HOME NEWS VIDEOS MARINE TECH NAVIGATION SAFETY CAREER LIFE AT SEA NAVAL ARCH

MORE Search In Marine Insight

How The Power Requirement Of A Ship

Is Estimated?

By Soumya Chakraborty | In: Naval Architecture | Last Updated on May 5, 2017

o 3.1K
SHARES j 3.1K s 2 f 49 h 7

One of the most important stages of a ship design process is the estimation, calculation and
optimisation of a ship’s power requirements. Why? Because the power of a ship is a deciding
factor for many other aspects of the industry- both the market and the environment.

A ship with more power requirements will automatically require larger amounts of fuel for each run
of voyage, resulting in a hike in fuel expenses for the owner. Also, according to the recent trends,
the overall efficiency of a ship is quantified by Energy Efficiency Design Index (EEDI). The lesser
the EEDI of a ship, more is the efficiency of the ship from an environmental and societal point of
view. As the EEDI of a ship being proportional to the power required, it is always preferred by the
designers to reduce the power requirements of a ship by every way possible. It reduces the EEDI,
and in turn, brings down the carbon footprint of the ship.

Before we go into the core topic of this article, let me tell you what this article is about. In this
article, we will first look at the ways of determining the power requirements for Marine Diesel
Engines and Diesel Electric Propulsion Systems (note that the methods of estimating the power
ratings for both the systems are quite different from each other). However different they may be,
the first step is always the same, regardless of the type of propulsion system that is to be used by
the ship, as discussed below.
The First Step: Calculate the Resistance of a Ship
In order to calculate the resistance of a ship, the first step is to conduct a towing tank test. In case
of new hullforms, a towing tank test is preferred. However, if the hullform of the ship in design has
already been tested in a tank, it is preferred to just follow the scaling method (Which we shall
discuss soon).

In a towing tank test, the resistance of the model scale is obtained in the computer of the carriage.
This is then scaled up to the ship’s scale by using a set of steps recommended by International
Towing Tank Conference (ITTC).

The towing tank however gives on the bare hull resistance of the ship. Air Resistance, Resistance
due to Appendages and a Correlation Allowance are added to obtain the Total Resistance of the
ship. This total resistance when multiplied with the ship’s velocity gives the Effective Power of the
ship (PE).

An Interesting Tip: If you now rate the ship’s engines to the obtained effective power, the ship
should move at the design speed. Right? But if you actually did, the ship would only be able to
operate at a speed lower than this. We will discuss the reason as we proceed.

The Second Step: Decide on the Type of Ship Propulsion

System
This is one of the most deciding steps of the entire process. Selection of the wrong type of
propulsion system may result in an economic catastrophe for the ship in future. Years of
experience and research has now provided us with a clear idea as to what kind of propulsion
should be preferred on different kind of ships.

Diesel-Mechanical Propulsion is preferred in most cargo ships which require low speed operations
and lower operating costs (operation costs for Heavy Fuel Oil used in Marine Diesel Engines is
lesser than operating costs of Diesel Electric Propulsion Systems). In the recent times, slow
steaming has become an efficient method to counter the effects of the maritime recession, and
diesel propulsion is hence preferred in most cargo ships (bulk carriers, oil tankers, container
ships).
Diesel Electric Propulsion is preferred in ships which require more electric power (for example
cruise ships require more electric power to run its facilities, drillships that require dynamic
positioning systems for most of the time of operation, etc.) and ships that require undisturbed
operations with varied torque (for example, tugs). This is one of the most notable advantage of
diesel electric propulsion. Where diesel mechanical propulsion systems do not offer high
efficiencies at all torques, diesel electric propulsion systems can operate at high efficiencies at
almost all torque variations.

