the impact of rotor sails on a vessel’s manoeuvrability

The impact of rotor sails on a

vessel’s manoeuvrability

Michael Forsberg

Bachelor's thesis

Degree Programme in Maritime Management

Turku 2020

EXAMENSARBETE

Författare: Michael Forsberg

Utbildning och ort: Utbildning i sjöfart – Åbo

Inriktningsalternativ/Fördjupning: Sjökapten

Handledare: Tony Karlsson

Titel: The impact of rotor sails on a vessel’s manoeuvrability

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Datum 07.05.2020 Sidantal 23 Bilagor 1

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Abstrakt

Högre bränslepriser nya utsläpps direktiv, driver fartygsägare till att söka innovationer för

att dra ner på bränsleförbrukningen. En lösning är rotorseglet, det är lätt att installera på

nästan viket fartyg som helst och kräver minimal servis. Men hurdan inverkan har ett rotor

segel på fartygs hanteringsförmåga?

Syftet med denna undersökning är att ta reda på om det förekommer någon skillnad i hur

ett fartyg beter sig med och utan rotor segel.

Undersökningen gjordes i simulatorn (Transas virtual shipyard) vid Aboa Mare, med rotor

kraft data from Norsepower. Simuleringen var uppdelad i två delar; med rotor segel och

utan. Simulationen bestod av fyra olika manövrar; vridcirkel, zig zag manöver, williamson

sväng och kraschstop. Varje manöver var uppdelad i fyra olika vind riktningar; nord, öst,

syd och väst. Vinden för varje simulation var lagt till 25m/s och fartygets startade varje

gång med kursen 000° och maskinerna fullt framåt.

Resultaten indikerar att det är en liten skillnad mellan ett fartyg som har ett rotor segel

jämför med ett som icke är utrustat med ett rotor segel. Främst, fartygets förmåga att bättre

bevara sin hastighet genom manövern.

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Språk: Engelska Nyckelord: Rotor Segel

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OPINNÄYTETYÖ

Tekijä: Michael Forsberg

Koulutus ja paikkakunta: Merenkulku – Turku

Suuntautumisvaihtoehto/Syventävät opinnot: Merikapteeni

Ohjaaja(t): Tony Karlsson

Nimike: The impact of rotor sails on a vessel’s manoeuvrability

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Päivämäärä 07.05.2020 Sivumäärä 23 Liitteet 1

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Korkeat polttoainekustannukset ja uudet päästödirektiivit ajavat alusten omistajia löytämään uusia ratkaisuja polttoainekulutuksen vähentämiseksi. Yksi ratkaisu tähän olisi roottoripurje eli ”flettner-roottori”, se on helppo asentaa ja vaatii vähän ylläpitoa. Mutta minkälainen vaikutus sillä on aluksen käsittelyominaisuuksiin?

Tämän tutkielman idea oli saada selville, onko roottoripurjeella varustellun aluksen käsittelyssä eroja alukseen, jossa sitä ei ole.

Tutkimuksessa käytettiin simulaattoria (Transas Virtual Shipyard), joka sijaitsee Aboa Maressa, sekä Norsepowerilta saatuja datoja roottoreiden voimasta. Simulaatiot oli jaettu kahteen osaan: ilman roottoria ja sen kanssa. Simulaatiot koostuivat neljästä ”standardi” liikkeestä; kääntösäde, siksakliike, Williamson-käännös ja hätäpysähdys, lisäksi jokainen liike tehtiin neljän eri tuulen suunnan mukaan; pohjoinen, itä, etelä ja länsi. Tuuli oli asetettu jatkuvaan 25m/s nopeuteen ja jokaisessa simulaatiossa aluksen lähtötila oli asetettu kurssiin 000° ja koneet täydellä teholla.

Tulokset viittaavat pieneen eroon näiden kahden kokoonpanon välillä. Suurimmaksi osaksi se liittyy aluksen kykyyn säilyttää suurempi nopeus koko liikkeen ajan.

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Kieli: Englanti Avainsanat: Roottoripurje

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BACHELOR’S THESIS

Author: Michael Forsberg

Degree Programme: Degree Programme in Maritime Studies, Turku

Specialization: Bachelor of Marine Technology

Supervisor(s): Tony Karlsson

Title: The impact of rotor sails on a vessel’s manoeuvrability

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Date 07.05.2020 Number of pages 23 Appendices 1

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Abstract

Higher fuel prices and new emission directives, drive ship owners to find new solutions to cut their fuel consumption. One solution to this is the rotor sail or “flettner rotor”, it’s easy to install and its low maintenance. But what kind of an impact does it have on the handling characteristics of the ship?

The idea behind this study is to find out if there is a noticeable difference in handling characteristics between a ship fitted with a rotor sail versus one without.

The study was carries out in the simulator (Transas virtual shipyard) at Aboa Mare, with rotor force data provided by Norsepower. The simulations were divided in to two parts: with rotor and without rotor. The simulations consisted of four “standard” manoeuvres; turning circle, zig zag manoeuvre, Williamson turn and crash stop, furthermore each manoeuvre was done with four different wind directions; north, east, south and west. The wind was set at a constant speed of 25m/s and for each run the ships starting condition was a course of 000° and engine at full ahead.

The results indicate that there is a small difference between the two configurations. Mainly the ship’s ability to retain more speed throughout the manoeuvres.

