Final Report of Capstone Experience:

Design and Construction of a Froude

Scaled Model DTMB 5415 Hull Form

By James Haller, University of Maine, in partial fulfillment for the

requirements of a Bachelor of Science in Engineering Physics, with a

Concentration in Mechanical Engineering

August, 2016

Project Advisor: Dr. Krish Thiagarajan, Alston D. and Ada Lee Correll

Presidential Chair in Energy and Professor of Mechanical Engineering

Report Submitted to: Dr. James McClymer, Associate Professor of Physics

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ABSTRACT

The Marine, Ocean, and Offshore Research Group at the University of Maine, in

cooperation with faculty and students from the University of Maine, the University of Southern

Maine, Maine Maritime Academy, as well as industry professionals from General Dynamics Bath

Iron Works, formed a working group in May of 2016 that was tasked with the design and

fabrication of a Froude scaled model DTMB 5415. The primary goals of the project were to foster

a working relationship among these partners, as well as develop and construct a model DTMB

5415 to be utilized as a validation case for the implementation of a towing carriage to be installed

in the state of the art Alfond W2 Ocean Engineering Lab, located at the Advanced Structures and

Composites Center on the University of Maine campus. The DTMB 5415 is a hull form for a naval

surface combatant that features the incorporation of both a transom stern and a sonar dome, and is

well regarded amongst naval architectural circles. In addition, due to its complex underwater

geometry, fluid flow on the surface of the 5415 has been thoroughly explored through the use of

Computational Fluid Dynamics simulations, as well as through experimental tests in towing tanks.

As such, the usage of a properly scaled model 5415 would serve as an excellent validation case for

any ship model basin. In the summer of 2015, one year prior to the commencement of the DTMB

5415 project, the MOOR Group had undergone a similar experience in model fabrication, through

the design and construction of a 1/120 scale FPSO model. A summary of the construction of the

FPSO is presented along with a description of the fabrication methodology used. Thanks in large

part to this background, many of the initial design considerations utilized in the design of the naval

combatant drew on experiences that were described through the construction of the model FPSO.

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As the design of the model 5415 was further refined, it became apparent that in order to

develop an effective validation case for the Alfond W2 Ocean Engineering Lab, tolerances for the

fabrication process needed to be restricted to a much lower level than those permitted in the

fabrication of the FPSO model. An outline of the specifics for each of these applicable model

tolerances is described, and each manufacturing method is examined in depth for its likelihood to

best adhere to these prescribed tolerances. An exploration of restrictions placed on model scale

due to the limitations in available tank size is presented alongside a discussion of expected

limitations due to the towing carriage; in addition, recommended scaling factors are suggested. A

scaling factor of 1/50 was decided upon, which corresponded to a model with a length between

perpendiculars of 2.84 m, a draft of 0.123 m, and a mass of 69.1 kg (ballast included). At Froude

numbers of 0.248 and 0.413, the model service speeds are 1.32 m / s and 2.16 m / s, respectively.

Creation of a CAD model was undertaken, using the publicly available DTMB 5415 .igs

file, found on the SIMMAN 2008 conference webpage. Using both SolidWorks and Rhinoceros

software, attempts were made to rectify the flawed surfaces that were originally provided in this

file. Eventually, through the use of curvature analysis tools, these surfaces were combined in a

manner which minimized flaws in the surface geometry of the hull form. Given the small

tolerances required in the fabrication of the model, as well as the lack of capabilities to construct

the hull form out of a single, continuous piece of material, a recommendation is given to have the

model constructed by a professional model builder, or alternatively, fabricated out of one piece of

material. A dual pronged approach was taken in the development of two final model designs: If

provided funding, the preferred method is to construct the model as one piece; if not, the model

will be fabricated through the assembly of numerous 7.5 cm thick CNC cut foam slices, along with

a 3D printed bow section, affixed together through the use of epoxy, fiberglass, bolts, and plywood.

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DEDICATION

This work is dedicated to Mr. and Mrs. Steve Garland. Had

it not been for the awe-inspiring nature of your incredible

generosity, I would never have been able to embark upon

the journey that has led to the culmination of this degree.

Thank you.

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ACKNOWLEDGEMENTS

While the work presented on the following pages may inadvertently lead a reader to assume

that the efforts described herein are the results of just one individual, the reality is that nothing

could possibly be further from the truth. First and foremost, I am in debt to Professor Krish

Thiagarajan, who was not just merely willing to offer me, at the time an unknown student, this

opportunity on such short notice, but also to serve as a mentor throughout my introduction to the

subjects of naval architecture and model design. Of course, nothing in this project would have been

accomplished without the ongoing efforts of Razieh Zangeneh, whose expertise in offshore ship

structures and experience in the design and construction of the model FPSO has been invaluable

in ensuring the successful development of this project. I would also like to thank Professor James

McClymer, who has been more than generous in both his willingness to be flexible with the

requirements for this project as it continued to evolve, as well as in his offering of advice when

presented with difficult scenarios. I also need to thank Professor Alex Friess, whose assistance was

crucial in the ability to manufacture the 3D test print, and whose motivation towards helping me

find solutions to troublesome flaws in the CAD geometries was unparalleled. Thanks are also in

order for Mr. Stephen Abbadessa, who was willing to sacrifice blood, sweat, and tears if we needed

to use his CNC router for fabrication purposes. Finally, I must thank the other members of our

working group: Professor Andrew Goupee, Dr. Doug Read, Dr. Michael Davis, Mr. Russ

Hoffman, and Mr. Paul Friedman. Without the benefit of your insights, this project would still be

getting off the ground.

