Hull Form Development for a High-Speed Trimaran Trailership

Hull Form Development for a High-Speed Trimaran Trailership

I. Mizine Computer Science Corporation, Advanced Marine Center, Washington, DC, USA

S. Harries & M. Brenner Friendship Systems GmbH, Potsdam, Germany

ABSTRACT: The paper describes the hull form development of an innovative High-Speed Trimaran Trailership (HSTT), capable of carrying about 160 53ft-trailers in the speed range from 26 to 32 knots. Calm-

water performance, in particular the trade-off between cruise speed and top speed, is studied on the basis of first-principle methods, utilizing non-linear free surface potential flow and boundary layer calculations. A

baseline comprising the center hull and the side hulls was created in a traditional design process. These

shapes were utilized as input to a partially parametric model by which displacement distribution, bulb shape

and the side hull stagger could be changed. The design space was investigated by means of various Design-

of-Experiments to develop an appreciation of dependencies (exploration). A multi-objective genetic

algorithm was then repeatedly used to produce non-dominated solutions (exploitation). Energy consumption

at two prominent speeds was simultaneously improved, while observing important constraints such as initial stability. The proposed HSTT design addresses dual use as a fast ship for military mobility and as a high-

speed complement for trade along Interstate 95 at the US East Coast.

1 INTRODUCTION

High-speed sea transportation usually calls for

unconventional hull forms since the energy

consumption of large conventional monohulls scales

up too rapidly with an increase in speed. A trimaran

concept was therefore studied for both commercial

and military applications, introducing an innovative

High-Speed Trimaran Trailership (HSTT), see

Figure 1.

The HSTT follows the requirements set by America's Marine Highways (AMH), an evolving

US strategy, according to which commercial trailerships could be utilized to decongest traffic

along the US East Coast, in particular on Interstate 95 (Boston-Miami). High-speed sea transport over

more than 1 000 nautical miles would need to be covered.

The HSTT also supports the requirements associated with capability for military mobility in

many inter and intra theater Sealift and Sea Base

scenarios. Again fast ships would be required but

with a range of unrefueled voyages of up to 9 500

nautical miles.

Consequently, a dual use concept was worked

out which would hold the potential for the US armed

forces to lease necessary and capable ships for their

missions that, otherwise, would be used regularly to

shift transport from road to sea.

Figure 1: Hull form of the High-Speed Trimaran Trailership

The paper focuses on the hydrodynamic

development of the trimaran hulls on the basis of

simulation-driven design, i.e., a process in which

first-principle methods, here for numerical flow

simulation, are intensively utilized not only to

IX HSMV Naples 25 - 27 May 2011 1

analyze a few competing alternatives, but rather to stir the creation of many variants by searching for

favorable performance (Harries 2008). The paper's layout is as follows: Principle ideas

and requirements for the trimaran design are covered

in section 2. The partially parametric modeling

approach taken for creating variants and the flow

simulation used to assess their hydrodynamic

performance are outlined in section 3. Section 4

elaborates on the formal studies to understand the

design space and to identify promising candidates.

Section 5 presents the conclusions along with an

outlook.

2 DESIGN RATIONALE

The HSTT implements the type of the hull forms

developed as prototypes for costal trimarans by

(Vom Saal et al 2005) and Heavy Air Lift Support

Ship (HALSS) by (Mizine et al 2009). The HSTT

hull form has most of the displacement in the center

hull, with small waterplane area (SWA) side hulls providing stability, as show in Figure 1. This ship

carries 160 x 53ft trailers in eight bays or 240 x 40ft trailers in 10 bays on two decks. The upper deck has

11 rows while the second deck has 10 rows to provide room for structure (pillars) and access.

All diesel engines are in the central hull with the main engine aft and the diesel-generator sets

forward. The total installed power is about 66 MW,

providing 54 MW at the propellers. Diesel engine

exhausts are led through the crossover deck from the

center hull to the stack as shown in Figure 2.

