Materials & Factor of Safety

[brian eiland]

courtesy of SailingScuttlebutt....

INDUSTRY RESPONSE by Eric Hall
I feel compelled to respond to Dr. Jacobs' letter regarding materials and factors of safety. The marine industry has borrowed many of the same composite materials and processes used in the aerospace industry.Today aerospace-quality prepreg carbon fiber and autoclave curing are readily available in sailboat products. Well-designed and processed marine industry laminates are excellent in tension, compression, shear and fatigue and regularly meet or exceed aerospace standards. Regarding fatigue, compare Dr. Jacobs' paper clip to an equal size carbon rod. As he says, the paper clip breaks quickly in fatigue; bending the carbon rod, you will fail in fatigue before it does.

Granted, our industry is not as "precise and scientifically based" as the aerospace industry and nor does it claim to be. But we regularly use such powerful tools as Finite Element Analysis and do a surprising amount of testing. Incidentally, the same aerospace industry takes a simpler approach to safety factors than we do: They place limits on G-loads, gust velocities and forward speed, among others. If a dynamic load in a thunderstorm rips the wings off your plane, post crash reports will simply say you exceeded one of the plane's G-load, gust, or flight speed standards.

Imagine imposing those kinds of standards on a sailboat. Racing sailors expect rigs and hulls to take any wind and sea condition. Arguably, designers of boats and hardware could set not-to-exceed, wind, sea and G-load standards, and conceivably warn crews. Yet, in elite racing's competitive environment, who would listen?

There is a reason for seemingly inconsistent answers to questions about factors of safety. Since there are no universally accepted composite analysis methods in our industry, most of us have our own calculation programs and sets of safety factors. For instance, here at Hall Spars & Rigging, we wrote an esoteric local crippling program to analyze elastic instability in composite tubes. Using this particular software, we design to a factor of safety of 2.0. Why 2.0? As in the aerospace industry, it's a number that reflects our confidence levels in the program's formulae and baseline values derived from our experience and testing. With better mathematics, more experience, and more testing this factor may (or may not) go down. Precise and scientifically based? Not exactly. But 'guesstimation?' Hardly.

Call a designer, fitting manufacturer or spar maker. Ask your questions, then take the time to listen. You will get honest, technical, and, if you're patient, satisfactory answers. And, as Dr. Jacobs correctly says, you may very well learn something (and have fun doing it).
- Eric Hall

[AVMan]

Brian, good article. Do you have the letter from Dr. Jacobs' that this is in response to? I read it yesterday, but the email got deleted...

[brian eiland]

GUEST EDITORIAL
There is something seriously wrong when multi-million dollar maxi's are
literally breaking up at sea under difficult but hardly hurricane
conditions. As one who has previously designed and built a racing 40
footer, and has designed and is currently building a custom 7777 mm sloop,
the heart of the problem lies with an item known in engineering as the
relevant "safety factor". One can reasonably accurately determine static
loads. From these, one can then establish relevant local stresses, and then
determine, based upon the choice of materials, how much "stuff" is needed
to withstand those stresses.

However, these are what might be termed the "static stresses". Dynamic
stresses, such as the loads imposed when a sailboat "falls off a wave" are
much more difficult to quantify. Consequently, prudent design calls for an
appropriate "factor of safety" to account for the finite...and all too
real... possibility that "sooner or later" a substantial dynamic stress
will be encountered. Ah...as Shakespeare might say "there's the rub". Make
the factor of safety too large, the boat gets too heavy and is not
competitive. Make the factor of safety too small and we see what happens in
the latest Sydney-Hobart race, or even to some recent IACC boats.

Simply stated, the new materials ... Kevlar, carbon fiber, Vectran, etc.
are remarkable and have some truly amazing properties, especially tensile
strength, tensile modulus, and creep resistance. Unfortunately, they are
NOT especially terrific with respect to shear strength and fatigue
resistance...in fact...regarding the latter they are not even close to that
ancient boat building material known as wood! A paper clip will not break
if you bend it once. But bend it back and forth a dozen times and you will
have two halves of a paper clip. How many bending cycles does a racing
sailboat encounter over a long distance ocean race?

If you want to have fun the next time you encounter a yacht designer, spar
designer, yacht builder or spar builder ask them two questions: "What
factor of safety did you use"? And then "why did you pick that value"? Not
only will you likely learn something, you will also begin to understand the
amount of "quesstimation" that is involved in what many people naively
believe is a precise and scientifically based business.