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Language: English Key words: Rotor Sail, Flettner Rotor

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Foreword I would like to thank: Norse Power, especially Tuomas Riski and Ville Paakkari, for

providing me with the force data for the study. Aboa Mare, especially Bo Lindroos and Per

Westerholm for assisting me with the simulations. And Joacim Johansson for providing

insight into simulating rotor sails.

Table of Contents

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

1.1 Objective ...................................................................................................................................... 1

1.2 Research method ...................................................................................................................... 1

1.3 Delimitation ................................................................................................................................ 1

2 Rotor Sail ............................................................................................................................................. 2

3 Forces .................................................................................................................................................... 2

3.1 Centre of lateral resistance ................................................................................................... 2

3.2 Force from the Rudder ........................................................................................................... 3

3.3 Rotor Force ................................................................................................................................. 4

3.4 Magnus effect ............................................................................................................................. 4

4 Assessment of handling characteristics .................................................................................. 5

4.1 Terminology ............................................................................................................................... 6

4.1.1 Advance ............................................................................................................................... 6

4.1.2 Transfer ............................................................................................................................... 6

4.1.3 Tactical diameter ............................................................................................................. 6

4.1.4 Steady diameter ............................................................................................................... 6

4.1.5 Speed loss ........................................................................................................................... 6

4.1.6 Directionally stable ......................................................................................................... 7

4.2 Manoeuvres ................................................................................................................................ 7

4.2.1 Turning circle .................................................................................................................... 7

4.2.2 ZigZag ................................................................................................................................... 7

4.2.3 Crash stop ........................................................................................................................... 8

4.2.4 Williamson turn................................................................................................................ 8

5 Method .................................................................................................................................................. 9

5.1 Simulations ................................................................................................................................. 9

5.2 Force data .................................................................................................................................... 9

5.3 Virtual shipyard ..................................................................................................................... 10

5.3.1 Test manoeuvres ........................................................................................................... 10

5.3.2 Ship ..................................................................................................................................... 11

5.3.3 Checking Base Performance ..................................................................................... 11

6 Results and Analysis ..................................................................................................................... 12

6.1 Turning circle .......................................................................................................................... 12

6.2 Zigzag ......................................................................................................................................... 16

6.3 Man overboard manoeuvre “Williamson Turn” ........................................................ 18

6.4 Crash Stop (with software failure) ................................................................................. 20

7 Conclusion ........................................................................................................................................ 21

8 Discussion ........................................................................................................................................ 22

9 References ........................................................................................................................................ 23

Appendix

1

1 Introduction

Higher fuel prices and global trends lead to ship owners seeking for new and innovative

solutions for lowering fuel consumption and becoming more environmentally friendly. One

such solutions is the rotor sail. Not a new concept by any means but is staring to gain some

traction thanks to sophisticated electronics and software to optimize thrust in almost every

weather condition. Norsepower has jumped on the band wagon and are modernizing the old

flettner design in to the 21-centuri.

Everybody talks about fuel efficiency and performance; although little is talked about how

this will impact the handling characteristics of a vessel. Should the vessel perhaps undergo

new sea trials because of this?

With the help of Norsepower (force data) and Aboamare (simulators) I intend to get a

glimpse into this, to get a base understanding of how a vessel’s handling is affected.

1.1 Objective

The objective of this study is to find out what kind of an effect rotor sails have on the ship’s

manoeuvrability in Transas virtual shipyard. And should the navigating officer have to pay

any special attention to this?

1.2 Research method

The study is done in Transas “virtual Shipyard” with force data by Norsepower (30m * 5m

model). The data provided by Norsepower is used to create “virtual forces” on the vessel

corresponding to the prevailing wind conditions.

1.3 Delimitation

This will only be a basic study into how the handling characteristics are affected, and to open

new doors for further study and discussion.

The study will focus on how the rotor sail will impact the handling characteristics of the

vessel and it will not handle the question of efficiency or lower fuel consumption.

The simulated forces will not take into account any turbulence that is caused by the ship’s

hull or superstructure. To eliminate as many external factors as possible the simulation is

2

done in open waters without any waves. The only external forces on the ship are the wind

and the force from the rotor sail.

2 Rotor sail

A rotor sail is mechanical sail, i.e. it requires mechanical power to rotate a cylinder (the sail)

and thus to produce any thrust. This is explained by an effect called the magnus effect; a

rotating cylinder, placed in a moving fluid it will generate thrust perpendicular to the motion

of the fluid. (Seifert, 2012)

The main features of the rotor sail, according to Norsepower, (besides the fuel savings); most

vessel types can be fitted, be that a retro fit or on a newbuild. The system is designed to be

highly automated and be able to optimize for a variety of different wind conditions.

3 Forces

The main forces acting on a ship’s hull and superstructure are the aerodynamic and

hydrodynamic forces, the later usually being the dominant one. Consequently, aero forces

are usually ignored when talking about ship handling and only regarded as disturbing forces.

How the ship movers in response to these forces, manly tree axis of motion need to be

considered, linear motion in the x and y axis (horizontal plane) and rotation around the z axis

(yaw). (Molland, 2008)

3.1 Centre of lateral resistance

The centre of lateral resistance is the point at which a lever arm leans (its origin,

geometrically speaking). When the ship is subjected to lateral force acting at the centre of

lateral resistance, the ship will only experience lateral movement, no rotation will be induced

(since the length of the lever arm is zero). The centre of lateral resistance is dependent of the

centre of gravity, underwater surface area and any pressure fields around the hull. At rest the

centre of lateral resistance is located between the centre of gravity and underwater surface

area. Once the ship stars moving through the water, drag forces induced by the water will

create pressure waves around the ship (high pressure in the direction of travel and vice versa).

