Discontinuous vs. Continuous Rigging & Spar Compression Loads

[UNCIVILIZED]

Guys, need to borrow a bit of brain power on this one, as tragically, I'm completely out of chocolate covered expresso beans: AKA nitrous oxide for my thinker.

So, here's my question. Given 2 equal spars for the same boat, would the one with discontinuous rigging have a lower vertical compression loading on it due ot the rigging than the one with continuous? And if so, why?

[Don C L]

Not sure if this helps but when I queried my old friend who got his degree from MIT about compression loads I can expect, the topic of rigging, discontinuous or continuous, never came up. He spared me the math and said it roughly boiled down to approximately the displacement of the boat. I'll have to give him a call again.

[UNCIVILIZED]

Thanks, I'll be curious to hear what he has to say. Although I'd prefer it if he can site the references & math involved. As on the majority of racing boats, the compression load on the tube is a lot higher than what the boat weighs. Sometimes by several multiples there of.


For example, here's the spec's on a 2-tonner fitted I had, that was with continuous rod rigging.
Cap shrouds, Headstay, & Backstay: -17 rod
Intermediates, 2 per side: -12 rod
Lowers: -17 rod
Babystay: 10mm 1x19 wire
Runners: 5mm Amsteel Blue
Check stays: 4mm Amsteel Blue


Boat displacement: 9t


So if you tension all of the above shrouds & stays to 25% of their breaking strength, the compression load comes out as being double that of the boat's mass. And this is far from a unique case. For example, it's fairly common to have 50-100 ton hydraulic jacks to assist with mast tuning on maxi's & large racing boats.

[ggray]

If the rigs are otherwise the same--mast height, spreader positions and lengths, chainplate positions--I don't think there would be any difference.

Mast compression would be determined by more than just displacement. Ballast, draft, mast height, sail area, width of shroud base, length of spreaders also factor in mast compression.

[Snowpetrel]

From what I remember the major advantage of discontinous rigging is slightly lower weight, windage and less stretch/deflection of the mast. I am just guessing that it may result in less pretension induced compression, but the sailing compression loads are going to be so variable anyway that any small differences are lost in the noise.

I would be very reluctant to change a spindly discontinuous rig over to a continous rig more because of the deflection and mast stability issues than due to outright static compression loads. I also don't think you are going to wind everything up to 25% UTS. the intermediates are often nearly slack.

Interestingly in my rigging loads spreadsheet a major unexpected factor in the compression loadings was prebend. More prebend meant the runners and checkstays have to do more work to hold stop the mast moving forward. Amd they added significantly to the compression loads. If you have spept spreaders this factor gets pretty high, but without enough prebend the risk of inverting the rig is too high.

The other interesting thing is the loads and stretch on the intermediates when reefed, esp when bald headed. A discontinous rig would have the advantage here due to the larger caps helping reduce stretch.

This load gets extreme if a staysail is flown in strong winds. Pretty much all the force gets loaded onto the mast near the intermediates if you dump the main in a puff.

The worst case is a deep reefed main only. The lowers take a pounding in this case. When running the fore and aft loads also get pretty big. Esp if you have swept spreaders.

Many pre 1980 boats still used continuous rigging with masthead rigs. After that very spindly fractional rigs took over, forcing discontinous rigging to deal with deflection, and excess deflection leads to very high rigging and compression loads.

[Snowpetrel]

I guess to answer your question directly, instead of the above ramble. I would say yes very slightly less compression loads overall due to less deflection on the discontinous rig. This keeps the rigging angles slightly wider at any given load. It will also more effectively unload the less strained wires in the rig.

But then less stretch results in higher shock loads, so the true dynamic loads are all pretty hard to calculate without access to some pretty serious FEA analysis in the real world.

[Eigenvector]

So unless I'm not understanding the problem the answer is that it makes no difference as shown. (Pardon the scratch, I was in speed mode)

The external forces don't change, therefore the compression of the mast is twice the vertical tension components for this 2D analysis as shown.

