Realizing Efficiency Gains

for Vessels in Operation: Guide to Dynamic Trim

Optimization

June 2011

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Contents

Summary .................................................................................................. 3

Introduction .............................................................................................. 3

Variations in trim ....................................................................................... 4

What is the ‘optimum dynamic trim’? ............................................................ 5

Hull Shape Development ............................................................................. 6

Trimming Using Ballast Tanks ...................................................................... 7

Trim Testing Methods ................................................................................. 7

Finding the Optimal Trim ............................................................................. 9

Dynamic Trim Optimization ....................................................................... 10

Example of savings with Dynamic Trimming ................................................ 12

Environmental affect ................................................................................ 13

Additional Information value ...................................................................... 13

Conclusions ............................................................................................. 14

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Definitions

trim Noun /trim/ The difference between a vessel's forward and after drafts, esp. as it affects its navigability

stat·ic Adjective /ˈstatik

Lacking in movement, action, or change, esp. in a way viewed as undesirable or uninteresting

dy·nam·ic Adjective /dīˈnamik

Characterized by constant change, activity, or progress

Summary

Since the introduction of currently non mandatory initiatives to improve environmental

performance such as the Energy Efficiency Design Index (EEDI) and Ship

Energy Efficiency Management Plan (SEEMP), shipping owners have been

under increasing pressure to reduce their carbon output and take steps

towards more sustainable seafaring as well as remain profitable.

Although the EEDI will focus on new vessel builds, the EEOI can be enhanced by

applying best practices for fuel efficient operations as well as deploying latest

technological devices for existing vessels. The introduction of initiatives such as slow

steaming, weather routing, anti-fouling and trim optimization, can lead to a reduction in

fuel consumption for existing vessels as well as contribute to an improvement of ship life

cycle environmental performance.

The purpose of this guide is to explore the area of trim optimization. It will identify

current trim testing methods, the direct impact of dynamic trim on fuel savings and how

onshore cost efficiencies can be made for the long term by translating real time data into

continuous benchmarks for performance improvements and decreased emissions.

Dynamic Trimming Saves Fuel and the Environment

Introduction

It is widely known that the optimization of trim can improve vessel performance in terms

of better speed and lower fuel consumption. Experienced seafarers, through trial and

error, have been able to identify a suitable trim at which their vessel is believed to

perform at optimum level.

The growth in vessel size over recent years has resulted in various developments in

structure and hull form. The performance of the vessel as a result has increasingly

become sensitive to trim. This means that the trim of the vessel has a direct relation to

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the way the vessel performs. It is therefore now important to identify the trim (or

optimum dynamic trim) at which the vessel performs at its best. Measuring trim (or

static trim) when it is in port is easy and accurate. However to measure and monitor the

exact trim (or dynamic trim) when the ship is underway is difficult due to various factors

that influence the vessel such as speed, wind, sea state, swell, hull deflection, depth etc.

Only when the precise dynamic trim is measured in real time taking into considerations

all of these factors can the vessel be trimmed to the optimum dynamic trim and reap the

benefits of enhanced performance.

Variations in trim

All large commercial vessels are designed to perform optimally at a certain speed, draft

and trim, known as the design parameters based on which the vessel was initially

constructed. Most vessels however operate outside the original design parameters most

of the time due to differing loading condition (cargo specifications, cargo requirements,

cargo parcel size, draft restrictions, ballast voyages, etc.). To optimize the performance

of the vessel when it is operating outside the designed parameters one needs to know

the optimum dynamic trim. For any given vessel, numerous factors including

displacement, water depth and speed make it challenging to find the optimum dynamic

trim. In short, there are many inter-related variables effects when modeling the

optimum dynamic trim.

Figure 1 Trim of a container vessel during various legs

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The figure 1 above displays the trim of a 5500 TEU container vessel during various legs

(times in port included). A negative value indicates that the vessel has trimmed by the

bow and a positive value indicates that the vessel has trimmed by the stern.