The centre of lateral resistance will shift if the direction of travel, usually less than 10% of

the ship’s length. (Cauvier, 2008)

3

Centre of lateral resistance should not be confused with pivot point or “apparent pivot point”

as Captain Hugues Cauvier calls it, (which is a fictional point along the ship’s centre line).

The “pivot point” is a result of lateral movement and a moment of rotation around the centre

of gravity. Somewhere along the ship’s centreline (might also be ahead or abaft the ship in

line with the ships centreline), there will be a point where the ship doesn’t seem like its

moving so to say, this is rightly called the apparent pivot point. It is only an apparent

phenomenon and doesn’t have any physical properties. (Cauvier, 2008)

The apparent pivot point will shift depending on where the lateral force is acting. The closer

the sideways force (e.g. from the rudder pushing the vessel to starboard) is to the centre of

lateral resistance the further way the apparent pivot point will be (Cauvier, 2008). This can

be demonstrated with a regular ruler places on and even surface that isn’t to slippery. To find

out where the centre of lateral resistance is, push on the long side of the ruler, the point at

which the ruler only moves sideward and doesn’t rotate is the centre of lateral resistance

(should be somewhere in the middle). Then push just to the side of that point, what you will

notice is that the ruler turns slowly and is pushed to the side quite a bit, try to find the apparent

pivot point, it should be on the opposite side of the centre of lateral resistance (from where

the force is applied) and around the tip of the ruler. Then let’s do the experiment again, now

push on the tip of the ruler, you will notice that the ruler almost exclusively rotates, and only

moves a slight bit to the side. Here the apparent pivot point is close to the centre of lateral

resistance.

3.2 Force from the rudder

In a turn, the ship is acted upon by a radial force pushing the ship towards the centre of the

turning circle. This force arises when the ship is moving through the water at an angle or in

other words; the ship has an angle of attack to the flow of the water. The rudders purpose is

to be able to create enough force acting on the hull to be able to sustain the ship’s angle of

attack through the water. For the rudder to be able to generate a good amount of leverage

and still be of a reasonable size, it is advantageous to place it as far away from the fulcrum

(Centre of lateral resistance3.1) as feasible. Furthermore, by placing the rudder in the

propeller stream, a short increase is the engine rpm, or a “kick” will increase the force from

the rudder when the ship is moving slowly through the water. (Molland, 2008)

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3.3 Rotor force

If we were to imagine the ship equipped with a rotor sail standing still in the water, and the

wind veering around it (changing direction in a clockwise manner), the total force produced

by the rotor will be constant, but the thrust would oscillate in a sinusoidal fashion between

full thrust forward and then full thrust back ward. By reversing the rotation of the rotor just

as the thrust is about to become negative, the forces are flipped 180°, and thus, the rotor is

almost always able to produce positive thrust. As pictured in Figure 1 Force on the ship, each

time the thrust goes to 0 the rotation of the rotor is flipped and the thrust will increase again.

Figure 1 Force on the ship

Since the force produced by the rotor is constant, and the thrust generated is the sine of the

wind direction, the transversal force is the cosine. Meaning that the transversal force is 90°

(true wind angle) rotated is respects to the thrust. This leads to moments where the ship will

experience no thrust and large amounts of transversal force.

How the ship will react to the force generated from the rotor is dependent on where the rotor

is placed. Placing the rotor further away from the centre of lateral resistance will (as pointed

out earlier) move location the apparent pivot point, but more crucially have a bigger moment

arm. The effect of this is that the force applied will act in the same way as the rudder (yaw),

either it can work to push against the rudder or work with it. This in essence, will alter the

effectiveness off the rudder.

3.4 Magnus effect

A cylinder which has its axis of rotation perpendicular to a flowing fluid (like air) will

produce a net force at a 90° angle (in the direction of rotation) to the flow of the fluid. This

effect is called the magnus effect. Friction between the fluid and the rotating body, will

0 50 100 150 200 250 300 350

Rotor Force On The Ship

Thrust Transverse Force

5

cause the fluid to speed up on the side moving with the fluid and slowdown of the side which

is moving against the flow. According to Bernoulli's principle if a fluid is sped up the

pressure will drop and vice versa for when the fluid slows down. This means that there is a

pressure differential across the cylinder, the high-pressure zone on the side turning in to the

flow direction pushes the cylinder to the low-pressure zone on the side turning away from

the flow. The magnitude of the force is manly a function of the size and the rpm of the rotor

and the flow velocity of the fluid (Seifert, 2012)

Figure 2 Rotor Thrust by Michael Forsberg

In Figure 2 Rotor Thrust, the rotor is turning anti clockwise while the wind is blowing from the starboard side of the ship, producing a net force forward.

4 Assessment of handling characteristics

It can be difficult to rigorously assess a ship’s manoeuvrability, e.g. cost, repeatability,

reliability, and accuracy of tests are big limiting factors. Preferably one would carry out a

full-scale assessment in the real-world, using a number of common manoeuvres to get good

data, but that comes with huge costs. At a cost of real-world accuracy (inherently complex

to model), using computers to model a ship’s handling characteristics is fast and inexpensive.

(Molland, 2008)

6

4.1 Terminology

To be able to quantify the performance of the ship, a set reliable data points need to be used.

By taking data from key points along the planned manoeuvre, it’s possible to set a standard

for which to compare other tests against. The following parameters are commonly used when

describing a ship’s manoeuvring capabilities.

