Sail Loading on Rig, Rig Loading on Vessel

[brian eiland]

Sail Loading on the Rig, Rig Loading on the Vessel

Several weeks ago as I was searching thru the internet for analysis of 'rigging loads', I ran across a couple of subject threads on this BoatDesign.net forum, that reinforced my observation as to what an inexact science this appears to be. One was entitled "mast loads" and another entitled "loads for swept spreader rig". I begin this posting with some excerpts from those threads.
shu:
"they (Larsson and Eliasson) don't provide a means of determining the shroud loads due to headstay tension"

terhohalme:
"Exact headstay tension is impossible to determine."
"Multiplier 15 includes all guestimated loads...."

shu:
'The tricky part in Larsson and Eliasson is that you calculate "real" athwartships static loads on the rig ..., then apply separate safety factors for the other components based on some multiplier. What was the assumed factor of safety for the forestay?' (Ed:not the exact quote)

terhohalme:
"Impossible to know exactly..." (Ed: about the Nordic Boat Std)
" Where do you really need exact numbers? Tightening sagging of foresail ...will mix the whole calculating process"

shu:
"However, this all assumes that you have a permanent backstay.... I see no accounting for the additional loads the shrouds must take to oppose the forward component of the load in the forestay."

"...changing the sag of the forestay can make large differences in the resulting shroud tension. I don't know if I can predict the sag as a function of the foresail loads."

tspeer:
"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...."

shu:
"assuming half the propulsive force acts at the hounds"

SailDesign:
"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."

"you just need to apply the sailing loads to each length of rigging, calculate the stretch, and plot it…" (Ed: how does one determine these loads with any exactness)

gonzo:
" I have observed that many of the rigging formulas don't take bending into consideration"

shu:
" 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 3 shrouds all came out at very different ultimate diameters for safety, when their working loads were much closer"
____________________________________________________


What really surprises me about these few quotes, and many other discussions on engineering a sailing rig, is the total dependency on the use of 'guesstimates' and a variety of 'multipliers', some of almost unknown origin and application.

I recently purchased a copy of Larsson & Eliasson's Principles of Yacht Design, specifically to investigate their analysis of these rigging loads. But what I found at the very opening paragraph of their chapter on Rig Construction, "in dealing with the dimensioning and construction of the rig, over the years different methods have evolved, ranging from old rules of thumb…to sophisticated computer models for exotic composite materials. We will take a middle line (approach) using accepted practices (old rules of thumb?).….." Page two (text 202) of their chapter, "It is common practice that the transverse and longitudinal stability are studied separately"
And this is supposed to be a modern analysis? Later in the chapter (text222), "another factor which improves performance is the rake of the mast. Although not numerically proven…"

In this modern computer age why have they chosen to ignore the "sophisticated computer models"? Are sailboat rigs such a complicated structural problem to analyze?? Even the more simplistic steady-state ones (minus some of the more complicated dynamic questions)?

I guess my frustrations with understanding and defining the actual true loads on the rigging of a sailboat is best summed up at this Classic Marine website,

http://www.classicmarine.co.uk/Articles/rigging_loads.htm
"Rigging Loads- a study in guess work, or a tale of scientific progress?"

I will quote a few of the more notable passages from his very interesting summation:
a) He opens with a quotation from Douglas Phillips-Birt, "Masts are tricky things. It is not for nothing that Lloyd's, which is ready to specify the scantlings of nearly every other part of a yacht, washes its hands of them altogether and plants the responsibility for their size and shape squarely on the designer's shoulders.... suggesting that mast are perhaps a little beyond rational analysis."

b) For all its crudeness, this rule (a particular one) at least recognizes that the strength of the rigging relates more to the size of boat rather than the size of the rig.

c) You can see that these factors will bear on the issue, but the more you look at it, the less you can understand why they are combining in the way they are.

d) With a method so opaque in its assumptions, you never know what the range of validity is in terms of rig type, or arrangement of stays.

e) From Skene's book, the 'long method' is based on SAIL LOADING. Good Heavens! That is the first time it has been mentioned, which considering that it is the sails that load the rig, must be an improvement. Don't get too excited though. How much is the mast loaded and where? The answer is that nobody really (seems to) knows.

f) It provides not a real life start point for some rigorous analysis, but a common assumption that can be used to compare craft with each other, and/or with empirical data. To try to rationalize an assumption like this is at best pretentious—an attempt to ennoble guess-work, at worst dangerous—someone might believe it.

g) You may be getting the impression that this is not much advanced on earlier efforts.

h) Interestingly that the NBS method does not specifically relate mast loads to shroud tensions, but starts again with the righting moment.

i) Secondly, however numerate the rules appear, in practice they are all founded on empirical data.

j) So we are a long way from a complete picture of the loads in a rig, particularly tradition rigs. Why? For a start, the more sophisticated approaches have developed during the age of the BERMUDAN rig. The usual assumption that shrouds can be analyzed separately from fore/backstays probably holds better for Bermudan, than for gaffers, where there will be a complex interplay between peak halyard, runners, mainsheet, bowsprit, shrouds and so on.

k) And finally and MOST SIGNIFICANTLY, none of the methods derive loads from the force of the sails, which is after all what is loading the rig!!! Such an analysis could be fiendishly complex, but with ever more powerful tools and computers, I think it is not an unrealistic thing to attempt.