Each voyage leg (indicated by green rectangles) is plotted separately. It is apparent that

the vessel was operated at different trims during the different legs.

What is the ‘optimum dynamic trim’?

Optimum dynamic trim means the trim angle at any particular displacement and speed

where the propulsion power used is lower than the propulsion power used for any other

trim angle at the same displacement and speed. Operating the vessel at its optimum

dynamic trim can result in the vessel sailing at a higher speed and/or lower propelling

power. This translates to savings in fuel as well as other economical and environmental

benefits.

Figure 2 Propulsion energy decomposition

Figure 2 above shows a break-down of the factors affecting propulsion power on a

vessel. Whilst the majority of the power is used for propelling the vessel, non-optimum

trim accounts for a significant amount of power that is wasted. This lost or wasted power

could have been saved if the vessel was optimally trimmed. The other affecting factors

on propulsion power are the prevailing sea conditions of the voyage. Although the

optimum trim will vary in different conditions, trimming dynamically means that it should

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be possible to find the most efficient trimming angle in any sea condition that the vessel

finds itself in.

Hull Shape Development

As the average ship size is growing and more modernized hull designs (i.e. especially

bow and stern designs) come into play, vessels have become more sensitive with the

optimum trim range becoming narrow band.

Ensuring that trim can be measured accurately becomes even more crucial to create

maximum fuel efficiency as the vessel should know the exact trim to keep it within the

narrow optimum trim range.

An effective bulbous bow modifies the way the water flows around the hull, reducing

drag and thus increasing speed, range fuel efficiency and stability. Large ships with

bulbous bows generally have a 12 to 15 percent better fuel efficiency than similar

vessels without them.

Bulbous bows have been found to be most effective when used in vessels that meet the

following conditions:

The length along the waterline is more than 15 meters (49 feet).

It is assumed the vessel will operate most of its time at or near its maximum

speed.

The bulbous bow is at the precise depth below the water line.

Figure 3 Bulbous bow modifies the flow of water around the hull

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Only large vessels that sail most of the time near their best speed will benefit from a

bulbous bow. This would include cargo vessels, passenger vessels, container vessels,

tankers and supertankers. These vessels tend to be large and usually operate within a

small range of speeds close to their top speed.

However the point to note here is that the bulbous bow is most effective at a precise

bow draft and not at other drafts.

Trimming Using Ballast Tanks

Significant improvements to fuel consumption can be achieved even through minor

changes in the operation of the vessel like economic steaming, super slow steaming etc.

Optimum trimming is one such area where savings can be made simply by adjusting the

water in ballast tanks.

It’s crucial for the officers on watch to be aware of the bunker and/or water transfers

onboard and the affects of this activity on the trim. Ballast operations can sometimes be

carried out by another crew on watch and information is not passed on during the watch

changeover which can add to the discontinuity causing the vessel to be operated outside

the optimum trim.

Quite often there is a perception in the shipping industry that the lighter the ship, the

lesser power is required to propel the vessel efficiently. However this is not always true.

A vessel sailing with normal ballast at optimum trim can perform better than the vessel

sailing just on minimum ballast. This is because the water ballast can be used to obtain

the proper immersion of the bow and stern of the vessel which allows the propeller and

bulbous bow to be effective.

Trim Testing Methods

Static, virtual and dynamic testing

Optimal trim has been traditionally explored at the design stages with model (tank)

testing, the results of which are validated during the delivery sea trials. The output of

the model testing on various trims is summarized in a trim chart, or matrix, showing the

optimum trim for a reasonable number of speeds and drafts. These charts provide the

crew with a valuable indication of the necessary trim adjustments. There are however

some limitations in the procedure, one of which is given by the fact that the actual speed

and draft of the ship is not always one of those used during the tank testing, thus the

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need of interpolating in the matrix; the interpolation assumes a linear variation between

the values given by the charts, and this assumption is not always true, mainly for drafts

and trims close to the bulbous bow submersion point.

