Rig tuning physics

[Davidasailor26]

I know for a "fact" that my forward and aft lowers have less force than the cap shrouds. All I have to do is look at the chainplates. Nice beefy ones going to a serious piece of the hull for the cap shrouds. The lowers have a couple of bolts through the deck to hold the chainplates (if you could call them that) on plus the wire and fittings are smaller.

I agree with that too. But the idea that the cap shrouds have greater tension is counter to the free body diagram showing that the cap shroud only needs to hold the top of the mast, whereas the lower needs to hold the relatively more loaded (by sail area) middle of the mast plus the compression of the spreader holding the more loaded cap shroud. I can’t resolve that discrepancy in my head.

 

[Ed Caldwell]

View attachment 197204

This drawing is in equilibrium as is your mast.

Looking at point "A", the the tension in the upper shroud must equal the tension in the lower shroud because the spreader does not offer any vertical force TO THE CABLE. The shroud merely slides in a groove at the tip of the spreader. For those of you really on the ball, in construction there is a very slight upward tilt to the spreaders which bisects the angle formed at point "A".

Sorry Ralph, "boats are different" alert! My Selden B&R rig has DIScontinuous shrouds, which terminate at the spreader tips of each of my two spreaders. No sliding, no grooves. Your points in the quote are for continuous shrouds. Just adding to the cacophony in this thread to further confuse the casual reader

 

[jssailem]

LeeandRick, post:
"I know for a "fact" that my forward and aft lowers have less force than the cap shrouds."

• There is no disagreement. Different boat designs have different shrouds with different tensions. What I was trying to identify is that the tensions you set with your gauge at the beginning of the process, while at the dock, are considered static. They are intended to place the boat/shrouds/mast/deck under tension. You will need to monitor all four of these factors as you attempt to reach the tension you read in the manual. The variance in boat design is evident as you change size. As an example, my boat with it's masthead rig has all 6 chainplates (3 each side) exactly the same. They attach to the hull in the same manner (a strong bulkhead which is tabbed to the hull). So my tensions will be different than say a B&R rigged boat.
• Smaller sized shrouds are intended to handle smaller forces. Shorter distances up the mast will require less strength to be applied to keep that section of the mast in column. The mast itself will provide some of the strength for the shorter distance to stay in column. The shorter shroud length will likely have similar tension as the tension applied in this case is to remove some of the wire stretch.

Ok, so it would seem that forces on the lower shrouds, either a single shroud or a pair, would be greater than forces on the cap shrouds? Equal? or less than? (in the dynamic condition with sails under wind load)

• I opt for Yes. Maybe. Less than.
• Once you set the rig in place, you need to sail the rig and make adjustments as needed to keep the mast in column. If you are sailing with a maxed out sail plan in very strong conditions, with the boat battering through the waves, the amount of rigging forces will be much greater than on a quiet condition with the wind coming off land and smooth water.
• If you were racing you would adjust the rig to take full advantage of the conditions you panned to sail. As we are not all racing and are a bit lazy, we compromise the rig tensions and seek a balance of tension and release that can accommodate the conditions we frequently enjoy.

 

[Davidasailor26]

Yes, uppers are usually set tighter than lowers because the upper wire is longer and subject to more stress. (Calder)

What I see in Calder is “Do not tension intermediate shrouds quite as much as cap shrouds; tension lower shrouds about the same as cap shrouds.”

 

[Ralph Johnstone]

Google fact checks?

Google, Facebook, they're both the same thing. Same thing as shrouds and stays. Don't you know nothing ?

Anyways, maybe it was Facebook, but an acquaintance of mine is often censored for posting anti-vaxing propaganda:

That is definitely what SBO needs .

 

[Ralph Johnstone]

Just adding to the cacophony in this thread to further confuse the casual reader

And a job very well done at that .

 

[Stu Jackson]

I'm sure many of us have contributed to earlier detailed discussions about complicated boat systems.

One of the issues Ralph raises is quite valid, and whether you call it disinformation or basic stupidity may well be your call.

However, what I've found is that two things work much better than trying to explain complicated details in a forum: links to authoritative sources (like Maine Sail) or GASP! books (which is why they wrote them in the first place - it takes time and space to explain complicated things).

If some of the folks asking basic questions about rig dynamics would spend half that much time cracking a book, it might reduce the amount of expressed abject ignorance a long way.

 

[Scott T-Bird]

I know for a "fact" that my forward and aft lowers have less force than the cap shrouds. All I have to do is look at the chainplates. Nice beefy ones going to a serious piece of the hull for the cap shrouds. The lowers have a couple of bolts through the deck to hold the chainplates (if you could call them that) on plus the wire and fittings are smaller.

Could the combined load on 2 lower shrouds be equal to or greater than the single cap shroud?

 

[Davidasailor26]

However, what I've found is that two things work much better than trying to explain complicated details in a forum: links to authoritative sources (like Maine Sail) or GASP! books (which is why they wrote them in the first place - it takes time and space to explain complicated things).

And to be fair to the OP, the original request was for “a link explaining the physics behind rig tuning” to “educate myself on the why and how”.