4.1.1 Advance

Advance is the distance travelled by the ship’s centre of gravity, in the original direction of

travel, from the time that the rudder is placed over. Usually this distance is quoted as far as

till the ship is at 90° to the initial course. (Tupper, Introduction to Naval Architecture, 2004)

4.1.2 Transfer

Transfer is the lateral (sideways) distance travelled by the ship from the original path until a

heading change of 90° (Tupper, Introduction to Naval Architecture, 2004)

4.1.3 Tactical diameter

Tactical diameter or TCD is the lateral distance the ship has moved over a heading change

of 180°. The value of this is usually quoted as a ratio between the tactical diameter and length

of the ship or TD/L, a TD/L of 4.5 would be regarded as good. (Tupper, Introduction to

Naval Architecture, 2004)

4.1.4 Steady diameter

After the initial application of rudder, the ship will have a period of temporary motions due

to the sudden application of forces on the hull. After a while, the ship will reach a new steady

state where the speed, drift angle and turning diameter reach steady values. This state often

occurs after the 90° heading change mark but it might take up to a 180° heading change

before the ship reaches a steady state. (Molland, 2008)

4.1.5 Speed loss

For the ship to be able to turn, the hull needs to produce lift to induce a turn. This is achieved

by the ship having an angle of attack or drift angle in relation to the water. The lift that is

produced comes at the expense of more drag through the water, meaning that if the power

7

setting is not increased the ship will slow down. In most cases the ship will reach a steady

speed after 90° of heading change. (Molland, 2008)

4.1.6 Directionally stable

A ship is considered directionally stable when after being deflected from its straight-line

path (e.g. wind gusts or waves), it returns to a new straight-line path, in layman's terms the

ship tends to go straight. This comes at a cost though, with high directional stability the

ship’s ability to manoeuvre is low. On the flipside directional instability means that the ship

has lacklustre course keeping but good manoeuvrability. Usually a compromise between the

two must be accepted when designing a ship, depending on the use case of course. (Molland,

2008)

4.2 Manoeuvres

4.2.1 Turning circle

The turning circle test is done with the ship at full speed and with the rudder set to 35° (port

or starboard), the manoeuvre is complete once the ship has turned through 360° in one

direction (e.g. starboard), then the same is done in the other direction (port). On ships with

one propeller, a difference between the port and starboard test is expected, due rotation of

the propeller pushing the stern of the ship sideward.

From the test one can find out the TCD, Transfer, advance, drift angles and heel angles. For

merchant ships a TCD of 3-4 times the length between perpendiculars is normal, with a

bigger rudder this value can be lowered. (Barrass, 2004)

4.2.2 ZigZag

The zigzag manoeuvre is designed to test how the ship responds to rudder movement or its

“controllability”, a larger rudder yields a quicker response. To start the ship will sail at a

steady course, the rudder will then be moved over to e.g. 20° starboard, this is kept till the

ship’s heading has responded and moved 20° to starboard, then the rudder is placed to 20°

port. It will take a bit of time for the ship to respond to the new rudder input, this is called

overshoot. On a directionally stable ship, the smaller the overshoot value and the shorter the

time between the rudder orders the more efficient the ship’s response. (Barrass, 2004)

8

4.2.3 Crash stop

Large ships (especially in loaded condition) can take a long time to arrest their forward

momentum. The crash stop test is done to determine the distance travelled in the original

direction (head reach), the sideways movement (side reach) and the time taken. (Tupper,

Introduction to naval architecture, 2013)

A crash stop test is carried out as follows: (note, rudder is kept at midships throughout the

test)

1. Ship proceeding at full speed and an order is given to go from full ahead to engines

stopped. Subsequently the ship will start to slow down due to friction with the water.

2. Propeller shaft speed drops to zero slip and zero thrust is produced.

3. After a specified time (dependent of the type of machinery installed onboard), reverse

torque can be applied to the propeller shaft. The shaft will stop turning (wind milling)

and then start turning in reverse and producing backwards thrust.

4. Astern power should now be applied gently until maximum reverse power is reached

and held till ship speed is zero.

The test will tell the overall distance it took for the ship to come to a standstill and, how

much lateral deviation has occurred. Lateral deviation is caused by hydrodynamic

imbalances and can be a bit unpredictable, but the expected lateral deviation is about a third

or a fourth the stopping distance to port or starboard. (Barrass, 2004)

4.2.4 Williamson turn

The Williamson turn is a so-called man overboard manoeuvre, the purpose of the manoeuvre

is to make such a turn that the ship ends up on its original track (with as little deviation from

it as possible) going in the opposite direction, without stopping. The general starting point

for executing the manoeuvre is; the rudder is turned hard over (port or starboard), once the

course has deviated by 60° the rudder is placed hard over to the opposite side, the rudder is

placed a midships when the heading is about 20° from the opposing initial course. The

headings for rudder placements will vary slightly from ship to ship and is therefore vessel

specific. (Karvinen, 2017).

9

5 Method

The simulation is done in Transas Marine “virtual shipyard”, version 2.93.3651.0. At a

dedicated simulator at Aboa Mare

Virtual Shipyard is capable of preforming a set of standard manoeuvres (no human steering

input needed), these were used to eliminate variability between runs. The software is precise

enough, so that if the circumstances stay the same there is virtually zero difference between

runs. Therefore, I decided that each case only needed to be done once.