Another significant limitation is the impossibility of taking into account the vessels

deflection and effects of dynamic, real time conditions like wind, as well as the effects of

squatting; in other words, the crew has no precise indication on the actual drafts of the

ship while at sea, but only know the exact drafts prior to departure.

Moreover, determining the success of the model tests and comparing them with daily

real time and full size operations can be difficult due to the lack of tools and effective

feedback.

As an alternative to towing tanks, hydrodynamic simulations are run at full scale which

requires running through the conditions at which the vessel normally encounters

(loading, weather etc.) and collecting data to form more precise trim charts than with

model testing. However such tests are very expensive and time consuming, and this

method still suffers to a certain extent with the limitations of model testing, given that

enforcing it on daily operations may not reflect the constantly changing prevailing

conditions. In addition, these methods rely on testing in specific and often optimal

conditions, which do not reflect the precise varying circumstances which the vessel will

face in daily operations.

Virtual testing or computational fluid dynamics (CFD) is carried out by computers

simulating the water flow around the 3D model of the vessel. CFD works well to evaluate

changes to a design quickly without having to build a new model nor to carry out

expensive runs in the model basin; it is therefore a good solution to either integrate the

trim charts obtained in the model basin or to build more detailed trim charts, thus

reducing the error introduced by the interpolation The accuracy of CFD testing is rapidly

improving, even if some challenges remain mainly in the simulation of the turbulent flow

field around the stern of the ship.

The information gained from CFD testing is therefore a more detailed trim chart when

compared to the one obtained through tank testing, however the difficulties of knowing

the actual draft of the ship while at sea and the impact of external conditions are the

same of those given by tank tests.

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Figure 4 Snapshot comparing static and dynamic trim testing methods

Finding the Optimal Trim

The question remains as to the best way of finding the right trim during sailing. Many

vessels have draft indication systems installed at the bow, midship and stern area of the

hull. These sensors are usually quite reliable while the vessel remains static, but once

the vessel is moving the hydrodynamic flows affect the water pressures underneath the

vessel thus affecting the measured results.

The deviation in trim measurements is often overlooked using these techniques and

sometimes the real trim angle is a revelation as it can be way out from the static trim.

When comparing optimal trim in static conditions with dynamic trim, Eniram has

observed differences of half a meter or more.

Another surprising factor to some operators is the effect of dynamic hull deflection,

meaning that the vessel is sagging or hogging dynamically due to sea, swell and other

affecting factors. Significant squatting results in increased drag and changes the vessels

dynamic trim. As a result, the calculated static trim is compromised. Hull deflection is

measured to be up to 70cm. Eniram has measured up to a 90cm change in the trim of

the vessel due to squatting which in turn has a significant effect on the vessel’s overall

performance.

Logging wind has also shown similar effect. From a slight tail wind to a head wind

Eniram has observed a measured trim change of around 40 cm.

Static Trim Tables Dynamic Trim

Variables 2

Speed, draft

5+

Speed(s), draft, wind, sea

state, bending(s), list

Model Basis Tank tests / CFD Real-life

Full scale sea trial method

Feedback by the real trim

measurement

No, only set value Yes, set value and actual value

Trim Accuracy Varies Minimum 5cm

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In some circumstances, certain vessels may also have a dual optimal trim, due to

different optimals being required for bow and stern. This makes it even more

complicated to find the optimal trim without a real time system.

Figure 5 Factors affecting optimal trim

Dynamic Trim Optimization

Dynamic trimming is a method based on collection and multidimensional analysis of real-

time data on a vessel.

Dynamic trim technology takes into account all changing variables to calculate an

optimal trim. These include hydrodynamic forces, such as squat, propeller thrust and

maneuvering rudder angles. It is also necessary to include effects due to additional

weather conditions such as wind, rolling, surging etc.

After taking into account the entire range of factors affecting the vessel at sea, a value

for the optimal trim is then calculated and displayed in real time. Continuous data

collection, filtering and analysis is then used to constantly improve the accuracy of the

optimum trim.