Figure 2: Main propulsion arrangement

Several machinery options were evaluated, with the

following combination identified as most promising:

• Center hull: 19.2 MW medium-speed diesel engine geared to a fixed-pitch propeller,

aligned with a contra-rotating 14 MW Azipod, arranged to recover wake losses.

• Each side hull: 12 MW electric motor geared to a waterjet, powered by diesel-generator

sets located in the center hull to provide good

access for maintenance.

There are two main reasons to select contra-rotating

propulsion. Firstly, there is improved efficiency by

absorbing rotational losses with the aft propeller.

Secondly, contra-rotating propulsion allows the

beneficial distribution of thrust load over a larger number of propeller blades in a confined space. The

added total blade area then results in improved cavitation characteristics so that more power can be

fed to a contra-rotating system with smaller propellers. This is especially important for a sealift

asset with requirements for shallow-draft port accessibility. Without restrictions on the diameter,

the benefit of a system with less noise and cavitation

remains. Finally, an azimuthing electric propulsion

unit yields excellent maneuvering characteristics

The selection of waterjets in the side hulls is

based on overall fuel efficiency in consideration of

the dual use profile. When operating at speeds up to

26 kn only the center hull propulsion would be

utilized. For higher speeds the side hull waterjets

would have to be brought into action. Hence, the

configuration avoids substantial wind milling at

speeds below the cruise speed.

The HSTT hull forms were developed in

consideration of arrangement requirements, stability constraints and hydrodynamic optimization. A major

factor in the power requirement of a trimaran is the interaction between the wave train produced by the

center hull and the wave trains produced by the side hulls. Ideally, the two should counteract each other

at the primary speed(s) of interest. Lessons learned from HALSS numerical analysis and model testing

confirmed that wave interference and optimization

are the key issues for the proper design of trimarans

(Mizine at al 2008, 2009). Hence, they were

intensively studied within the HSTT project.

3 MODELING AND SIMULATION

The formal process of hull form development was

completely set up within the Computer Aided

Engineering (CAE) software FRIENDSHIP-

Framework, making use of the interfaced flow code

SHIPFLOW. The FRIENDSHIP-Framework served

as both the geometric modeling engine and the

controller for simulation-driven design

(FRIENDSHIP 2011).


3.1 Partially parametric model

Since an advanced initial design had been established in Rhino prior to the systematic hull

form development, it was decided to utilize the existing lines and apply partially parametric

modeling for variation. Contrary to a fully parametric model in which the entire geometry is

hierarchically built up from scratch, e.g. (Harries 2010), the partially parametric model takes an

existing shape and only specifies the desired changes

by means of parameters. The general idea is that of a

chamber of mirrors in an amusement park: Distorted

mirrors lead to images that are different in shape and

yield more or less favorable variants.

Figure 3: Example variations of center hull without skeg

(top: baseline, middle: highest increase in displacement,

bottom: largest inflation of bulb)

Figure 4: Example variations of side hull position

(top: baseline, middle: most forward position of side hulls,

bottom: most outward position of side hulls)

Here a longitudinal shift of sections was applied for the trimaran's center hull, employing the Generalized

Lackenby approach of swinging sections longitudinally in a highly concerted manner (Abt

and Harries 2007), while a surface delta shift was used to inflate and deflate the bulb, Figure 3. The

side hulls were not changed in shape but shifted longitudinally and transversally while maintaining

starboard-port symmetry, Figure 4. Up to seven parameters of the partially

parametric model were controlled as free variables

during the investigations, three of which altered the

displacement volume and longitudinal center of

buoyancy of the center hull, two modified the bulb's

volume and profile while the last two changed the

position of the side hulls.

3.2 Computational Fluid Dynamics

Typical options of Computational Fluid Dynamics

(CFD) for calm-water hydrodynamics currently

encompass potential flow analysis of the free wave

system along with thin boundary layer calculations

and, alternatively, fully viscous flow analysis

including the free surface. The right choice of CFD

depends on the specific trade-off between accuracy,

speed and usability.