Since the software doesn’t have native support for rotor sails, I imposed a virtual force on

the ship, to simulate how the rotor sail impacts the handling of the ship. The direction of the

virtual force is set relative to the ship in a x, y and z coordinate system. X and y being the

horizontal plane and z being the vertical (z was not used in the simulations). Likewise, the

location where the force is being applied is set by x, y and z coordinates.

5.1 Simulations

In each simulation the ship’s staring condition will be set to the following: an initial course

of 000°, steaming at ~15 knots and the engines at full ahead. The wind will be set to blow in

one direction at 25 m/s, to limit the variables the simulations take place in open waters (depth

1000 * the draft) and the sea state set to 0 (no waves).

Each of the selected manoeuvres will be divided into two sets of four simulations. One set

will be with the force of the rotor applied to the vessel, and the other without any rotor force

applied. Each set is then simulated four times with the wind from either north, east, south or

west. In the set with the rotor force applied, the produced rotor force will be manually

updated every 005° of true wind angle, till the test is done.

5.2 Force data

The thrust produced by the rotor is dependent on the wind speed, the angle of the wind and

the ships speed. The calculations for this are very complex and beyond this study, therefore

I contacted Norsepower to give me a hand with the force data.

Norsepower, a Finnish company that makes rotor sails, provided their proprietary force data

for this study. The data they provided was for their 30m high, by 5m diameter rotor. The

data was given at an interval of 15° true wind angle, up to 180°, at 1 m/s intervals up to

10

25m/s and for every four knots (ships speed) up to 16 knots. As mentioned, the data is

proprietary and will therefore not be included in this paper. To somewhat improve the

accuracy, the data was linearly interpolated to every 5° true wind angle in excel.

The force that is produced by the rotor is divided into longitudinal force (thrust) and

transversal force. This enables the force to be represented in a x, y coordinate system, and

can thus be inserted as a virtual force in Virtual shipyard. Since the ship will be turning in

the wind and performing manoeuvres, the x and y components of the virtual force will

continuously change. Therefore, they need to be updated frequently to get an accurate result

(every 5° true wind angle in this case). The point at which the force acts upon on the ship is

50m forward of midships (x), on centreline (y) and 15m above the water plane (z), the z

height will have little effect on the results.

5.3 Virtual shipyard

Virtual shipyard is a powerful tool that enables engineers and designers to develop and edit

ship motion-, engine- and propulsion- models, as well as controlling and documenting the

results. For these tests, the “test manoeuvres” feature was chosen, as it eliminates human

error interfering with how the ship manoeuvres, thus being very accurate and repeatable.

Once the test is started the simulation will run for a few seconds to stabilize the ship before

the actual test begins. During the test, the software performs the given manoeuvre

compensating for any external forces present on the vessel. When the test manoeuvre is done

the test automatically ends and a test report is presented (this can lead to some issues with

the reported time to finish the manoeuvre, as can be seen in the crash stop test with northerly

wind). It is on this report that the comparison between the runs is done. The test reports

presented by the software contain the most significant data relevant to the test.

5.3.1 Test manoeuvres

In total five different manoeuvres were tested. Turning circle to starboard, Turning circle to

Port, both with 35° of rudder angle. Zigzag manoeuvre with 20° of rudder angle staring to

starboard., Crash stop and Williamson turn staring to starboard. The manoeuvres were

performed automatically by the software, as selected through a drop-down menu.

11

5.3.2 Ship

The ship chosen was an oil tanker 235 m in length and 42 m in breadth, fully loaded with a

displacement of 120750 tons and a draught of 14,5m. This was chosen manly due to

availability reasons, I wanted a hull type that was common, original idea was a bulker but

there were some incompatibilities with the software versions.

Figure 3 Crude Oil Tanker

5.3.3 Checking Base Performance

As with any program, there are going to be bugs in the software. Discussing with the

simulator instructors at Aboa Mare, it became clear that the ship model needed to be tested

and validated, to ensure that there aren’t any problems with the model. To ensure that the

vessel will handle as is expected, I used a feature in Virtual Shipyard that performs a sea

trials test. This test can then be presented as a wheelhouse poster. Upon inspection of the

results, I did not find anything abnormal and was satisfied that the ship model was fit for use

in the simulations.

12

Figure 4 Wheelhouse Poster

6 Results and Analysis

The results are compiled in excel (data is appended), and then compared to each other. The

difference between with rotor versus without rotor is then given as a percentage. Positive

(blue) meaning that the result with the rotor on, is bigger than the result without a rotor and

vice versa.

As can be observed, the two turning circle manoeuvres are quite similar, therefore the rest

of the test were done to one side (port or starboard). A general picture can nonetheless be

interpreted of the effects. To get a better picture of how the handling is affected take a look

at the track plots.

In the tables; Blue = higher value with rotor than without, Red = lower value with rotor

than without.

6.1 Turning circle

The results seem to indicate that the wind direction doesn’t have a big impact on the end

result. As a result of the added thrust from the rotor the ship is able to retain more speed

throughout the manoeuvres, with the gap only increasing as the test comes to an end. On

13

average the vessel had a final speed about a half a knot faster with the rotors on versus

without rotors.

The higher speed also means that the steady diameter was grater and the time it took to

execute the manoeuvre is lower. And average steady diameter without the rotor was about

974 meters, while with the rotor 1003 meters, a difference of about 30 meters. The time to

complete a 360° turn without a rotor took on average 803 seconds versus with the rotors an

average of 772 seconds, a time reduction of about 30 seconds.