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TABLE OF CONTENTS Abstract………………………………………………………………………………………………………..…………...………iii

Dedication.…………….………………….………………………..………………………….........................................................v

Acknowledgements……………………………………………………………………...………………….………………….vi

1 Introduction.................................................................................................... ......................................................................1

1.1 Overview of Marine, Ocean, and Offshore Research Group’s Prior Work…….......................1

1.2 MOOR Group’s Role in Development of the Alfond W2 Ocean Engineering Lab..………2

1.3 DTMB 5415 Background and Overview……………………....………………………...………………….5

1.4 Project Goals for the Construction of a Model DTMB 5415……………...….………………..…..6

2 Model Design Process……………………………...…………………………………………………………...…..7

2.1 Original Design Considerations…………………………………………….………………………………...7

2.2 Exploration of DTMB 5415 Models Constructed by other Research Facilities….….……..8

2.3 Selection of Scaling Factor for Construction of a Model DTMB 5415……………………….10

2.4 Initial Development of SolidWorks CAD Model..............................................................................11

2.5 Designing for Manufacturability…………………...…………………………………………………….15

2.6 Methods Available for the Cutting of Model Foam…………...…………………….……………....17

2.7 Revision of the Model Design Process……..………………………….…….……………............................19

3 Conclusions and Recommendations for Future Work…………………..………...…................................21

3.1 Proposal for Additional Model Funding……………………….………….……………………...………21

3.2 Analysis of Surface Curvature and Fairing of the Hull’s Surface……………………………...….22

3.3 Next Steps in Project Development….…………………...………………………...…………………...23

3.4 Conclusions…………...…………………………...…………………….…………………………...………………25

Works Cited………………………………………….…………...………………………………………………………………26

4 Appendices…………………………………………………………………………………………………………….28

Appendix A – Engineering Constraints…………………………………………………….…………………….28

Appendix B – DTMB 5415 Full Scale Particulars……………………………..………….……………………..30

Appendix C – Model DTMB 5415 Construction Options and Recommendation………..…………31

Appendix D – Quote Obtained by MOOR Group for Construction of a Model FPSO……....…..32

Appendix E – Weights Table for Scaled Model DTMB 5415……………………………………………33

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1 Introduction

1.1 Overview of Marine, Ocean, and Offshore Research Group’s Prior Work

The Marine, Ocean, and Offshore Research Group at the University of Maine, in

conjunction with staff at the Advanced Structures and Composites Center, also located at the

University of Maine, have been researching various aspects of FPSOs, such as the heading stability

of FPSOs under wind-loading conditions, as well as unidirectional and bidirectional seas

(Zangeneh et al, 2016; Zangeneh and Thiagarajan, 2015). Designed for the deep ocean extrication

of underground oil, a Floating, Production, Storage, and Offloading vessel consists of a converted

tanker hull which is then attached to a passive mooring system. This configuration allows the

vessel to remain fixed in place, which, since the engines do not need to be running in order to do

so, provides significant cost savings, while still allowing the vessel the freedom to move slightly

under the motion of the waves and winds. If storm conditions are predicted to become severe, the

vessel is able to detach from its mooring system in order to protect against breakage and accidental

discharge of raw crude into the ocean environment. A visual example of such a mooring

configuration is shown below in figure 1.

Figure 1: Artistic rendering of an FPSO weathervaning around its turret mooring system.

The black lines in groups of three are the mooring lines, while the other colors correspond

to flexible hoses and pipes utilized for oil production and extraction purposes.

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1.2 MOOR Group’s Role in Development of the Alfond W2 Ocean Engineering Lab

In addition to its research on FPSOs, the MOOR Group has also been instrumental with

the development of the new Alfond W2 Ocean Engineering Lab at the Advanced Structures and

Composites Center. This new, state of the art laboratory combines a programmable 16 paddle wave

generator along with a moveable wind tunnel in order to simulate coastal and deep ocean storms

on offshore structures. The wave tank features a wave-absorbing beach on the far end, as well as

a movable floor which allows the depth of the tank to be adjusted from 0 to 5 meters. The lab is

also outfitted with extremely high frame-rate cameras, which, in combination with optical tracking

points placed on the surface of the structure, are able to effectively monitor and report on a vessel’s

motion in all six degrees of freedom (surge, heave, sway, roll, pitch, and yaw). In 2015, the MOOR

group designed and constructed a model FPSO, at 1/120 scale, which has thus far been utilized to

perform seakeeping tests while operating in a moored capacity under conditions of unidirectional

and bidirectional seas. The FPSO was fabricated by hand, using plywood bulkheads that were cut

by the Laser-Cutting machine at the Advanced Manufacturing Center at the University of Maine

(see figure 2a, below). These plywood bulkheads, each describing the transversal cross section of

the ship, were then aligned along the length of the hull. Foam blocks were cut and added to fill in

the spaces between these stations, and were glued to the bulkheads (see figure 2b, below).

Figure 2, a: Plywood bulkheads for FPSO, b: Addition of foam to outside of bulkheads.

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These blocks were eventually all stacked together, and sanded to their proper shape. The

group constructed both the bow section and the stern section in a similar manner, although the

construction of the bow utilized the incorporation of a high density spray foam in addition to the

block foam, which can be seen as the yellow foam in figure 3. To provide structural support,

fiberglass was then applied to both the outside and top-side deck surfaces of the model.

Figure 3: All three sections (stern, middle body, and bow) attached together and sanded.

Afterwards, the model was coated with primer, during which imperfections in the surface was

observed. These large imperfections were filled in with a significant amount of model putty, for

which Rubbing Glaze Compounds, Bondo(R) Professional Glazing & Spot Putty was utilized.

