AC 2009-233: TEACHING SHIP STRUCTURES WITH SHEET METAL
William Simpson, United States Coast Guard AcademyDr. William M. Simpson, Jr. is a faculty member in the Engineering Department at the U.S. CoastGuard Academy. He has a Ph.D. in Aerospace Engineering from the University of Maryland, aMasters in Naval Architecture and Marine Engineering from Massachusetts Institute ofTechnology, and a Bachelor of Science from the U. S. Coast Guard Academy. He is a registeredProfessional Engineer in the State of Connecticut. He served on active duty in the U.S. CoastGuard from 1965 to 1992 and had assignments in Marine Safety, Naval Engineering, Acquisition,and Research and Development.
© American Society for Engineering Education, 2009
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Teaching Ship Structures with Sheet Metal
Abstract
The design and analysis of ship structures is taught to seniors majoring in Naval Architecture and
Marine Engineering as a part of their senior design course sequence. In the Ship Structures
course the students build on their basic knowledge of structures from their sophomore level
mechanics of materials course and add ship specific knowledge about hull girder bending, plate
bending, shear flow, and buckling. These techniques are applied to their senior ship design
project that is also being developed in the parallel courses of Principles of Ship Design and Ship
Propulsion Design. As an additional opportunity to apply their knowledge of ship structures and
to practice design, the student design teams are tasked to design and build a barge from sheet
aluminum with the goal to carry 120 pounds of weight. The weight is restricted to a 9 inch by 12
inch hopper to create a more or less concentrated load. The students must carefully plan the use
of their limited material just as any ship builder does, and they must also apply their knowledge
of ship hydrostatics and stability. The barges are tested in a tank of water and the students
receive credit for the amount of weight they are able to carry without structural failure, sinking,
or capsizing. For the past two years, corresponding to their senior project to design an
icebreaker, the students have also been tasked to pull their barges across/through a piece of ¼
inch foam to simulate icebreaking. Through the barge project the students get direct feedback on
the quality of their naval architecture and structural design work and experience the importance
of workmanship in metal fabrication. There is some positive student feedback regarding the
barge project in the student course evaluations. Objective course assessment tools do not show a
definitive impact for the barge project, but it is felt it is a positive contribution to the course.
Introduction
The course sequence for Naval Architecture and Marine Engineering under graduate majors at
the U. S. Coast Guard Academy includes a one-semester course in ship structures in the fall of
their senior year. The prerequisite for the ship structures course is a mechanics of materials
course taken in the fall of sophomore year that includes the normal introduction to structures.
This is followed by a sequence of courses in naval architecture starting in the spring of the junior
year with a general course covering the basic principles of naval architecture. This is followed in
the fall of the senior year with three parallel courses in ship design, ship propulsion, and ship
structures1. These three courses share the same ship design senior project that is worked on in
groups of typically 3 or 4 students. The ship design projects are carried over to and completed in
the spring semester culminating in a final presentation to invited industry professionals. The
design projects are selected by the instructors to ensure they can be completed during the two
semesters available and to ensure the desired breadth of ship design experience will be achieved.
Ships Structures
The approach taken in the ship structures course follows the traditional approach by addressing
what is referred to as primary, secondary, and tertiary stresses. The primary and secondary
stresses are beam stresses evaluated using the beam theory learned in the sophomore mechanics
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of materials course. Primary stress is the stress coming from the longitudinal bending moment
on the hull girder acting as a box beam. The longitudinal bending moment arises due to a miss
match of weight and buoyancy over the length of the ship. The computation of hull girder
bending and the associated hull girder stress is introduced to the students in the basic naval
architecture course in the junior year and continued in the ship structures course. The ship
structures course adds analysis of hull girder shear using the concept of open box beam shear
flow. The secondary stress is the beam stress on the hull stiffeners cause by hydrostatic pressure
on the outside of the hull and/or internal liquid pressures, structural weights, cargo, etc. The
tertiary stress is the plate stress caused by the net pressure loads on the hull plating. Addressing
this loading and stress analysis requires introduction of plate theory that is new to the students in
the ship structures course. A good bit of course time is devoted to plate theory as it is necessary
to go beyond elastic plate bending and cover membrane stresses with and without plastic
deformations (often referred to plastic “hinges”). A significant amount of course time is also
devoted to buckling. Both beam and plate buckling are addressed from an application point of
view. Analysis of the possible critical modes of beam, stiffener and plate, and plate buckling are
covered. In addition to this coverage of first principle analysis the students are introduced to
maritime classification society structural rules. This is done primarily through use of the
American Bureau of Shipping rules for aspects of their design projects.