Dr. Paul F. Jacobs, Saunderstown, RI

BTW, you might visit their site, and even sign up for their eMail newsletter.
http://www.sailingscuttlebutt.com/

Dr. Jacobs Has completly covered the raging debate on another thread about racing boats safety in a way that does not upset me or polarize me. Jacobs for President.

[brian eiland]

from Sailingscuttlebutt:

Guy Buchanan comments:
Eric Hall's response to Dr. Jacobs is thoughtful and
well reasoned. I particularly like his example of Hall's "esoteric local
crippling program" since it exemplifies the very empirical data that lies
at the heart of even the most rigorous finite element analysis in the
yachting industry. I do take exception to the notion that one would never
impose not-to-exceed standards on sailing hardware. During our brief
existence, (1991 - 2000,) we regularly imposed such limits on mega-yacht,
AC, and turbo-sled rigs and structures. Probably the only yachts we did
that had no limitations whatsoever were Whitbread 60s.

Mega-yachts had very stringent, very explicit tables of wind and sea
conditions versus configuration. AC boats had very explicit limitations on
rig loading, sea state, and winch loading. Turbo-sleds, for many years,
operated under very strict limitations carried in the heads of the
successful boat captains. These limitations were well understood by the
successful rig designers. (I'm guessing Eric remembers.) Failures often
occurred when something "new" was tried, falling outside of what was
"expected" or "understood". (I remember well a turbo-sled crew blowing out
a rig in their first race by vang sheeting a blast reach, something an
experienced owner/ captain would have never allowed without consulting the
designers/ builders.)

[brian eiland]

From Paul H. Miller:
Uncertain loads and variable material properties are
the big unknowns in yacht structural design. FEA does a great job if good
information is used. I've successfully used factors of safety as low as 1.1
(battens, halyards, daggerboards) and I've seen failures with factors as
high as 8 (mast tubes, rudder stocks, hulls, keels). Most failures were
traced to poor communication between designer, engineer, builder and user.
Examples were builders who didn't communicate that they couldn't build the
structure "as-designed" (too difficult to build, material substitutions),
owner/operator modifications without checking (bigger rudder, more righting
moment), operators exceeding their stated limits (running aground!), and
engineers who through lack of sailing experience or education did not
estimate the loads correctly (underestimated boat speed, missed a load
case) or used the wrong analysis.

Competitiveness has led to yacht structures that will not survive extreme
(but likely) conditions. The old ABS Guide worked within the limits of its
development. Unfortunately yachts today exceed those bounds, and
extrapolating the Guide takes more engineering understanding than most
designers are trained for. If strictly interpreted, ABS is conservative and
safe. Unfortunately that strict interpretation is rarely done for race
boats today. Additionally, in November ISAF voted to relax the enforcement
standards.

Just a small correction to Dave Few's comment. I had the pleasure of
working with Carl on the structural design of many of his designs and while
Carl admired UC Berkeley, he was a proud graduate of the excellent
architectural engineering program at Cal Poly.

[BDC]

I am new to the thread and am very intereted in the topic. I am a recent mech. engr. grad and am particularly interested working in the field of marine design...structural, system...

I would like to mention two things. First off, I agree that it is hard to set limits on loads. Much of sailing is self governed and relies on personal judgment for when to sacrafice speed for safety (judgement commently known as seamanship). An example of this is Sir Peter Blakes mid 90's around the world record trip in Enza. He sailed into Cows with a sea anchore out so that the boat would not crash through or over potentially hazardous waves. With boats being pushed harder and harder it seems that some of this seamanship has been lost.

Secondly, it would seem to me that the only way to safely reduce the safty factor is to understand more. With fiberoptic stress/strain guages being experimented with, would it be possible to get a realistic model or a better idea of what yachts go through when subjected to extreme sea conditions, or is this technology already being widely used?

Like I said, I am new to the field, find the topic interesting and am trying to learn and hear as much as I can.

BDC

[roamdeep]

Help pleas

whitch would be stronger

450 csm ec460
820 rw kc175
450 csm eu 450 90deg
etf 750 0 deg
820 rw 30 foam
450 csm
klever on keel line
250 csm
490 finish
80 psi 25 mm foam

whitch has the best shear and impact { weight aside }
roamdeep@hotmail.com