After the larger of the visible imperfections were corrected for, the model was once again coated

with primer, and then painted. Figure 4a shows the various applications of the red putty across the

hull’s surface, while figure 4b shows a close-up of the large number of imperfections present in

the geometry of the stern section (de Oliveira Costa, 2015). This increase in the frequency of

imperfections in the stern is likely due to the larger number of foam block layers that were needed

to adjust for the geometrical changes of the ship’s stern. After the application of the putty, although

there were still imperfections in the shape of the hull, these were not considered significant since

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the FPSO model was only to undergo seakeeping tests under wind and wave conditions in the

Alfond W2 Lab. A photograph of the completed FPSO model is exhibited below in figure 5.

Figure 4: Imperfections corrected by putty across the FPSO’s a) hull surface, b) stern.

Figure 5: Completed FPSO. The model has been sanded, painted, and ballasted with lead

weights placed at strategic locations across the deck surface in order to properly simulate

the two load cases selected for testing.

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1.3 DTMB 5415 Background and Overview

The DTMB model 5415 is a model hull for a surface naval combatant that was originally

conceived in the 1980s by naval architects at the David Taylor Model Basin, which itself is a part

of the Naval Surface Warship Center, Carderock Division. Although no full-scale ship has ever

been built to the exact lines of the DTMB 5415, the hull form did serve as a forerunner for what

eventually became the DDG-51 Arleigh Burke Class of naval surface combatants, which is

currently comprised of 62 ships (Cope, 2012). The design of the DTMB 5415 incorporates a

transom stern, as well as a sonar dome, which is a variation of a bulbous bow intended to house

sonar equipment. This complex underwater geometry, as well as the availability of towing-tank

test data (Stern et al, 2000; Olivieri et al, 2001), makes the DTMB 5415 an excellent benchmark

for the validation of Computational Fluid Dynamics (CFD) simulations and programs, as well as

other hydrodynamics research. As such, numerous studies of both numeric and experimental

nature have been performed on the DTMB 5415 (e.g., Diez et al, 2005; Jones and Clarke, 2010).

At full-scale, the DTMB 5415 has a length of 142 meters between the forward and after

perpendiculars, a beam of 19.06 meters, and a draft of 6.15 meters. The ship also has a block

coefficient of 0.507, and a midship section coefficient of 0.821 (SIMMAN, 2008). The lines plans

for the ship are presented below in figure 6, with the remainder of the full-scale particulars

described in appendix B of this report.

Figure 6: Lines Plans for the DTMB 5415 naval surface combatant.

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1.4 Project Goals for the Construction of a Model DTMB 5415

Given the experience of the MOOR Group in the construction of a model FPSO, as well as

the on-going development of a new towing carriage to be implemented in the Alfond W2 Ocean

Engineering Lab, a working group was formed to foster the design and creation of a similarly sized

ship model. The DTMB was selected primarily for its usefulness in validation purposes, as well as

for its popularity amongst ship-building circles. Thus, the primary objective of this project was to

design and manufacture a Froude-scaled DTMB 5415 hull form, to be affixed to the newly

constructed towing carriage at the W2 facility. The model was intended to serve as a test platform

for future research for the Marine, Ocean, and Offshore Research Group, the Department of

Mechanical Engineering at the University of Maine, and the Advanced Structures and Composites

Center. As a part of this primary objective, upon construction and distribution of the proper ballast,

the model will be utilized to provide validation data for the tow carriage that to be implemented at

the W2 facility. As a secondary objective, through collaboration amongst the various partners

involved in this project, a working relationship was formed amongst both industry professionals

in addition to university researchers at multiple institutions across the state.

Figure 7: Photograph of the Alfond W2 Ocean Engineering Lab at the Advanced Structures

and Composites Center, displaying the wave paddles (foreground) and wind tunnel (far end).

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2 Model Design Process

2.1 Original Design Considerations

The project officially began in late May of 2016, when the first meeting of the project’s

collaborators was held at the University of Maine. This initial meeting included researchers from

the University of Maine and the University of Southern Maine, as well as naval architects from

General Dynamics Bath Iron Works. During this conference call, there was a brief discussion of

the history the MOOR Group had with the construction of the FPSO model, as well as the

construction methods undertaken in the fabrication of it. The main objectives of the naval

combatant project were laid out and agreed to, and the resources available for the project were

outlined, which consisted mainly of leftover foam from the construction of the wind tunnel nozzle

for the W2 Alfond Ocean Engineering Lab. It was also discussed how there were no funds

available to budget towards construction of the model, thus forcing expenses in materials and

fabrication to be kept to a minimum. The basics of the model tests to be performed were outlined;

it was decided that the model was to undergo seakeeping tests (similar to those performed on the

FPSO), as well as resistance tests, which would be utilizing the W2’s tow carriage. Russ Hoffmann

and Paul Friedman, senior naval architects at Bath Iron Works, confirmed the advantages of using

the geometry of the DTMB 5415 for validation data, and commented on how BIW routinely uses

5415 resistance data to help predict other forms; they also agreed that since the 5415 has been long

available in the public domain, it has been well explored and would make an excellent choice for

validation of the tow carriage, as well as being a useful data set to augment BIW’s own records.