We all just couldn’t help jumping in with less authoritative info .

 

[Scott T-Bird]

I agree with that too. But the idea that the cap shrouds have greater tension is counter to the free body diagram showing that the cap shroud only needs to hold the top of the mast, whereas the lower needs to hold the relatively more loaded (by sail area) middle of the mast plus the compression of the spreader holding the more loaded cap shroud. I can’t resolve that discrepancy in my head.

We seem to be conflating a few different concepts and I was merely mining into Jon's statement, for which I have not yet seen a definitive answer. His statement was "lower stay tension will always be more than upper stay tension". Since tension is expressed in psi (English units) and stress is also expressed in psi, I am equating tension with stress in the wire under load while sailing (dynamic). I am interpreting his statement to mean that while sailing, the lower shrouds are always in more tension than the cap shrouds. If the wires are the same diameter, that would mean that the load is greatest where the tension is greatest. According to Jon, that would be at the lower shrouds. If you have fore and aft lowers, I suppose that could also mean that the combined load could be greater at the lowers, but the individual lower shrouds could be under less stress because the load is divided by 2.

While under sail, there is undeniably (I think) a load distribution on the mast that can't possibly be distributed evenly, given the dynamics of wind and sail area. Intuitively, it seems likely that distributed loads are greater closer to the base of the boom, but that point may be confusing. I don't think wire stress has anything to do with the length of wires. Stress is the same at all locations on the wire no matter the length. Is that disputed? What does change over length is sag and stretch, right? The cap shroud will have more sag under the same stress as the lower shrouds. Also, under dynamic load the cap shroud should have more stretch (strain) than the lower shrouds.

If the purpose is always to keep the mast in column, wouldn't it make sense that the static setting should be more tension on the cap shrouds (reduce sag - possibly reduce strain as stress increases?) to prevent the opposite of what is desired as the wind load is applied to the mast? The mast can be put into column under any number of tension applications in the static setting. @jssailem already explained that. The purpose is to apply tensions in all the stays so that the mast reacts appropriately under load, which I think is bound to be distributed unevenly.

 

[Scott T-Bird]

And to be fair to the OP, the original request was for “a link explaining the physics behind rig tuning” to “educate myself on the why and how”.

We all just couldn’t help jumping in with less authoritative info .

Well he did get some good sources as well as pontification!

 

[jon hansen]

Sailing green bay, 10 to 14 out of the ssw.
Sunny 78, water 72.

More sail

 

[LeeandRick]

To the OP here is some light reading on your original question.

https://www.boatdesign.net/attachments/calculating-mast-and-rigging-pdf.105782/

 

[DArcy]

This is better than an anchor thread, I need more popcorn
@Scott T-Bird tension and stress are NOT the same. Tension is measured in pounds-force, stress is in psi. Stress is equal to tension divided by cross section area of the shroud.

 

[Scott T-Bird]

This is better than an anchor thread, I need more popcorn
@Scott T-Bird tension and stress are NOT the same. Tension is measured in pounds-force, stress is in psi. Stress is equal to tension divided by cross section area of the shroud.

Yes, I have that wrong. I believe I am thinking in terms of tensile strength, measured in English units, psi, which is basically stress. My basic point is that we are essentially discussing the static adjustment of tension in the shroud rigging to best maintain the mast in-column under the variable dynamic loads dictated by sail area, wind speed (& shear) and boat movement thru various states of water condition. In my mind, it is easy to lose track of the conversation (but maybe that is just me!) @LeeandRick posted an interesting reference but I have to admit that I was only able to absorb the 1st page. Beyond that, I was lost. But the part that I was most interested in learning seemed to be stated in the very first paragraph. "The outermost stays (V1, V2, D3 in Figure 1) usually carry the highest loads, and as a group are termed the cap shrouds. They work in conjunction with the spreaders to provide lateral support to the mast and transmit most of the forces from the sails to the hull. The diagonals often carry smaller loads as they support the middle regions of the mast, helping to carry the lateral load." (The figure portrays lower stays as diagonals). This was not intuitive to me and I was happy to see this defined. Good source, I think, @LeeandRick ! On my own boat, the diagonals have a smaller wire diameter than the cap shrouds. But then, I do have forward and aft lowers - a pair of lower shrouds (thus a larger combined cross-sectional area?) opposed to the single wire of the cap shroud.

I'll readily admit that the conversation can easily transmit misleading information. But that is what conversation is for. If everybody were an expert and the conversation has no controversy, how boring would that be in a forum? Who would ever learn anything in an informal setting if knowledgeable minds adopted a condescending attitude toward anybody whom is striving to contribute to the conversation? There would be no purpose to this forum, don't you think? I'm sure there are academic forums where know-it-alls can hang together!

 

[LeeandRick]

@Scott T-Bird Did you notice one of the first things, in that article, they computed was righting moment of the boat. I think a lot of people will set tension when the boat is static and forget it. I was this way for years. Now I always go out in wind that will give me a 12°-15° heel and sight up the mast and make adjustments. Even a tight boom vang can have some pretty unique effects on a mast.