Rig tuning physics

DArcy

.
Feb 11, 2017
1,691
Islander Freeport 36 Ottawa
ralph, lay you mast on it's side. you pick it up from the area around the spreaders while wifey picks it up at the head of the mast. who is carrying all the weight?

and ralph, most everyone is smarter than i am, but i did do my homework.
jon, you may be right about higher loads on the lowers but certainly not for the reason you used above. When the mast is upright gravity isn't pulling it to the side, the wind is. There isn't as much sail area up high as there is down low which I suspect increases the lateral load on the lowers.

I have a single spreader rig with forward and aft lowers. I set the tension in the uppers first then the forward lowers approximately equal to the uppers while keeping the mast straight laterally (athwartships) then I tighten the aft lowers. On my previous boat I just hand tightened the aft lowers but I find I need more than this on the "new" boat to keep the mast from pumping. The two lowers add up to more tension than the uppers.

@Ted I like the free body diagram. When you talk about compression loads at the mast base it is worth considering the tension in any halyards led to the deck. In the "pedantic" category, it may be worth noting the sail load is lower at the head and higher at the foot of the sail. A lot of the load is taken by the boom which is attached low on the mast. This would certainly increase the lateral load at the mast base. If your intent is to pictorially represent the loads it is a good illustration. If you are interested in calculating loads, it would be worth more accurately representing the sail loads.
 
  • Like
Likes: jon hansen
May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
the center effort on the sails is below the spreaders. the base of course holds it's share. the jib goes up to the top of the mast the force there increases the pressure on the spreaders.
of course the mast in my example is different, but it was to get yawl thinking where the loads really are.
then there is the controls on the Ascow which are changed on the fly around the race course.

i like firm control of my lowers. i then will set my uppers as needed getting the rig tuned up for the season. and that may change as the fall winds show up after a summer of gentle sailing. more bow in the rig for higher winds.

aeolus is a stiff vessel. does well in higher winds. rides better when pushed. and i sail with a crew, not short handed, always have.
 

Tedd

.
Jul 25, 2013
745
TES 246 Versus near Vancouver, BC
Thanks, @DArcy . You're right, it was not my intention to consider all mast loads at this stage, only to create an aid to visualizing forces related to lateral mast tuning.

....the sail load is lower at the head and higher at the foot of the sail.
I had attempted to convey that with the shape of my distributed sail load, but I agree that it's not accurate--again, only meant to convey notions about forces related to lateral mast tuning, not an analysis.
 
  • Like
Likes: DArcy
Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
fair enough, shrouds. they are all stays i think. shrouds are the side stays and then there are fore and aft stays, and so on.
Now that you know the difference between a shroud and a stay, lets proceed from there.

you pick it up from the area around the spreaders while wifey picks it up at the head of the mast. who is carrying all the weight?
A somewhat peculiar response to the matter at hand, but let's take a look at it.

Here's a force analysis diagram (as you have described above) with the mast in equilibrium (it's at rest and not moving).

Mast.jpg


It shows me supporting the total weight of the mast while wifey is not required to exert any force to keep the mast from moving. Now what the hell this has to do with with your idea of unbalanced tensions in a shroud on a vertical mast, completely escapes me.

Your original statement which caught my attention was:

lower stay tension will always be more than upper stay tension.
not assumption, but fact.:cool:
You are using "stay" when you should be using "shroud" so let's look at a force analysis of two opposing "shrouds" on a mast which is what I believe you originally intended.

Shrouds.jpg


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".

and ralph, most everyone is smarter than i am, but i did do my homework.
No you did NOT do your homework. You concocted a hare brained idea of a mast lying on its side to explain your equally hare brained theory of a shroud on a vertical mast with simply AMAZING properties. And to top it all off, you presented it to the members of SBO as "not assumption, but fact". It seems that the weirder the idea, the more you can expect it to be a "fact".

And it exists only in the:

Twilight Zone.jpg
 
May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
Now that you know the difference between a shroud and a stay, lets proceed from there.



A somewhat peculiar response to the matter at hand, but let's take a look at it.

Here's a force analysis diagram (as you have described above) with the mast in equilibrium (it's at rest and not moving).

View attachment 197197

It shows me supporting the total weight of the mast while wifey is not required to exert any force to keep the mast from moving. Now what the hell this has to do with with your idea of unbalanced tensions in a shroud on a vertical mast, completely escapes me.

Your original statement which caught my attention was:



You are using "stay" when you should be using "shroud" so let's look at a force analysis of two opposing "shrouds" on a mast which is what I believe you originally intended.

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".



No you did NOT do your homework. You concocted a hare brained idea of a mast lying on its side to explain your equally hare brained theory of a shroud on a vertical mast with simply AMAZING properties. And to top it all off, you presented it to the members of SBO as "not assumption, but fact". It seems that the weirder the idea, the more you can expect it to be a "fact".

And it exists only in the:

View attachment 197205
I’m pretty sure Jon’s point was about the lower shrouds, not the lower sections of the upper shrouds that you’re referring to in your drawing. It is safe to say that the full upper shroud has equal tension along its length.
 
Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
lower stay tension will always be more than upper stay tension.
not assumption, but fact.:cool:
jon, that makes no sense. Ralph showed you just why.

Maybe we should engage in some GASP! definitions here:
On my masthead rig, we have
--- shrouds (sometimes called upper or cap shrouds because they go all the way up to the masthead)
--- forward lowers (shrouds)
--- aft lowers (shrouds)

Some dinglefooss made up the name stays FOR RIGGING OTHER THAN THE forestay and backstay, don't know where it came from. I see it all too often. Like fingernails on a blackboard. Like toilet, bathroom and kitchen on boats.