During this meeting, the group also considered some possible modifications to the original

DTMB 5415’s geometry. Multiple options that were of interest to both Bath Iron Works and Navy

architects were discussed, such as the incorporation of an additional bow bulb above the sonar

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dome, the usage of stern flaps, and a potential increase in the length of the middle body of the ship

by 35 to 52 feet at full scale (10.7 to 15.8 meters). The primary motivation behind the first two

potential modifications was to see if either would help decrease the resistance of the hull, which

effectively lowers the amount of power required to propel the ship forward; while the interest in

the length modification was to explore the possibility of adding another engine compartment as

well as allow more deck space for military resources, a question which has existed in naval

architectural circles for a while, but has not yet been fully explored. It was decided to keep these

possibilities in mind moving forward, with the potential of including them in the model design.

2.2 Exploration of DTMB 5415 Models Constructed by other Research Facilities

Through background research and exploration, four different models of the DTMB 5415

hull were found to have been constructed and tested by researchers in the public domain. The

facilities known to have tested these models are the Istituto Nazionale per Studi ed Esperienze di

Architettura Navale Vasca Navale, INSEAN (Italy); the Maritime Research Institute Netherlands,

MARIN (Netherlands); the University of Iowa’s IIHR; and the Massachusetts Institute of

Technology’s Sea Grant Program. Each of these models were constructed to different sizes, and

tested in various capacities. For example, the MIT model was designed to be a free-running model,

capable of self-propulsion in the nearby Charles River (Cope, 2012). Certain models were found

to be designed with additional appendages included, such as bilge keels, stabilizing fins, or the

incorporation of propellers and propeller shafts, while others were restricted to just a bare hull to

be towed by a towing carriage. The models varied in lengths overall from 3.0 to 5.7 meters, and

appeared to be constructed from either wood or high-density foam bases. The fabrication of each

of the models seemed to be undertaken by outside contractors, and very few details of model design

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or construction were ever found. Even though some of the models featured additional appendages,

the hull form for each model was an identical geosym of the DTMB 5415’s original lines plans

(see figure 6). The exact parameters for each institution’s model, including scale factor, are

presented below in table 1.

Table 1: Main particulars for both the full-scale and scaled models of the DTMB 5415 built

at INSEAN, Instituto Nazionale per Studi ed Esperienze di Architettura Navale; MARIN,

Maritime Research Institute Netherlands; IIHR, Iowa Institute of Hydraulic Research;

MIT, Massachusetts Institute of Technology (SIMMAN, 2008; Cope, 2012).

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2.3 Selection of Scaling Factor for Construction of a Model DTMB 5415

Given that the original intent in the fabrication of this model was to build a model of similar

size in length to that of the FPSO previously constructed by the MOOR group, a proposed length

of approximately 3 meters was envisioned. However, in order to ensure the ability for the model

to be used as a proper validation for the tow carriage at the W2 facility, a more extensive look at

scaling limitations needed to occur. In R.F. Halliday’s paper, Making the Best Use of a Small Ship

Model Tank, he describes the limitations placed on model size in terms of the size of the towing

tank itself. Although there is an infinite number of tests that can be dreamt of utilizing a ship model

tank, the most clear-cut and basic experiment is that of a model being towed across a tank filled

with flat water exhibiting no current of its own. In this example, both the speed and size of the

model are scaled according to the proper dimensionless Froude number. The premise behind this

test is that both the model and the full scale ship are run at the same Froude number, which is a

relationship between the inertial and gravitational forces and defined as (1) 𝐹𝑁 ≡ 𝑣/(𝑔𝐿𝑃𝑃)1/2,

where 𝑣 is the velocity of the hull, g is the acceleration due to gravity, and 𝐿𝑃𝑃 is the length of the

ship from the forward to the aft perpendicular (Newman, 1977). This relationship causes further

Froude scaling in volume displaced, design velocity, and time scaling for ship model tests. For

example, if the scaling factor were to be described as (2) 𝛼 = 𝐿𝑠ℎ𝑖𝑝/𝐿𝑚𝑜𝑑𝑒𝑙, the volume would

then be scaled by 𝛼3, and design speed velocity and time would both be scaled by √𝛼.

With these scaling factors in mind, the main limitations on model size were restricted by

the dimensions of both the towing tank as well as the tow carriage. For example, if the cross-

sectional area of the tank was not sufficiently large enough in comparison to the cross-sectional

area of the model, significant wave reflections could occur off the side walls, affecting the results.

As such, practical experience dictates that the immersed cross-sectional area of the ship should not

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be larger than 1% of the cross-sectional area of the tank (Halliday, 1977). In addition, to avoid the

excessive excitation of unwanted nodes of wave formation due to reflections off the bottom of the

tank, the depth Froude number, defined as (2) 𝐹𝐷 ≡ 𝑣/𝑔𝑑, where d is the depth of the ship model

tank, should not be greater than 0.3 (Halliday, 1977). Since at the time of this report, the tow

carriage had yet to be constructed, the limitations of this were not as well documented. That being

said, practical experience for use of a ship model tank requires a minimum of three to five seconds

of data collection. The force that a model traveling at high speed (5 meters per second) would

subject onto the towing carriage itself had to be considered as well, and was ultimately the deciding

factor in terms of scaling limitations. As a result of these investigations, it was determined that a

scaling factor of 𝛼 > 49 was needed in order to effectively test the model in the Alfond W2

facility. With this in mind, the working group was presented with recommended scale factors, as

shown in table 2. Ultimately, a scaling factor of 𝛼 = 50 was chosen for the design of the model.

Table 2: Recommended scaling factors for the DTMB 5415 model

2.4 Initial Development of SolidWorks CAD Model

Originally, one of the team members that was to be involved in the project had an extensive

knowledge of CAD software, and had intended to serve as the lead developer of this portion of the

project. Unfortunately, shortly after the start of this project, this particular student had to rescind

his offer to participate, which subsequently created a new distribution of the project’s roles.

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Among them was that of the author taking up the lead role in development of the CAD model.

