Loads for swept spreader rig

I am designing a fractional swept spreader rig for a 38' light displacement sloop, and I want to support the headstay tension with the shrouds. I have looked in Larsson & Eliasson, and although they discuss swept spreader rigs and some general rules, they don't provide a means of determining the shroud loads due to headstay tension.

Does anybody know of good resources for determining the shroud loads (with appropriate safety factors) in swept spreader rigs?

[SailDesign]

Guest,
What you are describing is a pretty basic engineering calculation. If you lack the resources to figure this one, may I respectfully suggest taking the problem to your local neighbourhood designer? Getting it wrong could be expensive.
Steve

[gonzo]

You can use trig or figure it out geometrically with vectors.

[terhohalme]

Hello guest, it is all there in Larsson & Eliasson. Exact headstay tension is impossible to determine. They use RM at 30 degree to dimensioning the breaking load of forestay. Multiplayer 15 includes all guestimated loads like pretension in shrouds, main sheet tension and factor of safety. Just follow the method and it will work fine. The method is a part of Nordic Boat Standard and widely used in nordic production boats with swept spreaders (most common rig type in nordic countries). All with success.

[shu]

I must not have logged on properly when I started this thread. Thanks for the replies.

The tricky part in Larsson and Eliasson is that you calculate "real" athwartships static loads on the rig based on the righting moment at 30 degrees, then apply separate safety factors for each of the shrouds. However, Forestay breaking strength is based on a multiplier of 15 of the transverse righting moment.

What was the assumed factor of safety for the forestay? Having this, one could determine a reasonable maximum static load, do the trig and determine the corresponding upper shroud load to add to the transverse load in the shroud. Finally the F. S. for the shroud can be factored in.

Knowing the preference is to have the mast fall over the side than straight aft, A larger factor of safety for the forestay is wise. How much larger than 3 (safety factor for shroud)? I assumed the F. S. for an inner forestay was 3, Since Larsson and Eliasson give a multiplier of 12RM@30 for this, a back calculation gives a 3.75 F.S. for an outer forestay (which is all I have). The end result is that the upper shroud is the same size as the forestay, Which seems reasonable for shrouds swept 28 degrees, and a fairly wide staying base. What would you guys have done differently?

[shu]

Has anyone a response to my reasoning on this?

[terhohalme]

As I said the method is a part of Nordic Boat Standard 1990. Impossible to know exactly because wasn't in the committee. But it is a scantling standard made to follow.

Where do you really need exact numbers? Tightening sagging of foresail from 4 to 2 % will mix the whole calculating prosess. Replacing the wire to the nex bigger changes the factor of safety from about 3 to 4.

[shu]

Is the Nordic Boat Standard available in english? I would like to see the whole treatment of the rig there. The part that I see missing in the portion of the NBS that is in Larsson and Eliasson has to do with the component of the shroud load due to forestay tension.

As I see it, the method shown in L & E takes the righting moment at 30 degrees, applies that to either a deeply reefed mainsail (higher loading on lower and intemediate shrouds) or to a foresail only (higher loading on upper shrouds). With the foresail only, the method essentially has you heeling the boat 30 degrees by pulling athwartships at the forestay attachement point.

When I do the trig for this I see that for sweep angles up to 30 degrees, it really doesn't matter if the shrouds are swept or not, so long as the athwartships dimensions of the staying base and spreader lengths remain constant. However, this all assumes that you have a permanent backstay which can oppose the loads on the forestay. Although L & E gives certain minimum longitudinal angles for the shrouds if you do not have a backstay, I see no accounting for the additional loads the shrouds must take to oppose the forward component of the load in the forestay.

Am I nuts, or is there something missing from the NBS method, as shown in L&E? As you pointed out, Treholme (sorry if I messed that up, but I haven't figured out how to look back at the thread while I'm typing this), changing the sag of the forestay can make large differences in the resulting shroud tension. Without doing a detailed analysis of the stiffness of the entire hull-rig system, I don't know if I can predict the sag as a function of the foresail loads. Even if I did such an analysis, I would be very suspicious of my ability to model the real stiffness of the hull with any sort of accuracy.

[shu]

Ah, there it is -- Terhohalme. Sorry about the previous butchering of your username.
-shu

[tspeer]

Originally posted by shu
...What was the assumed factor of safety for the forestay? Having this, one could determine a reasonable maximum static load, do the trig and determine the corresponding upper shroud load to add to the transverse load in the shroud. ...

One thing missing from this is the mainsheet tension. The sheet tension goes up the leech of the main, pulls aft on the mast, and is reacted by the forestay. So it's not as simple as figuring the component of the righting moment that is borne by the forestay.

I suspect the factor of 15 applied to the forestay tension was intended to account for mainsheet tension as well, with righting moment used as an indicator of wind strength and thus sheet tension.

Vang tension is reacted by the mast, so it doesn't have the same effect on forestay tension.

[shu]

That's a good point, Tom. I think from a safety point of view I don't want to assume mainsheet tension, since I am trying to size the upper shroud. But it supports a higher safety factor like 3.75 for the headstay (see my assumptions in a post a few days back on this thread).

I am also trying to look at this from a downwind point of view. Wild deep reaching at a speed of about 20 kts. Using the Delft series resistance equations published in Larsson & Eliasson and extrapolating exponentially (excell has a nice feature that does this) to a specific speed of 2.5. then assuming half the propulsive force acts at the hounds. This verifies the shroud diameter previously arrived at, so I am starting to feel pretty good about it.

Correction:
Previously in this thread I said the shroud diameter came out the same as the forestay. Actually it came out one size larger (8mm vs 7mm) but I increased the forestay to 8mm since I didn't like it being smaller than the shrouds.

[SailDesign]

OK - it's time to throw the proverbial monkeywrench (spanner to you Brits) in the works.
Using RM, etc. to calculate loads is great, but what will the mast look like when it is loaded. All rigging stretches, and that stretch over the length of that piece will determine the shape of the mast when fully loaded. I have seen sticks that were designed to death from a safety factor point of view, but were pretzels when you sight up them under load.
So, especially with a "no-backstay" situation, it is important to estimate the finished shape, so that you don't cause it to invert. It is not difficult, you just need to apply the sailing loads to each length of rigging, calculate the stretch, and plot it with an exaggerated scale to see if you have a double-bend or not. Ideal shape would be a "fishing-rod" bend. There is a long word to describe that, but the fishing-rod is more universally-known ;-)
Steve

[gonzo]

I have observed that many of the rigging formulas don't take bending into consideration. A mast out of column will fail at a lower load. One of the causes is hull deflection. It has to be designed to take the loads without considerable form distortion, or design the rig with these changes in mind. The main problem is that the shroud and stays angles decrease as the hull deforms. It adds to the compression force.

[shu]

Thanks for the tip, steve. I had started looking at getting uniform unit stretch in all the shrouds to keep the mast more or less straight, but kinda gave up when the shrouds (D3, D2, and D1) all came out at very different ultimate diameters for safety, when their working loads were much closer. I'll give your recommendation a try.

Gonzo, we're back to the hull flexure problem again. I'm not sure I can model the stiffness of the boat (plywood over bulkheads and stringers) with accuracy. If you can ignore the interior joinery, it would be simpler. Modeling the joint stiffness between members would be tricky too.

[gonzo]

You can model the hull without all the interior joinery. It may add to stiffness, which will increase the margin of safety.