A slight difference is the advance and transfer values seem to emerge, depending on the wind

direction. This would mainly be due to how the rotors thrust is pushing the vessel.

14

Table 1 Turning Circle Starboard

Turning Circle Port With Rotor vs Without Rotor

Wind Direcion North East South West

Initial Speed [knots] 0.00 % 0.00 % 0.00 % 0.00 %

Speed at 90° [knots] 2.28 % 2.87 % 3.93 % 1.60 %

Time at 90° [s] -0.67 % -1.40 % -2.09 % 0.00 %

Advance [m] -0.56 % 0.08 % -1.13 % 1.61 %

Transfer [m] 0.05 % 1.05 % 0.46 % 0.89 %

Speed at 180° [knots] 4.60 % 7.10 % 5.88 % 5.49 %

Time at 180° [s] -1.77 % -2.44 % -2.23 % -1.34 %

Tactical Diameter [m] 0.71 % 0.40 % 2.08 % -0.16 %

Speed at 270° [knots] 8.59 % 9.43 % 8.31 % 6.52 %

Time at 360° [s] -3.84 % -4.52 % -3.95 % -3.31 %

Steady Diameter [m] 4.38 % 1.58 % 3.86 % 3.31 %

Final ROT [°/min] 7.29 % 8.57 % 3.36 % 6.32 %

Final Speed [knots] 11.96 % 10.22 % 7.43 % 9.96 %

Speed Loss -4.94 % -5.96 % -7.80 % -8.07 %

Table 2 Turning Circle Port

Turning Circle Starboard With Rotor vs Without Rotor

Wind Direcion North East South West

Initial Speed [knots] 0.00 % 0.00 % 0.00 % 0.00 %

Speed at 90° [knots] 2.26 % 1.54 % 3.75 % 2.76 %

Time at 90° [s] -0.67 % 0.00 % -2.06 % -0.69 %

Advance [m] -0.50 % 1.54 % -1.05 % 0.50 %

Transfer [m] 0.03 % 0.99 % 0.35 % 2.18 %

Speed at 180° [knots] 4.79 % 5.30 % 5.07 % 6.19 %

Time at 180° [s] -1.77 % -1.61 % -2.27 % -1.83 %

Tactical Diameter [m] 0.75 % -0.02 % 1.68 % 1.05 %

Speed at 270° [knots] 7.89 % 6.43 % 7.81 % 8.70 %

Time at 360° [s] -3.78 % -3.21 % -4.16 % -4.28 %

Steady Diameter [m] 3.16 % 2.08 % 5.07 % 1.08 %

Final ROT [°/min] 7.41 % 6.58 % 2.05 % 8.43 %

Final Speed [knots] 10.66 % 8.84 % 7.18 % 9.45 %

Speed Loss -5.64 % -8.17 % -7.76 % -6.30 %

15

Figure 5 Turning circle Starboard (Red=with rotor, Blue=no rotor)

16

Figure 6 Turning circle Port (Red=with rotor, Blue=no rotor)

6.2 Zigzag

In comparison to the turning circle test, the zigzag manoeuvre seems to be very dependent

on the direction of the wind. This is clearly evident in the comparison table below; northerly

and southerly results are positive (higher value with rotor than without) and the easterly and

westerly are negative (lower value with rotor than without). Although in all wind directions

the vessel was able to retain more speed at the end of the test. For the northerly and southerly

directions about half a knot, and easterly and westerly directions about a knot.

With northerly and southerly winds, where the forces of the rotor are mostly transversal and

acting against the turn. The ship has a tendency to overshoot about 1,5° more with the rotors

17

on. Leading to the time for completion to rise from 1334 seconds to 1432, an average of

around 98 seconds, even though the vessel had a half a knot speed advantage.

Conversely when the wind is Easterly or Westerly the force is mostly longitudinal and acting

with the turn, thus slightly reducing the amount of overshoot, but only by a few tenths of a

degree. With an increased final speed of about one knot, the time it took to complete the

manoeuvre shrank form an average of 1237 seconds to 1160 seconds, a difference of 77

seconds.

Table 3 ZigZig with 20° of Rudder

ZigZag 20° With Rotor vs Without Rotor

Wind Direction Notrh East South West

Initial speed 0.00 % 0.00 % 0.00 % 0.00 %

1st overshoot [°] 7.80 % -7.50 % 7.52 % 3.20 %

1st overshoot time 5.08 % -2.75 % 0.00 % -0.92 %

2nd overshoot [°] 10.88 % 2.26 % 8.78 % -5.15 %

2nd overshoot time 7.59 % -4.18 % 2.44 % -2.49 %

3rd overshoot [°] 19.84 % -5.13 % 10.94 % 4.35 %

3rd overshoot time 9.19 % -4.23 % 2.73 % -4.17 %

Period 11.43 % -5.85 % 2.25 % -6.51 %

Final speed 2.26 % 13.41 % 6.48 % 13.08 %

Final time 11.57 % -5.92 % 2.26 % -6.51 %

18

Figure 7 Zig Zag manoeuvre (Black=with rotor, cyan= no rotor)

6.3 Man overboard manoeuvre “Williamson Turn”

The Williamson turn is the least exact manoeuvre in this study, since the software does many

different iterations with varying rudder angles and times to figure out the best track. The

most telling result is how much the vessel deviated from the original track. Meaning the

distance between the original track at a course of 000° and the returning track at a course of

180°. Due to the high wind speeds in the test its clear that the vessel is going to drift off the

track (the automated program is not compensating for the drift). With all else being equal

there should still be a noticeable difference in the tests.