Given that this author had no prior experience with CAD or surfacing software prior to the

commencement of this project, a significant portion of time during the initial development stages

was spent accomplishing tutorials and learning how to utilize the provided SolidWorks software.

This long learning curve was later initiated a second time when starting to use a different software

program that specialized in 3D surfaces, known as the commercially available Rhinoceros 5.0. The

use of this software is discussed in depth later on in this report (see chapter 3.2). A file with the

surface geometry of the DTMB 5415 was found available from the SIMMON 2008 conference

website, which was a workshop on the verification and validation of ship maneuvering simulation

methods. However, this initial file, provided as an .igs file, proved to be very messy upon

incorporation into the SolidWorks CAD software. It was provided as just one half of the ship hull,

in which multiple faces of the surface were found to be in conflict, having disjointed intersections

at weird angles, which caused failures while attempting to convert the surface into a solid object.

Figure 8: A 3D rendering of the initial CAD file found available on the SIMMAN 2008

conference workshop’s website. Only one half (starboard of the hull was provided, and this

surface had numerous flaws preventing the conversion of the surface into a SolidWorks

solid part. These errors were exhibited by either gaps between faces, or intersecting faces

which did not share any common edges, such as between the bow and midship sections.

This rendering was created using the commercially available software, Rhinoceros 5.0.

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Figure 9: Intersecting faces found in the surface geometry of the original .igs file.

An example of these intersecting faces is shown in figure 9. The initial solution to this issue

was undertaken by the splitting and removal of a small portion of these faces using the SolidWorks

software, and the incorporation of a fillet to bridge the resulting gap. This particular feature was

utilized for a few reasons, chief among them being its ease of use – further development of the

project was unable to proceed without a working CAD model, and the length of overall geometry

that seemed to be affected appeared minimal; for comparison, at the upper deck surface where it

was the largest, the fillet represented only just 2.3% of the ship’s overall length (see figure 10).

That being said, it was noted that the fillet feature did utilize an automatically created radius to

bridge the gap between the two surfaces, and did not necessarily keep the curvature between them

continuous, although, it was also reasoned that it would be possible to fix any major errors by a

process of hand sanding of the surface, such as was performed in the construction of the FPSO.

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By proceeding with this plan of action, the issue of transforming the initially provided surfaces

into a well-faired hull in the CAD software became a secondary objective to that of creating a well-

faired hull through fabrication, while concurrently, further development of assembly and

construction methods for the model was able to take place.

Figure 10: A 3D rendering of the model’s hull surface created utilizing the SolidWorks

software. The highlighted region shows the area represented by the “fillet feature.”

After the flaws in the initial surface were corrected, the surface was able to be converted

into a solid object. This was done by first mirroring the surface across the neutral plane, and then

joining both surfaces together. This required some exploration of the knitting functions provided

by the SolidWorks software, and once again, progress was not gained easily. A large amount of

trial and error, in addition to some manual adjustment of some conflict areas the software could

not automatically correct for, eventually led to the creation of a solid mesh. After the addition of a

deck surface, the surface had finally completed its transformation into one solid object. Now that

the model was a working part recognized by the SolidWorks software, a much deeper exploration

of the geometry was able to be undertaken. Upon scaling the part to the decided upon scale, the

software was able to calculate moments of inertia, centers of mass, volume, and weight of each

piece. This allowed the group to estimate the weight of the bare model given the foam available

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for construction, in addition to determining how much mass was left available for proper ballasting

of the model. Thus, the working group was pleased to determine that approximately half of the

model’s weight was able to be reserved for ballast purposes, a condition which would allow for

considerable flexibility in correction and replication of moments of inertia, center of mass, and

other particulars used in the calibration of the model (for further details, see appendix E).

2.5 Designing for Manufacturability

Due to the lack of a budget available for model construction, it was determined that the

model needed to be constructed in the most cost efficient manner possible. It has already been

mentioned that one of the assets available for this project was the procurement of leftover foam

from a prior project, originally utilized in the construction of the wind tunnel nozzle for the Alfond

W2 Ocean Engineering Lab. Given that such resources can sometimes have a habit of disappearing

when left in storage at a center for advanced engineering, it was decided that for safekeeping, a

few of the sheets would be moved into the MOOR Group’s lab in Crosby Hall. During the process

of this move, a few important observations occurred. First and foremost, it was observed that each

of the foam sheets, while identical in length and width, varied in thickness by as much as 50% – a

detail which would invariably add complication to any design. Secondly, upon lifting up each sheet

individually, it was observed that while being supported at either end, the sheet suffered from a

severe deflection at the midsection due to its own load. Although none of the sheets cracked during

transport, this observation presented a concern in terms of the overall strength of the model in

relation to the force that would be exerted upon it by the tow carriage. Finally, while carrying the

sheets across campus, it was noticed that the edges of the material were prone to crumbling while

being handled or compressed. Even though these deficiencies in the primary model material were

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significant enough to be noteworthy, the use of this available foam was at the time still the

preferred choice in comparison to the cost of outsourcing the model. The issues presented by the

use of the leftover foam were rectified by certain considerations undertaken in the design process.

For example, given the various thicknesses of the sheets, it was decided that only sheets of the

thickest foam, at 7.5 cm, would be utilized for the construction of the model. This would allow the

thinner foam slices to be available for test cuts of the machine and refinement of the cutting

process. The concerns that were presented regarding the durability of the foam material were

mitigated by the incorporation into the design planes of a thin layer of fiberglass, spread across

both the entire outer hull surface as well as the length of the inner decks.

Figure 11: Division of the model into five sections, corresponding to a 7.5 cm sheet thickness.

