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/Artic...ging_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"