica.com/EBchecked/top ... elasticity

In a simple tension test, the elastic response of materials such as steel and bone is typified by a linear relationship between the tensile stress (tension or stretching force per unit area of cross section of the material), σ, and the extension ratio (difference between extended and initial lengths divided by the initial length), e. In other words, σ is proportional to e; this is expressed σ = Ee, where E, the constant of proportionality, is called Young’s modulus. The value of E depends on the material; the ratio of its values for steel and rubber is about 100,000. The equation σ = Ee is known as Hooke’s law and is an example of a constitutive law.

Hi walt,

Cable does react somewhat differently than solid bar stock but the linear relationship is the same. The non-linear component of force that causes shock loading is the transition from an un-tensioned state to a tensioned state, which is entirely different than the linear (actually logarithmic in the case of a boat) increase in tension that occurs once the system is under tension, which is what your references apply to.

The transition from un-tensioned to tensioned is the non-linear shock load. It is why the rig shudders when its hit from the side by a gust if your stays are not under tension.

If the rig is properly tensioned, this shock load never occurs, and then the fully linear increase in tension applies, which is what we want. This is why we pretension the rig.

Whatever fraction of under-tensioned the rig is is the fraction of shock load that will apply when the rig is suddenly loaded. So, if you've tensioned to 100 lbs. and the boat is suddenly hit with 300 lbs. of force, then you'll suffer 200 lbs. of shock load.

Whatever fraction of over-tensioned the rig is is unnecessary stress on the rest of the system and additional unnecessary force applied if the rigging should suddenly fail, which can bend masts and break spreaders.

The recommended pre-load (300 lbs.) is not some magical engineering number, it's merely a subjective point where a rigger has determined that some large fraction of sudden loads that are likely to be met will be below this value. Essentially, 99% of all sudden loads will be less than 300 lbs., so we will tension to 300 lbs. If we get hit with a sudden 500 lbs. load, the boat will suffer 200 lbs. of shock load and 300 lbs. of tension load.

The 300lb. tension recommendation is simply the point at which the rig doesn't shudder under normal expected sailing and movement conditions. If you do a Mac bump with your rig up, you'll go way over 300lbs. of load on the rig and see it shudder even when properly tensioned.

Once we know what tension force we want (300 lbs.), we would then select the cable size that is moderately tensioned (20% more or less) tensioned at that value. This is why 1/8th is specified. We choose a 20% number because we want the rig's failure point to be far away from our pre-tension number, but not so far that the rig is not actually under tension at all. If the cable is so large that it's only at a small fraction of its total tension value, it is not yet in the fully elastic regime of the cable and therefore transmitting rather than absorbing shock loads to other components. It's also unnecessarily heavy.

Shock loads create deformations at weak points, such as kinks at swage eyes, and those kinks dramatically lower the force resistance of the eye. A kink in a swaged eye lowers the holding power of the eye by about 50%, and subsequent shock loads can then overwhelm the holding power of the eye, leading to a sudden break and failure. By pretensioning the rig, we keep this kind of accumulating damage at bay.

Matt

[walt]

Matt, thanks for the discussion. I think one other thing about this is that if your racing and want peak upwind performance, you want to run that forestay as dang tight as is safe and I wonder if this somehow gets mixed in with the recomened % of breaking strenght for sailboat applications. Dont know myself..

It looks like my Hyde jib and furler just shipped.. which I am way excited about.

Some of you guys here should ask Dr. J for a commission.. LOL

[mastreb]

Matt, thanks for the discussion. I think one other thing about this is that if your racing and want peak upwind performance, you want to run that forestay as dang tight as is safe and I wonder if this somehow gets mixed in with the recomened % of breaking strenght for sailboat applications. Dont know myself..

It looks like my Hyde jib and furler just shipped.. which I am way excited about.

Some of you guys here should ask Dr. J for a commission.. LOL

Hi Walt,

Yes, of course you want as much tension as you can safely get on a forestay for pointing. Unfortunately on the 26M, that's "not much". The rotating mast, lack of a backstay, and need to be able to manhandle it onto the bow chainplate all conspire to keep the forestay relatively loose on these boats.

Frankly, considering all the compromises Roger had to engineer in to make these boats possible, it's astonishing that they sail as well as they do.

[Catigale]

Walt"..your S boat is way faster than the power sailors...even the white but gettin tension on rigging to eliminate slack is important. I have not gone to 5/32 for this reason.

[BOAT]

This is a pretty old post but since catigale has resurrected it I think I would like to add a point that was not brought up, upgrading the MAC rigging causes other trouble that can be dangerous.

It's not a good idea to upgrade the cleats on the deck of your boat so that the deck fails before the cleat does. Better to let the cleat rip off in a storm than a chunk of the deck. Same goes for other parts of the hardware: a stay should fail before a chain-plate, a goose-neck should fail before a mast, and so forth. Rigging needs to be sized to allow for the proper order of failure.

This was all taken into account when Roger built the boats. The factory rigging is just fine, these are very VERY light boats, not heavy ballasted keel boats.