[UNCIVILIZED]

Ben, thanks, actually the "ramble" helped quite a bit. The rig isn't at all spindly. I'ts a 3 spreader, with a solent, & the option of a storm staysail. Along with runners, & checks, plus a tuneable backstay. The main's huge, but then so are the other sails. And I think it's an early 90's design. Aluminum spar BTW, with a semi-narrow staying base. And a moderate amount of prebend.

Due primarily to cost, it'd be nice to make the change to Dux. Though that would mean new spreaders, or at least tips. And chainplates. Which, all told, it might just be simpler (& cheaper) to stick with rod. Even though I know that yes, in theory, it's possible to do a discontinuous Dux rig. But the price of the specialzed end hardware to integrate the Dux with the spreader tip cups drives up the price.

I never have liked discontinuous rod, ever since it first appeared on the scene years ago. Too many extra parts; to fail, as well as to buy. And you need a PhD to assemble the rig properly. Not to mention that there are a dozen different tip cup designs out there, so getting all the parts to fit together when it's time for a rerig is a nightmare. Ugh! Race boat rigs suck!
Frickin lab geek engineers designing spars using hyper exotic alloys. But... they are getting "paid to play" when an owner gives them a blank sheet & says "make it fast".

[Snowpetrel]

I hear ya uncivil. At least mine is not too extreme, but handling runners, and jib sheets on these sort of boats gets pretty old, pretty quick. Its one thing when you know the rigs not going to fall down if you stuff it up, as in a conservative crusing rig. Quite another when the noodle needs the extra staying to survive...

One benefit of a racing rig is they go the extra mile to strengthen them. Eg sleeving the lower panels and tapering. These enable a lighter mast to have the strength where it is needed.

At least masthead gives you that extra margin of safety over the crazy inline frac's. Ie a missed runner doesn't automatically end up with a rig in the drink.

Dux is interesting. Ive done the numbers to death it should work (and it does) the biggest worry for me is the loss of tension due to cold/heat. Its not a massive issue if your rig can cope with a bit of slack if need be. It seems to me like it would be a big issue if your rig uses swept spreaders for fore and aft stability. In your case you have runners and an inner forestay to control this aspect, and typically a triple spreader masthead rig has reasonable staying angles, at least down low. So it is reasonably stable athwartships.

[Adelie]

Discontinuous rigging gives a windage advantage for multi-spreader rigs.
For the bottom panel there is 1 vertical up to each spreader tip where the vertical attaches to a fitting at the tip.
From there a diagonal runs up the the base of the next spreader up, and another vertical (Smaller diameter) continues up to the next tip. Both have turnbuckles atop the spreader tip.
This repeats up to the top spreader where the vertical passes thru and becomes a diagonal.

There is a slight weight penalty since there are 1 extra turnbuckle at every spreader tip except the top one and all the turnbuckle weight is up the rig rather than at deck level

Total cross-sectional area of the vertical wires is about the same at every level so there is no weight advantage in the wire offsetting the turnbuckle weight.

There is a slight but more important windage advantage since doubling the area of a wire does not cause nearly the same windage as installing 2 wires of same original diameter.

For a triple spreader rig the windage gain is even bigger.

For anything more than a triple spreader rig it would be hard not to have discontinuous rigging since it would be hard to deal with more than 3 vertical wires coming down at the same place and needing to space out them enough to conveniently adjust the turnbuckles.

I would be on the fence for a triple spreader rig. For a double spreader rig there is not a good argument for discontinuous rigging unless the boat is a high-end racer.

The big downside to discontinuous rigging is you have to go aloft to adjust rig tension. That means going up the mast and out to all spreader tips except the top spreader tip to adjust turnbuckles while underway.

Or doing the same thing at the dock, going for a sail to check the mast alignment, then repeating until the mast is as desired.

[Snowpetrel]

The windage question is an interesting one, and very valid, athough offsetting the gains are the fact that normally the extra wires are aligned fore and aft, so they slupstream each other somewhat. And the windage of the bottle screws aloft needs to be considered.