The trimaran hulls being rather slender and the

wave making being of paramount importance at speeds of 26 kn and 32 kn (corresponding to Froude

numbers of 0.32 and 0.39, respectively, at a reference length of around 180 m), a non-linear

potential flow simulation with free sinkage and trim was considered to give meaningful results for the

ranking of variants. Therefore, the optimizations were based on SHIPFLOW, capitalizing on the fast

turn-around time of the software's zonal approach

(FLOWTECH 2004, 2009).

Figure 5: Wave generation of center hull at

26 kn (upper part) and 32 kn (lower part)

Figure 5 shows the wave generation of the center

hull without the side hulls at the two speeds of

interest, namely 26 kn and 32 kn. The changes in

wave lengths and the distinct interferences can be


nicely observed by comparing the upper and lower halves in Figure 5. The different positions and

pronunciations of crests and troughs readily give rise to the assumption of favorable and unfavorable side

hull positions but also directly suggest that a

beneficial side hull position for one speed might be

less advantageous for the other; cp. e.g. (Yang

2001), (Brizzolara et al 2005).

Both the potential flow and boundary layer

calculations done with SHIPFLOW are relatively

fast, typically 5-10 minutes of CPU time on an

average workstation.

4 FORMAL STUDIES

4.1 Design with and without skeg

The proposed design featured a conventional

propeller plus a contra-rotating POD drive in tractor mode. An important question for the hull form

development therefore was whether the characteristics of the center hull's propulsion train

would have a significant influence on wave making that needed to be taken into account during the

optimizations. Two designs with and without skeg

were considered as options. Figures 3, 4 and 6 give

an impression.

For clarification a first Design-of-Experiments

(DoE) was conducted in which only the center hull

was altered, while the side hulls were omitted. For

each form variation the two alternatives, one with

and the other without skeg, were concurrently

analyzed at cruise speed and at top speed.

Figure 6 displays the pressure distributions over

the center hull for a representative variant. It can be

seen that close to the skeg the pressure distribution

changes, as is expected, while closer to the free

surface along the hull, i.e., the regions that stimulate

wave generation the most, very little differences

occur. Figure 7 exemplarily depicts the overlay of

waves for a variant with and its counterpart without

skeg. The isolines of wave height that appear very

close to each other, in particular, aft of the hull, stem

from the alternative aftbodies. In the forebody there

is practically no difference to be seen while the

deviations in the far field are rather subtle. When plotting the wave resistance for all variants from the

DoE for the designs with skeg (ordinate) vs. the designs without skeg (abscissa), Figure 8, it becomes

apparent that the ranking of designs is not really affected. For the lower speed of 26 kn the variants

almost line up on a straight line (blue diamonds). For the higher speed of 32 kn there are some minor

oscillations for variants (red rectangles) that perform

about equally well.

Figure 6: Pressure distributions on center hull

with and without skeg at 26 kn (side hulls omitted)

Figure 7: Overlay of waves generated by center hull with and without skeg (side hulls omitted)

Figure 8: Ranking of variants with regard to wave resistance

for center hull with and without skeg (side hulls omitted)

It was therefore concluded that wave resistance optimizations could be undertaken either with or

without skeg and then reasonably generalized afterwards. It was decided to build further

investigations on the baseline with skeg. Nevertheless, the correlation seen in Figure 8 gives

the comforting thought of not having to repeat the


full exercise if a vessel without skeg would be pursued in the end.

4.2 Exploration

A comprehensive DoE with 250 variants was run for

the trimaran configuration with the following free variables acting on the shape of the main hull and on

the positions of the center hull:

• deltaCP: change of prismatic coefficient within the interval [-0.01, +0.035]

• deltaXCB: change of the longitudinal center

of buoyancy [-1%, +1%] of LPP

• midTan: change of tangent of the shift function at the main section, [-60°, + 60°]

• bulbTipDz: vertical movement of the bulb tip

within the interval [-1m, +0.8m]

• bulbFullnessFactor: increase of bulb volume

via surface shift within [0, 2]

• sideHullDx: longitudinal position of the side

hull wrt the baseline, [-10m, +20m]

• sideHullDy: transversal position of the side

hulls wrt the baseline, [-2m, +2m]

The different resistance components and some

hydrostatic properties were evaluated for every

variant, with the total resistance being the main

objective. In terms of speed this investigation was

limited to the cruise speed of 26 kn. The purpose of

the DoE was to develop an understanding of the

systems’ behavior within the design space and to

identify promising regions for further exploitation (see following section).