Another major area of initial concern for the manufacturability of the model was the

incorporation of the sonar dome featured on the lower section of the bow of the 5415. Given the

complex geometry of this part, which is shown in figure 12, it was decided that it could be difficult

to replicate this section in foam. With that in mind, Dr. Friess, of the University of Maine, as well

as later Dr. Read, of Maine Maritime Academy, generously offered up the use of their own 3D

printers. Although neither of the printers were large enough to print the bow in one piece, by

incorporating the use of interconnected slots into the printing process, the part could be printed as

multiple pieces and then assembled together. Another advantage of 3D printing the bow was the

added possibility of making the model modular. This would open up the opportunity for the model

to be used as a test platform for future research; for example, by potentially swapping out the bow

section for another bow, the effects of differing resistance characteristics could be explored.

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Likewise, if the stern section were to be 3D printed, then the original stern section could be easily

swapped out for another with differing characteristics. In regards to the goal of keeping the design

of the model as modular as possible, it was decided that these parts would be attached via a bolt

and nut connection, while the outer surface connection would be water-sealed. Even though the

prospect of adding an additional middle body into the design was still appealing to the working

group, it was decided to be cost-prohibitive and excluded from further design considerations.

2.6 Methods Available for the Cutting of Model Foam

The original methodology intended to be utilized was that of the CNC water jet cutting

machine available at the Advanced Structures and Composites Center. However, cost of operating

the machine proved to be slightly prohibitive. In addition, although one of the members of the

team undertook training on the use of the machine, she was only trained in 2-dimensional cuts.

Further exploration on the use of the water jet also led to an understanding that it had a possibility

of potentially creating small ripples in the surface edge, when operated near its most shallow

angles. When it was considered that resistance experiments were a primary objective for the testing

of the model, it became imperative to ensure that the model design satisfied the requirements for

a well-defined model. This is described by having a smooth outer surface, equivalent to that

achieved with a 300 to 400 grit wet and dry sandpaper, by ensuring the geometric tolerances are

within ± 1.00 mm for breadth and depth of the model and within ± 1.5 mm for its length, and by

also ensuring that expected areas of flow separation, such as the boundaries of bulbous bows or

transom sterns, remain well-defined (ITTC, 2011). With all of these considerations in mind, an

exploration began to look for different methods to utilize for the cutting process.

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After a brief period of exploration of different methods, it was decided that the usage of a

5 axis CNC router was the preferred method. This would allow for greater precision in terms of

the cuts to be performed, which would increase the success of fabricating a well-defined model.

After a search of various CNC options, it was determined that due to the options available

financially, there was still a significant limitation placed on fabrication by only being able to cut

one slice of foam at a time. This would inspire the need for a reference point, which would have

to be common on both on the cutting table, as well as on each individual sheet of foam to be cut.

Given the limitations of the CNC router tables available for usage, it was also decided that only

the middle body would be cut by the CNC machine, while the bow section would be 3D printed

using multiple parts. The stern section would then either be 3D printed as well, or could be cut

from foam as an individual section and affixed afterwards by either bolts or epoxy.

Figure 12: The complex geometry of the DTMB 5415’s bow section.

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2.7 Revision of the Model Design Process

After further consideration of the original design intent, it was determined that a

reevaluation of the goals for model construction was needed. Given that the model was initially

intended to serve as a validation tool for the W2’s tow carriage, it became apparent that in order

to successfully accomplish this, the model needed to be constructed within a very strict tolerance.

Even though the cutting for each slice of foam by CNC would be very precise, it was determined

that it would be extremely difficult to not accidentally introduce minute errors during the process

of stacking and epoxying the sheets together for an end product. In addition, it was determined,

through the use of surface curvature analysis tools available in Rhinoceros, as well as through the

construction of a small 3D test print of the entire hull surface, that the filleted feature introduced

through SolidWorks was not a precise enough feature for the tolerances required.

Figure 13: Utilizing Rhino’s zebra analysis tool to analyze the curvature of the CAD file

created using the filleted feature. Note the discontinuities that are visible in the stripes,

especially near the upper deck surface, as they cross from the bow into the mid-section.

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With all of these factors in mind, a two pronged approach was developed for the revision

of model design process. The first, and preferred approach, was the preparation of a proposal for

financial assistance to have the model constructed by either a professional model maker, or cut as

one block of foam by a large CNC machine capable of such a cutting depth. The second approach

was to continue forward with the construction plans that had been developed thus far, and would

use the CNC machine graciously offered up by Stephen Abbadessa, manager of the Crosby Lab at

the University of Maine, to cut each slice individually. Afterwards, all of the foam sheets would

be stacked together, with a layer of epoxy laid in between each pair of foam sheets. In order to

maximize the amount of time that was left available before the beginning of the fall semester, it

was decided that the development of both approaches were to be undertaken simultaneously, while

also attempting to fix the surface flaws that were inherent in the filleted version of the CAD model.

Figure 14: A rendering of the hull using the surface curvature analysis tool available in

Rhinoceros 5.0. In this image, the filleted feature has been removed, and the model has

been rectified in a different location, exhibited by the gap that appears in the hull’s surface.

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3 Conclusions and Recommendations for Future Work

3.1 Proposal for Additional Model Funding

Since the MOOR group had previously obtained estimates in 2015 for the construction of

a professionally built model FPSO, which would have been similar in length and displacement to

the size of the scaled model DTMB 5415, it was relatively easy to prepare a brief document which

outlined the advantages of a professionally fabricated model, or alternatively, having the hull

surface cut from one continuous block of material. The advantages of a well-constructed model,

especially in terms of its usefulness as a validation case for the tow carriage to be implemented at

the Alfond W2 Ocean Engineering Lab, were quite significant. As such, a document was prepared

which outlined these advantages, and was submitted to the development team at the center for

consideration. At the time this report was submitted, a decision had yet to be officially reported to

the team. A more complete discussion of this subject is described in appendices C and D.