Barge Project
As a way to give the students hands on experience in ship structural design, they are assigned a
task to design and build a sheet metal floating container (i.e. barge) to carry as much weight as
possible. For the last two years the tasking has also included transit through a sheet of floating
pink foam. Transit through the pink foam is used to simulate ice breaking as the students have
been designing icebreakers as their ship design project. The weight is placed in the barge in a
hopper (hopper = open top box) that is 9 inches wide and 12 inches long. No weight is allowed
outside the hopper so the weight is a more or less concentrated load with the associated structural
loading impact. The hopper can be placed anywhere on or in the barge. The barge may be open
topped or enclosed as desired.
The following materials are provided to the students:
1. Aluminum sheet metal 20 inches wide and 120 inches long. The aluminum thickness is
0.012 in. It is alloy 3105-H22.
2. Hot glue and glue gun.
3. Pop rivets, 1/8 inch.
4. Caulk/Sealant
No other materials are allowed. Only the originally provided piece of sheet aluminum may be
used. No additional sheet aluminum may be used, and the original piece may not be replaced or
traded-in for a new piece. This reinforces the need for the students to do careful design, analysis,
and construction work and to get it “right” the first time. The students have access to an
industrial quality sheet metal shear and a sheet metal brake as well as hand shears and drills for
installation of the pop rivets.
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Each design team of 3 or 4 students designs and constructs their own barge so there is a natural
competition to see who can do the best. However, the performance is judged against a fixed
standard rather than relative to other groups. The grade on the barge project counts for 10% of
the grade for the Ship Structures course. The project deliverables and the points allocation for
the barge grade are:
1. Design calculations
a. Structural analysis including MAESTRO model (20 points)
b. Predicted weight capacity of container and the limiting factor (i.e.: stability,
buoyancy, or strength?) (5 points)
2. Completed Barge (15 points)
3. Weight capacity test (1/3 point for each pound of weight carried, max 40 points)
4. “Ice breaking”, 24 inch pink foam transit
a. Successful transit (10 points)
b. Pulling Weight Points
Less than 30 lb 5
30 lb to 35 lb 4
35 lb to 40 lb 3
40 lb to 45 lb 2
45 lb to 50 lb 1
Greater than 50 lb 0
5. One page written summary of results and conclusions (5 points)
The barges are tested on the assigned date in the open portion of a free surface circulating water
channel (with no flow). The open surface is 4 feet wide by 11 feet long, and it provides an ideal
test environment. The barges are first statically tested to determine how much weight can be
carried in the weight hopper. The students are allowed to add as much weight as desired while
their barge is free floating. After the desired maximum weight is onboard, the barge must float
unconstrained for 1 minute without capsizing, or sinking. At the end of the 1-minute period the
pink foam (“ice”) transit is performed. The barge is required to transit a sheet of pink foam
floating on the surface of the circulating water channel. The pink foam that is intended to
simulate ice is approximately ¼ inch thick and the length to be transited is 24 inches. The clear
width of the foam is approximately 42 inches. The pink foam is constrained on each side, but it
is not constrained in the front or the back. The barge is pulled through (or across) the foam with
two snap hooks leading to a single line. The hooks are attached to the barge with whatever
attachment the students devise, and the line passes over the top of the pink foam. The pulling
force is supplied by a suspended weight (pulling force = suspended weight). During the
“icebreaking” the barge is required to carry its maximum weight. Full credit for the “ice
breaking” is earned once the barge proceeds the 24 inch distance from the back of the pink foam
regardless of the resulting condition of the foam. If the barge does not transit the full 24-inch
distance but remains afloat and upright, credit for the “ice breaking” is awarded proportional to
the distance traveled. The barge or line attachment arrangement may not be touched once the
transit begins. If the barge sinks, capsizes, or loses the weight hopper during the transit, there is
no credit for icebreaking. “Icebreaking efficiency” points are earned based on the line force
required to transit the foam. The icebreaking “towing” arrangement is shown schematically
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below. There is no opportunity for the students to test the “icebreaking” capability of their
barges prior to the graded trial.