Figure 9: Pressure distribution on trimaran with skeg at 26 kn

The results of the DoE showed a strong dominance

in the influence of the side hulls’ position, while the

influence of the main hull parameters, being much

smaller, basically vanished in the background noise.

As previously discussed the effect of the side hulls

on the resistance is mostly given by the favorable or

unfavorable interferences in the wave pattern, as

well as by the change in the pressure distribution on the main hull, as shown in Figure 9.

Figure 10: Performance of trimaran variants at 26 kn plotted

for longitudinal (top) and transversal (bottom) side hull

positions

Especially the longitudinal position of the side hulls

exhibits a strong linear relationship to the resistance

within the range of the design variable, as depicted in Figure 10 top. At the speed of 26 kn it is clearly

better to move the side hull forward, as long as the LCB position is not negatively influenced above

reason. The transversal position has a smaller, but

still significant influence on the resistance, see

Figure 10 bottom. It is favorable to move the side

hulls to the inside, closer to main hull. The deciding

constraint here is the stability.

4.3 Exploitation

Due to the experiences made in the previous design

space exploration, it was decided to separately optimize for the most favorable side hull position

first, taking into account the resistances at both relevant speeds. Because the relevance of every

single speed was not known well enough to obtain a meaningful weighting in a single objective function,

a multi-objective optimization algorithm was


selected. This would produce a Pareto set of non-dominated solutions, from which a suitable

compromise design can be selected afterwards. The selected algorithm was the non-dominated sorting

genetic algorithm NSGA-II (Deb 2002) with 13

generations and a population size of 12 per

generation. The mutation probability was set to 0.01

and the crossover probability to 0.9.

The first optimization run with the full range of

the side hull variables and constant center hull shape

produced ca. 150 feasible variants and a distinct

Pareto frontier, see Figure 11. The first conclusion

that was evident from this set of designs and the

additional data for the higher speed, was the

opposite influence of the longitudinal side hull

position on the two resistance values. Moving the

side hull forward is favorable at 26 kn, but detrimental for the resistance at 32 kn. Furthermore,

moving the side hull inwards has small influence at 26 kn but a more significant effect at 32 kn. An

interesting feature that could be observed was a hook-like local minimum in the upper half of the

Pareto set. The design located in the tip of this hook (c in Figure 11) and four other interesting designs

were selected as starting points for further

investigations. The other four designs were:

• The design (a) with the lowest power

requirement at 26 kn. The max. power

requirement (at 32 kn) is higher than for the

baseline.

• The design (b) with the same max. power

requirement as the baseline (the same engine

could be used) but a reduced power

requirement at 26 kn.

• The design (d) with the same power

requirement as the baseline at 26 kn, but

smaller max. power requirement.

• The design (e) with the lowest max. power requirement, but higher power requirement at

26 kn than the baseline.

A local multi-objective optimization (NSGA-II) was

started from each one of these designs. The design

variables for the center hull were included in these

optimization runs, while the range of the side hull

variables was restricted to the vicinity of the

respective current values. Each run produced 50 to

250 additional variants. The purpose was to find the

optimal center hull for every selected position of the

side hulls, allowing for some adaptation of this

position.

While the other runs mainly remained in the region of the Pareto set, the run started from the

variant in the tip of the “hook” produced the most

interesting results by further enhancing this local minimum.