My Catalina 34 manual uses those terms, in both the "How to tune your rig" section and in the diagram of all the running and standing rigging. I have owned three Catalinas, all with masthead rigs and all with the same nomenclature, i.e., proper terms. Somehow I think they'd know what to call them.

Oh, Nigel Calder calls them that, too, jon. You've heard of him? :)

So, can we please stop using a nonexistent term instead of the right ones? Thanks.
 
Last edited:
Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
I’m pretty sure Jon’s point was about the lower shrouds, not the lower sections of the upper shrouds that you’re referring to in your drawing. It is safe to say that the full upper shroud has equal tension along its length.
David, use of improper terminology will only serve to confuse newcomers and those asking questions.
Yes, uppers are usually set tighter than lowers because the upper wire is longer and subject to more stress. (Calder), and Catalina so recommends, too.
But that's not what he "said" either.
And you're right, of course: "...the full upper shroud has equal tension along its length." Not "safe to say," but simply correct.
 
Aug 1, 2011
3,972
Catalina 270 255 Wabamun. Welcome to the marina
I'm staying away from the strands and wraps bit of the conversation. The book says X on the Loos, before the boom goes on and that's worked well thus far! (of course, in classic Catalina style, you need both Loos to properly do the deed)
 
Apr 26, 2015
660
S2 26 Mid On Trailer
Somewhere we get carried away with all of this, yet there are thousands of sailors who never look at this forum and just go sailing, no matter the tension or engineering involved in the rig. Are we on this forum just trying to make ourselves feel like we know more than Joe Blow (sorry Joe). I've read for 50 years on how it's supposed to be, and now I take those numbers with a grain of salt and do what I think is best.

New people on this forum need to realize we are all experts (haha) and do their on research. Good ideas here, but think and search. Sailors are not sheep when it comes to sailing. Think!












some
 
Oct 26, 2008
6,045
Catalina 320 Barnegat, NJ
Just to chime in ... it was perfectly clear to me that Jon was talking about lower shrouds vs upper shrouds and I never did get hung up on the distinction between "stays" and "shrouds". That said, I was never under the impression that Jon was describing the tension of the upper shrouds above and below the spreaders. The diagram that Ralph posted didn't make any sense to me because it doesn't show the lowers and so I thought this is a complete disconnect! :what::what:

Putting all that aside, I'd like to know if it is fact that the forces on the lowers are always greater than the forces on the cap shrouds. Describing the tensioning of the rig under the static condition is different than describing the rigging loads in a dynamic condition. If riggers are recommending greater tension on the lower shrouds than the cap shrouds, it would seem intuitive that forces are greater on the lower shrouds. If it were otherwise, it would seem that keeping the mast in column, as desired, would not be achievable. It also seems to me that the mainsail creates a non-uniform load that is distributed most greatly below the spreaders. I'd agree that the lateral load on the mast created by the headsail would basically be a point load at the top of the mast (or where the forestay is attached on a fractional rig) which is resisted by the windward shrouds in some combination. The location of the point load would impact the resisting load distribution between the cap shroud and the lowers I think. This assumes that the base of the mast is fixed and does not impact the loads on the shrouds in any way except that it takes on roughly half (probably more?) of the total lateral load from the head sail.
 
Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
New people on this forum need to realize we are all experts (haha) and do their on research. Good ideas here, but think and search
Truer words never spoken. Always consider the source.

I have seen crap on this site which could do real damage to the uninitiated. I suggest installing a fact checking algorithm, such as that used by GOOGLE, be installed on this site to remove the worst of the chaff. Unless someone challenges the ridiculous ideas which we see published from time to time, SBO will acquire the sordid reputation of being little better than a power boating site.
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
it would seem that keeping the mast in column, as desired,
My understanding of the rig tensions is to keep the mast in column while sailing. The compression of the mast, exerted as the sails load against the tension of the bow/stern/and lateral stays (shrouds) will cause the mast to bow out in the middle. The lower stays (shrouds) and the spreaders are there to prevent that from happening.

The various Loos measurements, as I understand, vary among rigging, mast and boat design. The numbers in my experience are used as an initial point based on the properties of stay (shroud) used. You want to tension the stay to a certain level. At the same time you are monitoring the impact the tension is having on the mast and the boat. Note that you are not tensioning in a static condition. As you apply tension to the stay(shroud) you are increasing the tension over the hull and driving the mast down against the keel (for a keel stepped) or the deck (for a deck stepped) mast.

You may be seeking to reach a certain tension of the wire, but ignoring what that is doing to the mast and the boat would be unwise.

You can have a perfectly tensioned stay(shroud) and have a boat hull misshaped or a mast warped and crushing the keel.
 
  • Like
Likes: Scott T-Bird
Feb 26, 2004
22,760
Catalina 34 224 Maple Bay, BC, Canada
If riggers are recommending greater tension on the lower shrouds than the cap shrouds, it would seem intuitive that forces are greater on the lower shrouds.
But they are not.

If you have Calder's "Bible" you might want to read the section on standing rigging.
 
Oct 26, 2008
6,045
Catalina 320 Barnegat, NJ
My understanding of the rig tensions is to keep the mast in column while sailing. The compression of the mast, exerted as the sails load against the tension of the bow/stern/and lateral stays (shrouds) will cause the mast to bow out in the middle. The lower stays (shrouds) and the spreaders are there to prevent that from happening.
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)
 
Apr 26, 2015
660
S2 26 Mid On Trailer
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 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.
 
  • Like
Likes: shemandr