Figure 1:Variable torque operations for Diesel Engines and Electric Propulsion (Image Credits: Image by Dr. Hiroyasu KIFUNE)

In the above graph, it is evident that the torque supplied by the Engine (diesel engine) varies with
speed. That is, higher torque can only be obtained at higher operating speeds. But if higher
torques (of propeller) are required at lower speeds (which is the case of tugs and dynamic
positioning systems), an electric motor (which is used in diesel electric propulsion systems) stands
out.
The Third Step: Estimate the Engine or Diesel Alternator
Ratings
For Diesel Mechanical Propulsion:

Remember the question we had raised in the tip above? Why would the ship not operate at design
speed if we rated the engines at the effective power?

> The resistance calculated from the towing tank tests is only the bare hull resistance, that is, the
effect of the propeller is not considered in the above. Whereas, when a propeller operates behind
the ship, there is the following we must consider here:

Resistance of the ship increases from the value calculated in bare hull condition. The propeller
must operate at a torque that is sufficient to overcome this augment in resistance and also enable
the ship to overcome its bare hull resistance.

Hence, due to the losses in the propeller, the power delivered to the propeller (PD) at the shaft
output should be more than the effective power (PE). The ratio of the effective power to the
delivered power is called Quasi Propulsive Coefficient (QPC). QPC usually ranges from 0.55 to
0.65.

> The power at the engine output (i.e. shaft input) is not fully obtained at the shaft output. This is
because of the frictional and heat losses that occur along the length of the shaft. These are
termed as shaft losses. Shaft losses are usually taken as 2 percent.

> In case of smaller ships where high rpm engines are generally used, reduction gearboxes are
used to reduce the shaft rpm, or operated at various RPMs. The losses induced by the gearbox
are classified under gearbox losses. Gearbox losses range from 4 to 5 percent.

> The resistance estimated during the design phase does not consider the effect of waves. Due to
the action of waves, the actual resistance on a ship is higher than that in calm water conditions.
Hence, a margin of 15 percent is considered as sea margin, and the engine power is rated so that
it overcomes the sea margin.

> It is always desirable to keep specific fuel oil consumption as low as possible. For marine diesel
engines, the Specific Fuel Oil Consumption (SFOC) is minimum when the RPM of the engine is
corresponding to 85% of maximum continuous rating (MCR). Which means, that the design speed
should be attained, not at the rated MCR but at 85% of the MCR. So to obtain the MCR, the
corresponding factor of 0.85 is considered.

The following table shows a calculation that is used to obtain the rated engine power from the
effective power of twin-engine ship using the above factors:
 Effective power of twin-engine ship

For Diesel-Electric Propulsion:

I will discuss the basic components of a diesel electric propulsion just to an extent that makes is
possible for a reader at a preliminary stage to understand what we will be discussing on the
estimation of power rating of a diesel electric propulsion system.

The basic components of a diesel electric propulsion system are:

Diesel Generators, Transformers, Electric Motors and Loads. Now, the loads on the system may
be a electric motor driven propulsion pod, or a bow thruster, or any component of hotel load
(lighting, HVAC, etc.)

Figure 2: Basic Components of a Diesel-Electric Propulsion System

In this case, the electric propulsion motors, propellers and other loads together form the load of
the entire power plant. But it is to be considered that not all loads will be in operation in every
condition. For example,
In harbour, the propulsion loads will be absent, whereas the hotel loads will be present.

In case of Dynamic Positioning Operations, both hotel loads and propulsion units will be in
operation. So in this case, the load on the diesel generators will be maximum.

So, we will basically need to calculate the total power requirement before deciding upon the
number of diesel generators required to meet all the conditions. Once the total power is decided,
the number of diesel generators will be decided upon based on certain principles that we shall
discuss later on.

First, in order to calculate the total power requirement, designers prepare a Load Chart which lists
out all the electrical loads on the ship. And the load chart is prepared, taking into consideration
three operating conditions in general:

Sailing, Harbour and Maneuvering.

In the load chart, the power requirements of each electrical load on the ship is calculated by
multiplying the Maximum Rated Power of the component with two factors:

Load Factor: It is the ratio of the operating power to the maximum power rating of the component.

Utility Factor: It is a factor which determines the extent of operation of the particular component
in a particular condition.