Brian notes, maybe I am being a little naive here, but I find it hard to believe in this computer era that we can't set up a three dimensional 'map' of a sailing rig and be able to analyze the forces in the individual components, and how they interact, and how changing one component's size, strength, geometry, etc, affects the other components, at least in a steady-state environment

I would imagine that we must first redefine the actual load paths that the forces of the sails use to transmit their power to the rigging. And then how and where do the rigging loads get transmitted to the vessel itself? I propose to start a new tread on this subject,"Sail Loading on the Rig, Rig Loading on the Vessel" . And I think it most appropriate to put it under the "Sailboat" subject heading as there may be a number of forum attendees that are only power boat oriented.

We have previously bunched all the sail loads together and assumed they acted thru the sail's CE. Granted this might yet prove to be a reasonable assumption, but I'm not convinced we have included all components of this summation of force (are there some vertical components we have ignored, etc?). Certainly this summation force is not necessarily acting at a perpendicular direction to the sail surface at this CE point, and it's not necessarily at a horizontal direction parallel to the water's surface. And remember the sail cloth itself can not exert a forward force on either the mast nor the forestay, at least not in an upwind situation. So how are these 'sail forces' getting physically transmitted to the vessel?

I'm sure there will arise considerable discussions about the magnitude of these sail loads, but at least this could be dealt with as a variable aside from the question of load path. If we have the load paths defined, then we can play around with a variety of different load magnitudes and look at those new consequences. And then consider how the load paths can deform in direction under different loads.

I don't pretend to be any kind of an expert in these engineering/computer structural analyses. I would just like to get a clearer picture of how the sails actually transmit their forces to the vessel; at what points, and in what path(s)??

Noted rigger Brion Toss relates to the "Flow of Forces. This is perhaps the most critical component of thorough rig appreciation. The pull of the jib on its stay, for instance, stresses the stay and its attachment points. But it also stresses the backstay, and thus the stern. Depending on hull structure and load level, the stress on the stern can also affect the alignment of the prop shaft. But wait, there's more. Some of the jibstay load is lateral, so that the upper shrouds are also stressed, siphoning so much of the load away that one can almost always make the backstay smaller than the jibstay, which reduces windage and weight aloft, as well as reducing rig cost. The upper shrouds, in turn, compress the spreaders, and all the wires at the masthead compress the mast. And so it goes, with the force from that one sail flowing around corners, in tension and compression, and intermingling and interacting with the forces from other sails as it makes its way to the water. If you can see this flow, really see the rig as a system, you will automatically be in good shape….to avoid missing significant relationships as well as significant details"

I intend to send a copy of this posting to any and all parties that I think might be interested in this subject matter in hopes that they will join in on this forum discussion. I would urge all viewers to likewise send it to others they feel may have an interest. We just might attract a whole lot of new qualified participants to this BoatDesign forum as a consequence, and learn something in the process.

 

[Guest]

The analysis of 'rigging loads' is not hard, but it is far from trivial.

In general, the problems are statically indeterminate so any solution requires the inclusion and determination of the deformations of the various members.

In general, there are way to many "worse case" loadings to consider.

There are a lot of poor ways to approach the problems.

There is at least one good way, but it has not appeared in the literature yet so I can not point you to it. (I don't even know if it works.)

 

[SailDesign]

"Guest" says:
"There is at least one good way, but it has not appeared in the literature yet so I can not point you to it. (I don't even know if it works.)"

That's like dangling a carrot in front of a donley that's chained up. Cruel! ;-)

 

[Guest]

As an engineer I would like to let you know that computer modeling is full of guesstimates and if you have any intentions of creating a model of a rig, clear about 6 months on your calander. What about the effect of the boat deck shape on the airflow? Shouldn't you model the whole boat. Now you need to determine the shape of the sails and model them but as we all know the shape of a sail is infinitely variable. Most people don't have any idea how much time is required to create the models or how much the software cost to analysis fluid flow. There is also a million variable that have to be guessed at. Just the surface of the sail is going to have several factors to consider. Did you model the seams in the cloth? Was it a radial cut sail? What is the surface roughness of the fabric? Even if you could do all of the modeling and flow analysis needed you still haven't considered the acceleration loads created by the boat crashing through waves or loads create by gusts of wind.
This is a much more complicated problem than it may appear on the surface.