Barge “Icebreaking” Arrangement
Barge Design
In doing the barge design the students must consider buoyancy, stability, structural strength, and
optimum use of the limited aluminum sheet metal. The barge design project has been varied
over the last several years by changing the allowable location of the weight hopper. In the
current variant with placement of the weight being allowed in the bottom of the barge, the
limiting factors are buoyancy and structural strength. Additionally, depending on where the
students choose to attach the towline, the “icebreaking” can cause longitudinal trimming
moments that result in the bow or stern being submerged. In previous years there has been a
requirement for the weight hopper to be placed on the deck. That has made stability a critical
design factor as well. In doing the design the students must optimize use of the sheet aluminum
to provide all the hull plating and the stiffeners for their structure. This introduces the students to
the concept of piece part nesting used by shipyards to arrange the cut out of irregular ship
structural parts in such a way as to minimize the amount of material required. If the students do
not adequately address all the necessary factors in their design work before metal is cut, they
learn the “hard way” that beginning construction with an inadequate design can be costly!
A new tool added to the course in fall 2008 is the ship structural analysis computer program
MAESTRO3. This program was developed by Professor Hughes at Virginia Tech. and is
discussed in his text SHIP STRUCTURAL DESIGN2. This past fall the students used the
software to produce and analyze a computer model of their barge designs. The program is
specially set up to quickly produce a ship structural model with limited effort. In order to
facilitate the student use of the program, a tutorial specifically for producing a model of their
barge design was provided together with a generic barge model example. With these two aids
(and some instructor assistance) the design groups were able to build computer models that they
used to evaluate and adjust their barge structure. An example of a MAESTRO barge model is
shown below. As seen in this example, all of the designs were of the open top design as this
allowed development of the maximum buoyancy with the limited hull material.
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Student MAESTRO Barge Structural Model
As the MAESTRO software has only been in use for one semester, the instructors are still
learning about its use. One of the things observed with the first cycle of use was that the students
were much more aware of the need for adequate transverse structure at the top of the barges
because of the graphic output from MAESTRO. In the fall of 2007, before MAESTRO was
introduced, several of the student barges had inadequate framing and, in particular, inadequate
transverse structure at the top of the barge. The improved transverse structure in fall 2008 was
certainly at least partly due to the use of MAESTRO, but it was also likely partly due to the
availability of a file of pictures from the fall 2007 trials.
Barge Trials
All of the students had successful barge trials in 2008. An example of the testing of one of the
student barges is shown in the sequence of pictures below. At least part of the reason for the
success of the trials was that the project was designed for the students to be successful. The
requirements were carefully set so they would be attainable by the students, but the students
could not discern that from the beginning. The requirement to carry 120 pounds in a small sheet
metal barge seemed daunting to the students at the outset, but, from project refinement over
several years, it was known that carrying 120 pounds was achievable. All of the groups were
able to carry the 120 pounds. It also turned out that all groups earned the maximum
“icebreaking” pulling force points in fall 2008. This was partly due to a high guess in setting the
pulling force points rubric as this was the first time the “icebreaking” pull force was measured.
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Barge Being Loaded for Static Weight Test
Barge Begins “Icebreaking” Test
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Barge Proceeding Through “Ice” During Test
Successful Completion of “Icebreaking” Test
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An additional part of the success was the positive correlation between the MAESTRO model
results and the observations during trials. The lightweight aluminum structure in the barges
happily deformed significantly during the trials, and the students were able to see in their barges
the deformations they had seen on their computer monitors. Several of the barges exhibited
plastic deformation of the hull plating graphically illustrating the idea that plating can carry
pressures well in excess of the elastic plate bending yield point and not rupture. This is
illustrated in the MAESTRO deformed model and in the failed barge below. In this case, after
the required trials were completed, the student group elected to add additional weight to “See
what it could do?” The barge bottom plate displayed plastic deformation essentially as the
computer model predicted. The computer model also predicted large deflections in the lightly
built stern that resulted in the buckled plate seen in the actual barge.
Deformed Student MEASTRO Model
Student Barge Tested to Failure
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Student Feedback and Course Assessment
It is apparent from the student end of course survey comments (submitted anonymously) that the
Ship Structures course was not the students’ favorite course. As the reader is no doubt aware,
this can be due to many factors including such things as quality of instruction, student workload,
difficulty of material, presence of a final exam (the complementary design courses did not
include a final exam), etc. However, for at least some students, it would appear the barge project
may have been the “petunia in the onion patch”.
