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