 

[Bradonvashon]

This should be called the unvarnished truth about rigging design. I have the same (though not nearly as extensive and frustrating) experience. It seems to me that some of this ought to be more understandable, easier to grasp. Depending on who you listen to it is--then you hear someone else who seems to be saying something completely different.

Now, I approach this issue somewhat frettfully--I'm a lay person, and I'm thinking of the questions posed here from the standpoint of a lay person. For many reasons, I've tried to do much of the rigging and related work on my boat. Despite a lot of resources, it is often like trying to sort out the genome with a hand calcualtor and steno pad.

For the experts, I think there is more agreement among them than appears as relates to the hows and whats of this topic. Where they begin to diverge is in the multitude of uses, purposes, intents and other personal variables to which people put boats and rigging. I think it is certainly possible to calculate loads to the Nth, look at areonautics, but who will pay for it and can that desired result be achieved in some less exact, but equally servicable alternative, hence the "factors" and flow forces and other elements that tend to confuse the "yous" and "Is" of the world.

How many differnt boat designs are there in the world today--I don't mean sloop vs cat boat, I mean a hull of specifc demensions with a mast and rig of specific demensions. Now move that mast aft 6 inches, or use another spar builders section. are the loads the same i the same places? how long will it take you to figure that out? is it worth that? So maybe "good enough" is achieved with a kind of hocus pokus. The point is that good enough often is good enough and doesn't need to be that exact be cause what we use, even if it has no "standard usage", is good enough.

 

[rjmac]

I have been reading this thread and I felt that I would like to add my two cents if I could.

If you really want to understand the dynamics of what is going on with standing and running rigging you will need to do the following.

Take a class in dynamics (which help you understand standing running rigging loads) and thermodynamics (at Purdue this is three classes) then you will have the base to understand wind loading, and finally fluid power (viscosity, pressure systems). A class in aerodynamics couldn't hurt. (Believe me this is an art)

This is the basic understanding you will need to understand sail design. Next you will need to look at the various configurations (historical and contemporary), analyze them to get a good feel for what good rigg setup is.

I guess I have been doing this since 1976, when I learned to sail on a 720.

If you look at a simple rigg setup it will start to look like a bridge design, wind loading and etc., and doing an analysis like that you will always have members moving and deflecting.

Keep in mind, all things bend, compress, shear, twist..... The basics of life.

Also I imagine that companies that build sailboats know this stuff but if becomes proprietary, the competition thing.....

I am not a professional, I am an amateur, and I cannot expect the guys that do this for a living to do it for free….. If you have a spar $20,000 I would be happy to help you out, give you all the detailed calculations, spreadsheets, etc…..

That’s my two cents...

Oh,.... Checkout the thread on the mothboat......

 

[SailDesign]

rjmac,
Yup, you can calculate anything with enough time and energy, but in the final analysis it will only tell you one thing, what the loads are for the specific case you have looked at. beyond that are an infinitesmal number of cases that you haven't checked, and can't check. The "art" of rig design is in knowing when to apply what kind of safety factor, knowing what you want to fail first (usually, wires, spar, chainplate, hull in that order for a rig design) and knowing (or guessing for production builders) wha tthe customer is going to want to do with his boat and what his/her own limitations are likely to be. Then you have to assume they'll do something stupid eventually.
Bertie Reed, when aske dwhich of 5 or 6 sections he wanted on Grinaker (Open60 from 1989-ish) said "Don't throw numbers at me, I want to look out my window in the Southern Ocean and see @#$%ing tree trunk standing there..."
Good thoughts for the first ever Vendee.
Steve

 

[shu]

Steve,
I love that quote. Made my day. Thanks.

 

[rjmac]

SailDesign (Steve),

You make a very good point and I do not believe I have the background to dispute your assertions…. I guess my point is that in the design of a bridge structure or electronics, nothing is static. What you would call safety factor, is what I would call the ultimate limits, (exceeding the elastic and going plastic.)(That has such a poetic ring...) Rig design is not a static system but dynamic, for example.... The forestay, what 3 dimensional forces act on the wire or rod, depending on your application? The force vectors on the ends contribute to or are summed up to the force vectors in the middle wire. Would one design or specify the component requirements based on a force 1 wind or force 73+ wind…., I guess I would go for the later…. , isolating components within the system makes design manageable. As an amateur I have time to dwell on such things, I don’t have customers, please understand, I do have customers at my regular job so I can sympathize with your point of view.

I guess I view things in very simple ways....... I'm a simple kinda guy...