Figure 15: Curvature analysis of the hull form after the usage of the join surface feature in

Rhino 5.0, in addition to adjustment of end constraints, adjustment of the mesh, and the

manual translation of certain control points along the upper edge. This image was

reproduced using the same geometry depicted in [14].

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3.2 Analysis of Surface Curvature and Fairing of the Hull’s Surface

Through the use of the Rhinoceros 5.0 software, the surface curvature of the original .igs

file as well as the filleted version created via SolidWorks were both analyzed. In figure 13, it is

easy to see the discontinuities that were evident in the zebra stripes which were rendered on the

surface of the hull. These discontinuities were especially prevalent in the portion of the filleted

section near the upper deck surface, and the severity of the discontinuities visible in the stripes

roughly correlated with the severity of the discontinuities between faces of the surface. Figure 14

shows another analysis, in this case performed on the original surface file, although it was trimmed

using Rhino. The program also included a blend surface feature which was advertised as being

able to analyze and match the curvature between unattached faces of a surface. Perhaps due to the

complicated geometry of the DTMB 5415, even the best attempts to effectively utilize this tool

yielded only marginally improved results, as shown in figure 15.

Upon further analysis of the initial CAD file, a thought occurred. If there was the existence

of a point where the two faulty faces intersected, which was also coincident with the upper

boundary line of the ship, then perhaps each individual faulty face could be split along a line of

action that would run directly through this point, in a perpendicular direction to the keel. Through

a close investigation, such a point was indeed found, and a cutting plane was formed coincident

with the prescribed line of action, and perpendicular to the longitudinal direction of the ship’s hull.

This allowed each of the faulty faces to be split individually along this line of action, and

subsequently joined together in a more seamless manner. The result of this approach is exhibited

in figure 16, where it is clear that the resultant changes in the curvature of the surface due to the

adjustments of the initial CAD file have been minimized.

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Figure 16: By utilizing the prescribed line of action approach, minimization of any changes

in the curvature across the surface of the hull was successfully accomplished.

The line of action that was used can be seen through an analysis of the mesh presented in

figure 16. By starting at the surface mesh of the parallel middle body of the ship, while moving

forward in a longitudinal direction towards the bow, the described line of action can be found

where the first group of triangles in the surface mesh of the middle body reach their maximum

respective heights, immediately before being broken up into more “square shaped” triangles.

Through the use of this method, the only apparent differences in the curvature were found near the

upper portion of the surface of the ship; which, being located above the designed draft for the

model, would be irrelevant to most tests that would take place in a tow tank.

3.3 Next Steps in Project Development

Unfortunately, at the time of submission of this report, there was not yet an answer

regarding the potential for the model to be funded. While this question remained, development of

both fabrication options reached a temporary standstill, although significant progress in each

method was accomplished. The next logical step would be the fabrication of a larger 3D print of

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just the bow section itself, in order to visually and physically confirm the success of the splitting

methodology undertaken and described in chapter 3.2. Although the findings of this report clearly

demonstrate that the objectives for development and experimentation of this model would be most

effectively met through fabrication by the cutting or carving of one continuous block of material,

as described in the preferred method outlined in chapter 3.1. Even though this was the

recommended methodology, considerable milestones were still simultaneously achieved in the

secondary methodology, such as the development of the assembly design featured in figure 17.

This particular fabrication plan shows the three individual section bodies, the connection plates for

securing the 3D printed parts to the foam, as well as how to confirm the proper orientation of each

section through the usage of a three bolt pattern.

Figure 17: SolidWorks rendering of an assembly diagram, with 3D printed bow and stern.

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3.4 Conclusions

In this report, the initial design objectives were presented and constantly revisited

throughout the development of what would become the two final designs. Froude scaling was

utilized to accurately relate the particulars of the ship model to the full size ship. These factors,

along with limitations placed on the scaling factor due to the parameters of the ship model basin,

in addition to the limitations enforced by the particulars of the towing carriage, were all considered

in the selection of the 1/50 model scale. The arduous process of transforming the initially provided

CAD surface file into a workable model, complete with properly trimmed lines, was undertaken

and described in depth. A description of the development of the two final manufacturing plans was

presented, along with recommendations for fabrication. The next logical steps for evolution of the

project were outlined, and should be undertaken while awaiting an answer regarding model

fabrication. Thus, although construction of the model has not yet occurred, development of the

model remains on track, with the current schedule calling for experimental testing in early 2017.

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Works Cited:

5415 Hull (Bare). Copenhagen, Denmark: SIMMAN, 2008. IGS.

Cope, David M. (2012). Design of a Free-running, 1/30th Froude Scaled Model Destroyer for In-

situ Hydrodynamic Flow Visualization. Master’s Thesis, Massachusetts Institute of

Technology.

De Oliveira Costa, Daniel. AT Report – Informal Report on the Training. MOOR Group,

University of Maine, Summer, 2015. Print.

Diez, M., Serani, A., Campana, E. F., Goren, O., Sarioz, K., Danisman, D. B., Grigoropoulos, G.,

Aloniati, E., Visonneau, M., Queutey, P., Stern, F. (2005). Multi-objective Hydrodynamic

Optimization of the DTMB 5415 for Resistance and Seakeeping. Retrieved June 8, 2016,

from http://www.sname.org/HigherLogic/System/DownloadDocumentFile.ashx?Docum

entFileKey=130c0fc1-7dea-5c8b-c5fe-1a27c7b84fa7.