One big plus in my eyes to the discontinous rig is the way it solidly locks the spreader into place. No way it can slip up or down. I have a pet theory that in a knockdown spreader slippage up or down is a likely failure mode.

Another plus for the discontinous rig is the issue of wire movement through the spreader ends. A continuous rig can have quite a bit of slippage of the wire due to differential stretch. This gets interesting on a triple spreader rig with three wires running over the lower spreader. Which one do you lock off to the spreader end?

Of course discontinous rigging has lots of minuses, the big one in my eyes is that the failure of the v1 or v2 gets pretty messy very quick. At least with continuous rigging you have a fighting chance if you act quick.

I can envisage an ideal solution with Dux, and something like this is being used by the top fibre rigging companies with custom PBO and Carbon shrouds. A fully engineered one piece continous/discontinous rigging solution. Effectively you taper the cap shrouds and directly splice in the intermediates/diagonals to this tapered cap shroud/vertical. To get real smart the D's could be terminated on the mast with some sort of lanyard that could be lead to deck for adjustment, prehaps with a locking system of some sort aloft for when they are adjusted correctly. (Or you just go aloft and lash them off). This would be a very neat setup, but I am not going to be the test pilot, I don't have the disposable cash to play around with prototypes. It would need some load testing to find out the best splicing and tapering methods.

[Matsubob]

Just like the last rigging discussion, this is fascinating. We all know what we have seen in sudden gusts or when the running backstay's clam cleat suddenly pops and the stay goes limp. We know the hazards and the results of lack of concentration, but having actual figures and formulae is just really cool! And, of course, "some of it" is left to conjecture, but that is what is so cool about it.

[Don C L]

I sent an email to my naval architect/engineer buddy.. he got much better grades in elementary school than I did. I have never had rod rigging but I always wondered about the wisdom of forcing a bend in it at the spreaders. I do see the wisdom of locking in the spreaders though with the discontinuous... that would be worth the extra weight and complexity to me.. for cruising anyway. When I was quite young I wanted to get tapered rod rigging for my Columbia 24. Oh I am sure I would have gotten 2 or 3 more knots out of it if I had.

[Bluebeard]

To answer the original question -
From an engineering viewpoint a mast and it's rigging is a framework.
It is fully braced or triangulated and for all intents and purposes each member of the frame is pin ended.
So for any given configuration of the mast and rigging and any applied loading, the load in each part ( or member ) of the frame will be the same no matter what size or material that part is made from.
So if we consider that part of the rigging between the lowest spreader and the deck, the total load in that part of the system will be the same whether it is a single piece as in a discontinuous system or 2, 3 or more in a continuous system.
Then with the simplification that a rod and a rope from the same material will have the same cross sectional area, they will also have the same weight.
So any differences will only come from the simplifications in the above argument, which I will explain if anyone wants me to.
In the real world there will be no noticeable difference in weight, continuous to discontinuous.

As has already been pointed out, there is a difference in windage.
The formula for wind load on a cylinder, ie a rod, rope or mast is simple and the load is proportional to the projected area.
So for a rod taking the same load as three ropes, it will have a projected area of 1.732 of one rope, hence the total wind load on the rod will be 1/1.732 = 57% of that on the ropes of a continuous system.

[Elie]

In my opinion, stretch and windage are less important on a discontinuous setup.But compression on the spreaders against the mast section is more important, and therefore speaders and spreader support in the mast must be significantly stronger. Charleston Spar provided me with a discontinued rigging setup, and :
1: spreders much bigger and rigid than expected;
2- in mast spreader support very strong;
3- Wire size diminishing with each pannel from to top to lower.
4- The lower stay size is much larger because it collect all the tension and bring it to chainplate.
5- All considered, the total weight is reduced significantly, stetch is noticibly reduced, and ajusting rigging tension on each shroud is a real complicated affair...