The test took on average about 1094 seconds without the rotor and 1048 seconds with the

rotors, a difference of around 45 seconds. Northerly, easterly and southerly wind directions

had only small variations in their deviations from original track. With northerly winds the

deviation rose from 109,5m to 111,4m (Δ of 1,9m) with the rotor on. Easterly winds rose

with 8,5m (251,9 to 260,4) with the rotor on. Southerly winds saw a decrease from 123,2m

to 117,1 meters (Δ of -6,1m). The westerly wind direction had the highest deviation

decreasing from 69,1m without rotor to 33,1m with rotor (Δ of 36m). This is due to the vessel

being able to perform a tighter turn, thus getting back to original track faster.

19

Table 4 Williamson Turn

Williamson Turn With Rotor vs Without Rotor

Wind Direction North East South West

Initial speed [konts] 0.00 % 0.00 % 0.00 % 0.00 %

Full Time [s] 0.00 % -5.38 % -3.65 % -5.18 %

Deviation from initial course [m] 1.74 % 3.40 % -4.95 % -52.11 %

Time when rudder changed to 35 [s] 0.00 % 0.00 % 0.00 % 0.00 %

Time when rudder changed to -35 [s] 0.00 % 0.00 % 0.00 % -1.29 %

Figure 8 Williamson Turn

20

6.4 Crash Stop (with software failure)

When the rotor control software is functioning normally, Norsepowers system will

automatically detect and shut down the rotors in the event of a crash stop. However, in this

test, a system failure that leads to the rotor running at full thrust for the duration of the crash

stop is simulated.

This test highlights how much thrust can be generated by the rotor and the impact is has on

the handling characteristics. In both the easterly and westerly wind directions where the

thrust is at its peak, the ship will take considerably longer to come to a standstill, 67 seconds

(895,6s rose to 962,6s) and 152 seconds (491,6s rose to 643,6s) respectively. The added time

coupled with the wind blowing straight on to the vessels side meant that it drifted quite a

ways downwind before coming to a stop, as compared to the test without the rotors. The

easterly wind direction saw the vessel’s head reach (forward reach) increase from 2249m to

2372m and westerly wind direction an increase from 1963m to 2459m.

With southerly wind the vessel veered to the west much more than expected, about and extra

300 meters. Yet it came to a stop 40 seconds and 329 meters earlier in its travel direction

(down from 610s and 2418m to 570s and 2089m) with the rotor on than with it off.

With northerly wind the vessel followed almost the same track, but then kept on drifting

with the wind (going back south) for over 3 minutes before coming to a stop, leading to the

test reporting a lower head track and longer time till stop. This is a quirk of how Virtual

shipyard decides when to end the test. Although as you can see in the track plots below, it

had a shorter track reach down from 2418m to 2089m.

Table 5 Crash Stop

Crash Stop With Rotor vs Without Rotor

Wind Direction North East South West

Initial speed [knotd] 0.00 % 0.00 % 0.00 % 0.00 %

Time engine reach 0%ET [s] 0.00 % 0.00 % 0.00 % 0.00 %

Track reach [m] 11.99 % 18.85 % -6.43 % 27.98 %

Head reach [m] -15.52 % 5.44 % -13.61 % 25.28 %

Side Reach [m] 21.68 % 84.13 % 66.82 % 482.97 %

Final heading [°] -11.69 % 12.27 % -7.55 % 5.86 %

Time from order until ship stop [s] 32.30 % 7.48 % -6.40 % 30.92 %

21

Figure 9 Crash Stop manoeuvre (Red=with rotor, blue=no rotor)

7 Conclusion

Should one be concerned about how much the force of a rotor impacts the handling

characteristics of the vessel? With only one rotor set up on a bigger vessel, no. Bearing in

mind that these tests were done at a wind speed of 25 m/s, the impact of the wind pressure

on the vessel alone would lead to altered handling characteristics. Also, the intended use

case for these rotors is in open waters and not in narrow fairways or channels, where the

vessel should be able to act immediately and predictably. The navigating officer would

probably notice the effects but would intuitively compensate for them, as they would

compensate for anything else, e.g. currents and wind gusts.

22

The most telling tests are the turning circle and zigzag ones, in general the increased thrust

leads to higher exit speed. However, a higher final speed doesn’t always lead to decreased

manoeuvring time, as in some circumstances the forces from the rotor will cause the ship to

overshoot the turns more than usual.

With a multi rotor set up on the other hand, the forces could be quite high even at low

windspeeds. Then one should definitively be mindful of how this will affect the safety of

navigation in high traffic areas such as traffic separations schemes and its surroundings. One

should also consider the placement of the rotors in respects to the centre of lateral resistance

and how that will impact handling.

Should a vessel fitted with rotors sails undergo new sea trials? Perhaps not, there is always

the possibility of turning off the rotor sail. And on the other hand, if one wanted to figure

out how the handling is affected a good first step is perhaps the simulator.

8 Discussion

Inputting the force data by hand at every 5° is the most limiting factor in the accuracy of this

study. Virtual shipyard is compatible with python scripts, so writing a script to make the

simulation dynamic would increase the accuracy and reliability of the simulations and reduce

the time taken to do the simulation. Perhaps Transas should look in to integrating this feature

into Virtual shipyard in the future. With built in features like these, it would be very easy to

study the effects of multiple rotors acting in different places on the vessel.