For example, for a steering gear equipment:

Utility factor for a steering gear equipment

Note that the utility factor is 0.8 in Sailing and Maneuvering Conditions, but 0 in Harbour
condition, since in harbour condition the steering gear is not used. So the contribution of the
steering gear equipment to the total power requirement in Sailing Condition will be zero.
In the similar manner as illustrated above, the load chart is prepared for all the electrical
components on the ship. A sample of the same would look like the following:

Figure 3: Electrical Load Chart

Once the load chart is prepared, the total power requirement for each of the three conditions
(Sailing, Harbour and Maneuvering) are calculated by adding up the power requirement for each
component for each of the conditions (follow figure 4). Once this is clear, we will now refer to
Figure 4 to understand how the total number of diesel generators is decided.

Figure 4: Determination of Total no. of generators

The two rules to be followed in deciding upon the number of generators are:

• If more than one generator is operating in any condition, both the generators should
share equal amount of load.
• The load on each generator in any of the three conditions should not be more than
70 percent of the rated power of the generator. (Or, the maximum rating of each
 generator is decided based upon the condition that seventy percent of the maximum rating
is more than the load on the generator in any of the three conditions)
• One additional generator should always be included, which is for standby purpose.
Note that this standby generator will not share the load in any of the above three
conditions unless any of the working generators are out of order. So the standby generator
is not included in the above calculation, but it is usually of the same rating as of the other
generators.

This process is iterated by varying power ratings and varying number of generators until the above
first two conditions are satisfied, and a situation similar to the one in Figure 4 is obtained. It is
advised that you analyse the first two conditions using the above figure to understand it in first
hand.

Report an Error

Tags: Design process power generation on ship Ship Generator ship power generator ship power
requirement

Want to share your tips and advice? Got questions? Visit the community forum to ask
questions, get answers, meet people, and share your tips!
YOU MAY ALSO LIKE

Perfect Port Different Types of Plano de Negócios 3 Important

Distance Marine Propulsion... Excel Calculations Every...
Ad Seafuture Inc. marineinsight.com Ad luz.vc marineinsight.com

Idiomas Online & 12 Terminologies Various Methods Procedure For

App used for Power of... Used for... Designing A...
Ad Babbel.com marineinsight.com marineinsight.com marineinsight.com

MORE POSTS FROM THE AUTHOR

Hull of a Ship – Understanding Understanding Watertight Different Jobs in a Shipyard &

Design and Characteristics Bulkheads In Ships: Shipbuilding Industry
Construction and SOLAS
Regulations

WHAT ARE YOU LOOKING FOR?

Search In Marine Insight

JOIN 873,942 MARINERS

737K+ Fans 24.2K+ Followers

2,023 Members 10.2K+ Subscribers

24.9K Followers 75.2K+ Readers

COMBO PACKS AT 40% DISCOUNT

 Engine Room Dry Dock Deck Department
Combo Pack Combo Pack Combo Pack

SUBSCRIBE TO OUR NEWSLETTER

Get Free News Updates Delivered Directly To Your Inbox

Enter Your Email KEEP ME UPDATED

About Soumya Chakraborty

Soumya is pursuing Naval Architecture and Ocean Engineering at IMU, Visakhapatnam,
India. Passionate about marine design, he believes in the importance of sharing maritime
technical knowhow among industry personnel and students. He is also the Co-Founder and Editor-in-
Chief of Learn Ship Design- A Student Initiative.
Comments

Capt Arnab Sen says

FEBRUARY 10, 2016 AT 1:03 PM

Dear Soumya,
It was a pleasure to read about the power requirements. I am a Master Mariner and run a
Maritime Academy in Kolkata and recently completed a course on Naval Architecture. I have
desisigned a fiberglass 40 ft fishing trawler and would like to use in the ganges estuary with a
100 hp marine diesel engine with a chinese gear box. what would by the advantages if I were
to go for motor propulsion run by generator with one stand by and during trawling both
generators could be used for more power.