Figure 11: Effective power requirement of trimaran variants at

26 kn (horizontal axis) and 32 kn (vertical axis)

Figure 12: Design space of the side hull positions. Longitudinal

position on the vertical axis, transversal on the horizontal


Some more interesting information is given by inspecting the distribution of the variants in the

design space of the side hull positions, see Figure 12. Obviously, the local optimizations form

distinct clusters. The Pareto set is reflected by the

left and upper border, with the “hook” local

minimum in the upper left corner. All the designs

above the “hook” in Figure 11 are on the upper

border, the ones right and below in Figure 11 are on

the left border. This picture seems to give a good

indication for the specific shape of the Pareto

frontier.

As expected from the previous investigations,

the final optimized design has an inward and

forward position of the side hulls. The volume is

slightly decreased with almost no movement of the

center hull’s center of buoyancy. Furthermore, the bulb is significantly fuller, with a higher bulb tip.

See the details in Table 1.

Figure 13: Wave generation of trimaran at 26 kn (top) and

32 kn (bottom). Upper part of each picture: baseline, lower

part: favorable design

The favorable interferences lead to a greatly

attenuated wave pattern with a reduced energy

content, see Figure 13 for the waves at 26 kn and

32 kn. In comparison to the performance of baseline, the power requirement was reduced by 9.5% at

26 kn and by 4.2% at 32 kn. Overall, the optimized design behaves better than the baseline within the

complete speed range, as the computed speed-power curve shows (Figure 14).

Table 1: Properties of optimized design. All variable values are

equal to 0 for the baseline

Variable Value

deltaCP -9.265E-03

deltaXCB 1.921E-04

midTan -23.428 deg

bulbTipDz 0.795 m

bulbFullness 1.943

sideHullDx 19.994 m

sideHullDy -1.947 m

Figure 14: Speed-power curves for baseline and optimized design. Ranges of variation for different trim and sinkage

conditions indicated by arrows.

4.4 Robustness analysis

To ensure the robustness of the optimized design for

off-design conditions, a sensitivity analysis with respect to different trim and sinkage situations was

carried out. The drafts at the forward and aft perpendicular were set as independent design

variables and an ensemble investigation was

performed with variable values within the range of

+/- 0.5 m compared to the design draft. Aside from

the optimized design, the same investigation was

done for the baseline for comparison.


The ranges of variation with respect to the required effective power can be seen in Figure 14. It

is evident that the optimized design behaves in a very similar way to the baseline design. Therefore, it

is assured that the optimized design is not sitting on

a “knife’s edge” optimum. It will not suddenly

become much worse, if the ship is sailing at slightly

other than design conditions.

5 CONCLUSIONS

A new hull form was developed for a High-Speed

Trimaran Trailership (HSTT), utilizing the

FRIENDSHIP-Framework for simulation-driven

design. A baseline design was taken and

parametrically modified in order to bring about

optimal performance at two important speeds,

namely at 26 kn and 32 kn. The hull form of the

center hull and the side hull positions were changed

systematically in order to identify a Pareto-optimal

solution of minimum resistance while satisfying

various constraints on stability, weight and arrangement and complying with requirements on

overall beam, trim, position of longitudinal center of buoyancy etc. Hence, the best combination was

sought for low power, good stability and high loading efficiency with all cargo on two decks.

Hull form optimization of trimarans benefits widely from powerful tools comprising (automated)

hull form transformations, adequate simulation and

practical optimization strategies. This set of tools is

combined within the FRIENDSHIP-Framework. The

practical design case of an HSTT proved its

applicability. The numerical results naturally need to

be further validated in the course of model tests.

ACKNOWLEDGEMENT

The HSTT concept evaluation and test program have been sponsored by Center for Commercial Deployment

of Transportation Technologies (CCDOTT) and the

Office of Naval Research (ONR). The FRIENDSHIP SYSTEMS calculations and analysis have been

performed based on Cooperative Research Agreement

between FRIENDSHIP SYSTEMS AND CSC

Advanced Marine Center, CCDOTT and ONR.

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Abt, C.; Harries, S. (2007) Hull Variation and Improvement

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Hull Form Development for a High-Speed Trimaran Trailership

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