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Figure 6 Inclinometer sensors

This is done through the automatic retrieval of data from bridge and engine room

systems, as well as using purpose-built inclinometer sensors.

Since operating at the optimal trim requires less power, vessel operators could

potentially expect to see substantial fuel savings as well as additional operational

savings. These include less machine wear, lower maintenance, and fewer spare parts

required, as well as prolonged machine life.

The information the crew needs to trim the vessel accurately is given in a real-time

visual display which shows the actual dynamic trim at that moment and whether the

vessel is operating at optimal trim for the moment.

Figure 7 Optimal trim ‘traffic light’ display on the bridge

This allows vessel trimming to be a self-guided process, removing the need for manual

trim charts which have been found to be unreliable and inaccurate under dynamic

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conditions. It also allows planning and integration of trimming as part of the normal

operating procedures onboard a vessel.

Example of savings with Dynamic Trimming

Eniram has been measuring the deviations between static and dynamic trim on several

vessels and the results are as follows;

Squat: up to 200cm

Speed: up to 90cm

Wind: up to 40cm

Passenger movements: up to 7cm

The result of these statistics showed a Panamax size cruise vessel had been sailing 30-

40cm off the estimated trim. On average 25 cm off trim equates to 2% in propulsion

power on the vessel in question. Had the vessel been sailing at optimum dynamic trim

they would have been able to save 700 tons of fuel annually*. Based on HFO at $600 per

ton this equates to $420,000 savings per annum, a notable impact for just a few

centimeters.

* These calculations are based on an average panamax cruise vessel that consumes 65 tons of fuel

per day.

Figure 8 Optimal trim performance graph before and after DTA

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The black line in the figure above demonstrates the savings potential before and after

the dynamic trimming technology was adopted. It also shows in green the amount of

time the vessel spent at optimal trim. The time the vessel was sailed at optimum trim

increased to 80% from an average of 40% prior to the use of the dynamic trimming

technology.

Environmental affect

Being able to quantify energy output is a crucial factor towards meeting environmental

goals and proving commitment of environmental stewardship.

The breakdown of propulsion power data can be used for developing fleet wide energy

efficiencies as it can be used to analyse the energy usage of the vessel, such as the

overall propulsion power per voyage, per week or per month.

The information provides management with a further understanding of their fleet’s

carbon footprint, i.e. the extent to which dynamic factors such as fouling, speed and trim

affect the performance and total energy consumption of a vessel.

Given that non-optimal trim over time accounts for a significant margin of overall

propulsion energy use, vessel operators could hope to lower their fuel consumption by

several percent through sailing at optimal trim. As such, if burning one ton of fuel is

taken as producing 3.16t of CO2, the environmental impact of saving of fuel is clear.

Additional Information value

By collecting large amounts of data from various factors affecting the energy usage of

the vessels, it is possible to help the personnel both onboard and onshore to pinpoint

potential areas for improvement.

By those means they can, for example, monitor the power decomposition of a vessel or

compare the collective performance of the whole fleet.

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Figure 9 Dynamic sea margin across a specified time period

The chart above shows a week by week breakdown of the energy usage onboard a

vessel. This information could be used to monitor energy performance across one or

multiple vessels across the fleet.

Improving the understanding of vessels’ performance drives change through optimized

execution. This enables ship owners and operators to take corrective actions early and

find the most efficient and environmentally friendly methods for operating their fleets.

Conclusions

In the current economic times, where fuel prices are constantly fluctuating although

generally increasing, environmental regulations are beginning to crack down on the

shipping industry, and vessel sizes are ever-growing, cost control at sea is becoming

more and more important. Operating vessels at optimum performance is becoming a

significant way of reducing costs, increasing efficiency and being competitive. Trimming

dynamically is a considerable factor in shipping operations, due to it potentially

accounting for typically 2-3% of a vessel’s fuel consumption depending on vessel type.

As such, the ever-changing conditions both at sea, as well as globally, make trimming a

viable yet simple way of reducing costs and being able to prove in tangible terms their

commitment to the environment.