Positive student comments include:
“I really enjoyed the barge project, the more hands on things the better.”
“I think designing an icebreaker made this class particularly difficult. IACS is not nice. I really
like the barge project. Overall, I thought it was a great course.”
“Class was very difficult to follow. Text was not very good. Barge project was the best project I
have done my whole academy career. I learned a lot from it.”
“Testing the barges was the highlight of the semester. I enjoyed the hands on experience and
everything we learned in class and calculated on the computer became real.”
The positive comments were of course balanced by comments such as:
“Probably time for a major revamping of this class, I honestly learned next to nothing, with the
exception of what my classmates taught me.”
“Overall this course was very frustrating. …..”
“This course was challenging and confusing …..”
At the Coast Guard Academy as at all ABET accredited institutions outcome assessment is an
important part of program evaluation and improvement. The Ship Structures course is credited
with “demonstration of outcome” for two program outcomes and “significant knowledge
development” for several other outcomes. The student work in the Ship Structures course is
specifically evaluated for the following program outcomes:
1. An ability to apply knowledge of mathematics, science, and engineering.
2. An ability to design a system, component, or process to meet desired needs within
realistic constraints such as economic, environmental, social, political, ethical, health and
safety, manufacturability, and sustainability.
3. A knowledge of contemporary issues.
4. Demonstrate the ability to apply probability and statistical methods to naval architecture
and marine engineering problems.
The contribution of the Ship Structures course to demonstration of these outcomes is subjectively
evaluated in periodic course reviews involving all program faculty. In addition, numerical
student performance on specific assignments and exams in the course is tabulated and reviewed
to evaluate demonstration of outcomes.
The final exam in the course is considered a good indication of the overall student learning in the
course and is used as a part of the program outcome assessment. The exam is not the same from
year to year, but in general it requires a fairly comprehensive analysis of a simplified ship type
structure. A copy of the final exam from 2008 is included as Appendix A. The students use
Excel and the plate bending spread sheets from Hughes2 in completing the exam. The final exam
scores for the Ship Structures course for 2006, 2007, and 2008 are shown in the plot below.
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As is seen there is an upward trend in the scores. It is felt that perhaps the barge project has
helped to produce this trend, but, as the reader knows, there are many factors that influence exam
scores. For small samples such as this with varying exams from year to year it is not really
possible to do an objective analysis of the whys. Subjectively, things seem positive, but there are
also students that are not achieving the outcome level desired, and thus continued improvement
remains a goal.
The scores on the barge project are also used to assess outcome achievement, but as seen in the
plot below there is currently a need to reevaluate the grading of the project.
With the use of MAESTRO in 2008 that enabled the students to more completely and more
accurately analyze their designs, the performance based grading essentially became a pass/fail
criteria. This clearly shows the value of a computerized analysis tool such as MAESTRO in a
ship design project. The performance based grading did provide the motivation for the students
to design and build a barge that performed as required, but, as essentially a pass/fail standard, the
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project grade is not a good outcome assessment tool. Efforts will be made to adjust the grading
to improve the outcome assessment aspect while maintaining the barge performance motivation.
Regardless, the project value as an assessment tool will be limited by the fact that it is a group
project and that the grade may only reflect the efforts on one or two members of a three or four
member team.
Conclusion
The sheet metal barge project has added a dimension of realism and practical experience in the
Ship Structures course at the Coast Guard Academy. It gives the students a challenging but
achievable design, analysis, and construction task that requires integration of hull design,
structural design, and construction planning. It gives direct correlation between theory and
practice. The students learn from firsthand experience the need for care in metal ship
construction. They experience the ease with which single curvature can be used in metal
construction and how difficult it is to use double curvature. In doing the project the students put
to use their group interaction skills, but, because it is a group project, the knowledge and
experienced gained varies from student to student. The variation of student effort and thus
individual student gain is a real concern in any academic group project, but, as a group, the
students are rewarded for dedicated effort. The students also experience the consequences if
there is inattention to engineering principles and critical design factors. Best of all, perhaps, the
sheet metal barge project gives the students the opportunity to enjoy success in a hands-on
project that does not take a semester or more to complete. Those who teach ship design will
likely agree this can be valuable in a subject that typically requires a great deal of work for each
“well done”.