Halliday, R. F. "Making the Best Use of a Small Ship Model Tank." 1976. Print.

ITTC – Recommended Procedures and Guidelines. Publication no. 7.5-01 -01-01. International

Towing Tank Conference, Sept 2011. Web. Retrieved July 14, 2016, from

http://ittc.info/downloads/Archive%20of%20recommended%20procedures/2011%20Rec

ommended%20Procedures/index.pdf.

Jones, D. A., & Clarke, D. B. (2010). Fluent Code Simulation of Flow around a Naval Hull:

The DTMB 5415 (pp. 1-34) (Commonwealth of Australia, Department of Defence, Defence

Science and Technology Organization, Maritime Platforms Division). Fishermans Bend

Victoria: Maritime Platforms Division, Defence Science and Technology Organisation.

Newman, J. N. "Model Testing." Marine Hydrodynamics. Cambridge, MA: MIT, 1977. 8-47.

Print.

Olivieri, A., Pistani, F., Avanzini, A., Stern, F., & Penna, R. (2001). Towing Tank Experiments of

Resistance, Sinkage and Trim, Boundary Layer, Wake, and Free Surface Flow Around a

Naval Combatant INSEAN 2340 Model (Tech. No. 421). Iowa City, Iowa: Iowa Institute

of Hydraulic Research.

SIMMAN, "Workshop on Verification and Validation of Ship Maneuvering Simulation Methods,"

2008. [Online]. Available: http://www.simman2008.dk/5415/5415_geometry.htm.

Retrieved May 31, 2016.

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Stern, F., Longo, J., Penna, R., Olivieri, A., Ratcliffe, T., & Coleman, H. (2000). International

Collaboration on Benchmark CFD Validation Data for Surface Combatant DTMB Model

5415. Twenty-Third Symposium on Naval Hydrodynamics, 402-422. Retrieved June 16,

2016, from http://www.nap.edu/catalog/10189/twenty-third-symposium-on-naval-

hydrodynamics

Zangeneh, R., & Thiagarajan, K. P. (2015). Heading Instability Analysis of FPSOs. International

Society of Offshore and Polar Engineers.

Zangeneh, R., Thiagarajan, K. P., Urbina, R., & Tian, Z. (2016, June 06). Effect of viscous damping

on the heading stability of turret-moored tankers. Ships and Offshore Structures, 1-15.

doi:10.1080/17445302.2016.1165554

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4 Appendices

Appendix A – Engineering Constraints

Economic: Economic constraints were the most prevalent constraints in the development of this

project. As there were no specified funds set aside for the construction of the model, resources

were sourced at little to no cost. In addition, the usage of the manufacturing processes themselves,

such as the high pressure water jet cutter at the Advanced Structures and Composites Center, had

to be considered in the design process. Thus, it was crucial to minimize both the cost of materials

being used, as well as the cost associated with the fabrication processes. In addition, an exploration

was undertaken to consider the multiple ways in which this model can serve as a testing platform

not just for the current prescribed tests, but also for future work in the MOOR group. This would

allow the maximization of not just the efficiency of production, but also the overall value of the

model.

Manufacturability: The consideration of the manufacturability of the model was another primary

constraint. Depending upon the materials sourced, different manufacturing processes could be

used, and the design was constantly adjusted in consideration of facilitating the assembly of the

model as a whole. The tolerances for each manufacturing process was examined in depth, as well

as the limitations of each fabrication method of the full model. Designing for manufacturability

eventually became one of the most important considerations in the iterative design process, and

recommendations were made for model fabrication based on the abilities of each process to

manufacture the model to within the required tolerances.

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Sustainability: Although the sustainability constraints for this project were often secondary in

consideration to the primary constraints of an economic and manufacturing nature, they are

nonetheless still important to consider. The main sustainability constraints involved minimizing

the amount of waste created through production, in addition to minimizing the waste produced and

energy consumed by the manufacturing processes themselves. Further evidence of the

consideration of sustainability constraints can be found through the objective of maximizing the

life cycle of ship model.

Health and Safety: The primary safety concerns in the development of this model can be mitigated

through careful selection of materials as well as control over their manufacturing processes. To

mitigate these hazards, consideration was undertaken to choose the least dangerous and toxic

options available. Another potential safety hazard is due to the size and weight of the model. As

the model was designed to weigh approximately 70 kg when loaded with proper ballast, caution

will need to be observed in order to ensure that the model does not have an opportunity to fall over

onto someone during construction, during transportation to or from the Advanced Structures and

Composites Center, or transportation in or out of the wave tank basin at the Alfond W2 Ocean

Engineering Lab. A final safety consideration lies in ensuring that the model has the necessary

strength characteristics so as to not collapse under either its own load, or as a result of the forces

being subjected upon it when being dragged by the tow carriage at the W2 Lab. Since the use of

fiberglass and epoxy was previously chosen to provide this additional longitudinal strength; upon

the culmination of the decision for the final design, the calculation of the required number of

fiberglass layers needed to provide the appropriate additional strength becomes a trivial exercise.

As far as this author can see, there were no significant environmental, ethical, legal, social, or

political constraints that affected either the design or development of this project.

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Appendix B – DTMB 5415 Full Scale Particulars

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Appendix C – Model DTMB 5415 Construction Options and Recommendation

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Appendix D – Quote Obtained by MOOR Group for Construction of a Model FPSO

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Appendix E – Weights Table for Scaled Model DTMB 5415