Soumya says
MARCH 2, 2016 AT 2:52 PM

Dear Capt. Arnab,

It would actually depend on the kind of operation the vessel is meant for. In case of trawling
operations, there is usually a requirement of high torque at low speed regions, which would
make it advantageous to go for DG powered motor propulsion. You however need to compare
the calculations done for both cases, i.e. for MDE and Electric Propulsion. Based on the
efficiency achieved from each, the suitable propulsion system can be chosen.

Mr Cap says
MAY 9, 2016 AT 3:14 PM

Hi,

Can you explain how you got the numbers from figure 4?
Thank you:)

Virendra says
OCTOBER 15, 2016 AT 5:51 PM
 Kindly throw some light on Fig 4 findings.

Will appreciate same.

Thanks.

LATEST NEWS

Kongsberg Acquires Rights Of Raised Bayonne Bridge MAN Diesel & Turbo Expands
Two Innovative Underwater Roadway To Allow World’s Cooperation With MES
Glider Systems Largest Container Vessels
Pass Through

Drewry: Growth In Chemical ABS Supports DSME On Novel BIMCO: Upbeat Crude Oil
Tanker Fleet To Outpace CPP FLNG Concept Tanker Demolition Goes
Fleet Against General Trend

More Latest News

LATEST FEATURED POSTS

Top 10 Largest Cruise Ships in Hull of a Ship – Understanding Infographic: North America
2017 Design and Characteristics Freight Numbers In 2016
Stowaways On Ships: How and Why to Take Manual Life Raft on Ships: A General
Classification And On Board Sounding On Ship? Overview
Measures

Leave a Reply
Your email address will not be published. Required fields are marked *

Comment

Name *

Email *

Website

POST COMMENT

POPULAR NOW

Video: 6 People Killed In Crane Collapse at Samsung

Shipyard in South Korea
Real Life Incident: Wrong Helm Applied And Vessel
Grounds

Panama Canal Welcomes Largest Vessel To-Date

Through New Locks

Digital Resources for Deck Cadets and Officers

Gigantic Load To Be Shipped From Port Of

Gothenburg

Ulstein SX175-Designed Windea Leibniz Nominated

“Ship Of The Year 2017”

Practical eBooks for Marine Engineers

 Download 12 Free eBooks
for Seafarers

Enter Your Email

DOWNLOAD YOUR EBOOKS

We respect your privacy and have Zero Tolerance for

spam. Your Information is Safe & Secure with us.

LATEST FORUMS POSTS

somebody please tell me about the fuel

pump shock absorber?

Hello All , Soot blow in SECA Area is

Possible or not? our ship…

Hi marine insight forums member, I am

currently conducting a st…

Is S.I. Crewing, that offers jobs at sea, a

legitimate recruiting…

what type of coupling is provided in the

camshaft?? how many cams…

Click HERE To Ask A Question

MARITIME EVENTS

Canada LNG Conference & Exhibition 2017

16th May, Canada

UK Ports Conference 2017

 23rd May, London

Nor-Shipping 2017
30th May, Lillestrøm

View all Events

NEWS FROM NASDAQ

Huntington Ingalls Industries Delivers National

Security Cutter Munro to the U.S. Coast Guard

Vroon Extends 140-plus Vessel Contract for KVH

mini-VSAT Broadband Solution

Boskalis awarded EUR 120 million dredging

contract for oil terminal expansion in Brazil

TRENDING ON SOCIAL MEDIA

Hull of a Ship – Understanding

Design and Characteristics

Top 10 Largest Cruise Ships in

2017

Hamburg Süd And Maersk

Line To Continue Business As
Usual Until Q4 2017

Ulstein SX175-Designed
Windea Leibniz Nominated
“Ship Of The Year 2017”

Wärtsilä 34DF Engines

Awarded EPA Tier III
Emissions Compliance
Certification
Havyard To Deliver World’s
Largest Live Fish Carrier

57% Rise In The Number Of

Seafarers Affected By Piracy
In West Africa
Privacy Policy | Refund Policy | Contact Us | Disclaimer | About Us | Team | Advertise | Send Us News

Copyright © 2010 - 2017 Marine Insight — All Rights Reserved |