Disclaimer
The views expressed here are the author’s and not those of the U. S. Coast Guard Academy, the
U. S. Coast Guard, or any other government agency.
Bibliography
1. Taylor, Colella, & Simpson, An Integrated Approach to a One-Semester Ship Design Experience at USCGA,
ASEE Annual Conference 2006
2. Hughes, SHIP STRUCTURAL DESIGN A Rationally-Based, Computer-Aided Optimization Approach,
Society of Naval Architects and Marine Engineers, 1988
3. MAESTRO (http://www.orca3d.com/maestro/)
Appendix A
Ship Structures Final Exam (Fall 2008)
Shown on the next page is the starboard side of the midship section of a proposed ATON (aids to
navigation) barge design to be used by the 140 replacement vessels in the Great Lakes. The
structure shown is a first “guess” at what the structure should be.
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Overall dimensions are: Material
L = 320 ft ABS EH-36
B = 64 ft Yield point min. = 51,000 psi
Depth = 20 ft Tensile Strength = 71,000 – 90,000 psi
Impact properties 25 ft-lb @ -40º F
Scantlings:
Bottom Sides
plating, t = 15.3 # plating, t = 20.4 #
longitudinal stiffeners longitudinal stiffeners
spacing, s = 24 in spacing, s = 24 in
structural Tees structural Tees
web = 5 in x 3/8 in web = 5 in x 1/2 in
flange = 3 in x 3/8 in flange = 3 in x 1/2 in
transverse web frames transverse web frames
spacing = 72 in spacing = 72 in
Center vertical keel Deck
24 in x 1/2 in plating, t = 15.3 #
Keel rider plate longitudinal stiffeners
12 in x 1/2 in spacing, s = 24 in
Midship section properties (total values) web = 5 in x 1/4 in
neutral axis above baseline = 115.1 in flange = 3 in x 1/4 in
cross sectional area = 1058.2 in2
transverse web frames
moment of Inertia = 83,232 in2ft
2 Spacing = 72 in
section modulus deck = 7,999 in2ft
section modulus bottom = 8,675 in2ft
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1. (12 points) Compute the primary hogging and sagging stresses in the bottom and in the
deck given the following L/20 wave bending moments. Specify tension or compression.
a. Maximum Sagging Moment = 95,216 LT-ft
b. Maximum Hogging Moment = 75,400 LT-ft
(Note: The midship section properties (I, YNA, etc.) are given on page 2)
2. (12 points) The maximum hull girder shear is 1508 LT. Compute the maximum hull
girder shear stress. Assume the cross section at the location of maximum hull girder
shear is the same as the midship section shown. In computing the shear stress ignore the
contribution of the longitudinal stiffeners.
The midship section properties without the longitudinal stiffeners are:
Midship Section Properties (total values without stiffeners)
Neutral Axis above baseline = 117.8 in
Cross sectional area = 834 in2
Moment of Inertia = 67,109 in2ft
2
Section Modulus deck = 6,588 in2ft
Section Modulus bottom = 6,839 in2ft
3. (12 points) Determine the maximum hull bottom plating tertiary Von Mises stress
assuming uniform hydrostatic loading with a hydrostatic head of 6 feet above the deck
(i.e. 20 ft + 6 ft = 26 ft).
4. (12 points) Compute the maximum bottom structure longitudinal stiffener secondary
stress due to uniform hydrostatic loading with a hydrostatic head of 6 feet above the deck
(i.e. 20 ft + 6 ft = 26 ft).
5. (12 points) Find the maximum stiffener stress if the barge were to experience ice loading
on the side shell such that one longitudinal was loaded with a pressure of 80 psi over a
single length between transverse frames (72 in. longitudinally) and vertically over a 24 in
high area centered on the stiffener. In other words, it is assumed a single side
longitudinal stiffener and its associated plating is loaded with 1920 lb/in over a length of
72 in. between transverse frames.
6. (14 points) Evaluate the risk of buckling in the deck structure due to hull girder bending.
7. (14 points) Find the location and magnitude of the maximum combined primary,
secondary, and tertiary Von Mises stress in the bottom structure.
8. (12 points) Based on your analysis in 1 – 8 above what if any parts of the first “guess” for
the barge structure are inadequate? What changes would you make in the design of the
barge structure and why are those changes necessary? This part does not require revised
scantling values or analysis of revised scantlings, but you may include that if you choose.
For instance, if you feel the center vertical keel should be made thicker, just state that you
think it should be made thicker because …….. . Page 14.1150.14