The past 2 years or so, from I have read, the laws of MR. NEWTON, are being applied to the sails more specifically aerodynamics, not just bournolli's(?sp)... laws, the engineering community is not taking it well from what I can tell.

I guess over the past 6000 years (?)(Depending on your viewpoint) of engineering boats we have not been able to retain and process information as well as we have been able to do here in the past 25 years.

Computers are the best things since sliced bread....., Validation of the computer says is wisdom….

I am not familiar with the quote (Bertie first South African - and one of only a few yachtsmen in the world - to complete 3 single-handed circumnavigations.): ‘Bertie Reed, when aske dwhich of 5 or 6 sections he wanted on Grinaker (Open60 from 1989-ish) said "Don't throw numbers at me, I want to look out my window in the Southern Ocean and see @#$%ing tree trunk standing there..."’

You have me at a disadvantage, although I am very curious as to what he means……

Rjmac(Bob)

 

[SailDesign]

Bob,
My problem with rig design as an exact science is that it cannot be exact. The loads experienced by the rig are many and varied and all sorts of things can happen to damge the rig. What are the loads on an Open60 rig when the boat, travelling at 25 knots, hits a loaded container floating just sub-surface? The answer, of course, should always be "Enough to bring the rig down" as it is the best thing to sacrifice in that situation.
When you look at things from the point of view of catastrophic failure, then you are back to sagefty factors again, or "fudge" factors (lets' be honest here...)
As far as the static forces on a rig are concerned, you HAVE to start with the objerct that applies those forces - the hull. THe wind merely acts on the system to displace it. The hull, because it has righting moment, is the part that determines the size of the loads. There are loads from the sails acting on the mast, yes, and thy should not be ignored in a proper analysis, but when all is said and done, they are simply the weight of the railway track in your bridge analogy. Pretty close to being not worth worrying about.
Bottom line, you can spend fortunes onn rig design and build, but in the end it all boils down to the experience of the designer and spar-builder as to how big a fudge factor to apply for your case.
Damn - I feel cynical this morning...

I would be amazed if you had heard the Bertie Reed quote before, as he said it on the phone to me in 1989 or so. He was not interested in the numbers for strength, he just wanted to look out and see something that looked like it wouldn't fall down. So we gave him one, and it didn't fall down. ;-)

Steve

 

[tspeer]

As Keith Burgess said to me, "You analyze for the loads you know, and design for the loads you don't"

 

[Stephen Ditmore]

I must be missing something, but it doesn't seem quite as complex to me as is being suggested. Where transverse loads are concerned is all this sail force analysis is required when the limiting factor is the boat's maximum righting moment? Perhaps a rethink is required concerning longitudinal forces, but the top sparmakers have surely already done this, and if not couldn't real world data be collected using load cells or tension indicators? Where sail loading is concerned don't the top sailmakers have this pretty well mapped out? Perhaps what's really needed is a sophisticated approach to halyard sizing.... then everything else could be sized such that the halyard breaks first.

Has anyone checked with the Wolfson Unit or gone through back Chesapeake Symposium papers? People like Dirk Kramers (formerly of Hall Spars) have a good grasp of these issues, no? Do they need to be publishing more? Should Navtec publish more on this subject?

 

[SailDesign]

Stephen D - Dirk Krmaers is probably the best guy on the East Coast, at least, for this kind of thing, and for analysis of a rig, hje needs the geometry, and the max RM. Period.
Yes, you could do all the math, and come up with forces that are pretty close to exact for one condition, static, with all the sail loads added in, etc. You still do not have info on all the other conditions it will see, let alone the dynamic problems. Hence you calculate for a "standard" condition, max RM, and then apply safety factors gleaned through years of experience (which is what it all boils down to).
When you are trying to sell a mast, you want to make a profit. Spending weeks analysing the structure of said mast will only eat into the profit, and the difference between the "Actual" loads and the calculated loads will be very small.
Steve B

 

[SailDesign]

Forgot to add - Yes, if you are working for an AC syndicate that wants to shave the weight of the spar to a bare minimum, you will be allowed to over-analyze to a fare-thee-well, but the conditions when the boat will be used are very well defined, and no crashing through enormous waves is going ot happen. If it does, no-one is going ot be surprised when things break.
Steve

 

[Guest]

saildesign ---

It was not my intent to taunt you with an unpublished solution.

There are half a dozen sets of hull strength scantlings floating about. Somehow those engineers come up with a very concise set of design rules.

It should not be surprising that an engineer can/has come up with rigging strength scantlings.

I would wager every designer here has a set of scantlings he uses at least for a starting point. From the discussion here I see several different views of how the design loads might be estimated, but I don't see any method here that provides tight estimates. An I think the original question was looking for a method that provides tight estimates.