As shown by this study, the rotor sail will have some impact on how the vessel will handle,

but can this be used in our favour? Further studies could be done on how to optimize and

integrate the rotor software so that it aids in the vessel handling. Or perhaps, the feasibility

of using them as emergency propulsion, in the case of main propulsion gear failure.

23

9 References

Barrass, D. C. (2004). Ship design and performance for masters and mates.

Cauvier, H. (10 2008). The Pivot Point. The Pilot. Hämtat från http://www.pilotmag.co.uk/wp-content/uploads/2008/06/pilotmag-2951.pdf den 28 10 2019

Karvinen, A. (den 08 10 2017). Person overboard rescue maneuver. Espoo.

Molland, A. F. (Red.). (2008). The Maritime Engineering Reference Book.

Seifert, J. (den 14 09 2012). A review of the Magnus effect in aeronautics. Hämtat från http://www.homepages.ed.ac.uk: http://www.homepages.ed.ac.uk/shs/Climatechange/Flettner%20ship/Seifert%20Flettner%20apps.pdf den 03 09 2019

Tupper, E. C. (2004). Introduction to Naval Architecture (4 uppl.).

Tupper, E. C. (2013). Introduction to naval architecture (Fifth uppl.).

24

Table of Figures Figure 1 Force on the ship ...................................................................................................................... 4

Figure 2 Rotor Thrust by Michael Forsberg .................................................................................... 5

Figure 3 Crude Oil Tanker ................................................................................................................... 11

Figure 4 Wheelhouse Poster............................................................................................................... 12

Figure 5 Turning circle Starboard (Red=with rotor, Blue=no rotor) ................................. 15

Figure 6 Turning circle Port (Red=with rotor, Blue=no rotor) ............................................. 16

Figure 7 Zig Zag manoeuvre (Black=with rotor, cyan= no rotor) ........................................ 18

Figure 8 Williamson Turn .................................................................................................................... 19

Figure 9 Crash Stop manoeuvre (Red=with rotor, blue=no rotor) ..................................... 21

Table of Tables

Table 1 Turning Circle Starboard ..................................................................................................... 14

Table 2 Turning Circle Port ................................................................................................................. 14

Table 3 ZigZig with 20° of Rudder.................................................................................................... 17

Table 4 Williamson Turn ..................................................................................................................... 19

Table 5 Crash Stop .................................................................................................................................. 20

Appendix

Turning Circle Starboard

Initial engine telegraph setting 1.00 Initial engine telegraph setting 1.00

Initial speed 14.92 Initial speed 14.92

Speed at heading alteration of 90 degrees 10.17 Speed at heading alteration of 90 degrees 10.40

Time to heading change 90 164.30 Time to heading change 90 163.20

Advance 813.15 Advance 809.12

Transfer 470.97 Transfer 471.12



Tactical diameter 1102.24 Tactical diameter 1110.54



Steady diameter 932.52 Steady diameter 961.95

Final rate of turn 19.57 Final rate of turn 21.02

Final speed 5.16 Final speed 5.71

Speed loss 66.06% Speed loss 62.40%

Initial engine telegraph setting 100% Initial engine telegraph setting 100%













































Wind

180°

Wind

270°

Rotor Off Rotor On

Rotor Off Rotor On

Rotor Off Rotor On

Rotor off Rotor on

Wind

000°

Wind

090°

Appendix

Turning Circle Port






Transfer -469.57 Transfer -469.80







Final rate of turn -18.38 Final rate of turn -19.72
















































Wind

180°

Wind

270°

Rotor Off Rotor On

Rotor OnRotor Off

Rotor OnRotor Off

Rotor OnRotor Off

Wind

000°

Wind

090°

Appendix

Zig-Zag


1st overshoot 14.1 1st overshoot 15.2

1st overshoot time 118 1st overshoot time 124

2nd overshoot 14.7 2nd overshoot 16.3

2nd overshoot time 316 2nd overshoot time 340

3rd overshoot 12.6 3rd overshoot 15.1

3rd overshoot time 544 3rd overshoot time 594

Period 490 Period 546


Final time 1468.7 Final time 1638.6


1st overshoot 12 1st overshoot 11.1
























3rd overshoot 11.5 3rd overshoot 12





Rotor Off Rotor On

Wind

000°

Wind

090°

Wind

180°

Wind

270°

Rotor OnRotor Off

Rotor OnRotor Off

Rotor OnRotor Off

Appendix

Williamson Turn


Full Time 982.7 Full Time 960

Deviation from initial course 109.5 Deviation from initial course 111.41

Time when rudder changed to 35 0 Time when rudder changed to 35 0

Time when rudder changed to -35 98.3 Time when rudder changed to -35 98.3


Full Time 1104 Full Time 1044.6





Full Time 1237.1 Full Time 1192





Full Time 1051.2 Full Time 996.7



Time when rudder changed to -35 85.1 Time when rudder changed to -35 84

Rotor Off Rotor On

Wind

000°

Wind

090°

Wind

180°

Wind

270°

Appendix

Crash Stop


Time engine reach 0%ET 37.2 Time engine reach 0%ET 37.2

Track reach 1940.53 Track reach 2173.26

Head reach 1666.33 Head reach 1407.66

Side Reach -376.91 Side Reach -458.64

Final heading 290.14 Final heading 256.21

Time from order until ship stop 566.6 Time from order until ship stop 749.6



















Side Reach 64.23 Side Reach 374.44



Rotor Off Rotor On

Wind

000°

Wind

090°

Wind

180°

Wind

270°

the impact of rotor sails on a vessel’s manoeuvrability

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