Ship Stabilizers: A Handbook for Merchant Navy Officers

By W. Burger and A. G. Corbet

Ship Stabilizers: The Design and Operation in Correcting the Rolling of Ships presents the working principles and operation of various types of gyrocompasses and automatic pilots. This book discusses the fundamental principles of synchro systems and servomechanisms. Organized into 12 chapters, this book begins with an overview of the problem of the reduction of rolling. This text then explores the means of resisting rolling, which have been used or are still being used, including bilge keels, oscillating weights, anti-rolling tanks, gyroscopic stabilizers, and tilting fin systems. Other chapters consider the anti-rolling tanks that are divided into three types, including diversified tanks, free surface tanks, and U-tube tanks. This book discusses as well the small oscillations of a vessel in still water, which are simple harmonic in character. The final chapter deals with the stabilizing gear for smaller ships. This book is a valuable resource for marine engineers and naval architects.

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 28, 2014
• ISBN: 9781483180939

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Preface

This work forms the third in the series dealing with gyroscopic appliances on board merchant ships. The other two volumes— both by Burger and Corbet— explain the working principles and operation of different types of gyro-compasses and automatic pilots.

Although stabilizing tank systems are discussed in this book, the core here, again, has been built around the gyroscope which with the different types of control systems forms the most important part of the book. The effective action of the units— surprisingly small in size— contained in these systems, can change a rough passage into a smooth one and greatly reduce damage to ship and cargo.

The large machinery which acts upon signals received from the control system has been described only from a Deck Officer’s point of view; here is a domain where the authors fear to tread.

The fundamental principles of synchro systems and servomechanisms are briefly dealt with in Chapter IV, but it is generally assumed that the type of student who reads this work is familiar with simple electrical and electronic circuits such as valve and transistor amplifier devices.

It is the purpose of this book to make Deck and Engineer Officers of the Merchant Navy more familiar with the splendid installations in ships which are the outcome of much research (in this country at the Admiralty Research Laboratory and by William Denny & Bros.) and the combined effort of precision engineers, electrical engineers, marine engineers and naval architects.

Full acknowledgements and a bibliography are given at the back of the book.

W.B. and A.G.C.

Cardiff.

INTRODUCTION: A Short Historical Review of Stabilizing Equipment up to the Present Time

The problem of the reduction of rolling has been the subject of investigation for nearly a hundred years. On merchant ships extensive rolling may cause shifting of cargo and subsequential listing, carrying away of deck cargo and other gear; on passenger ships the comfort to passengers and smooth running of catering services are affected; in warships the accuracy of gunfire is reduced. In all cases rolling will increase the repair bill, decrease the speed and can be responsible for injuries and fatal accidents.

The schemes and installations which have been tried are:

1. Fitting of bilge keels

2. Movement of solid weights

3. Movement of water

4. Gyroscopes

5. External fin movement.

They can be divided into two classes, namely, the "passive systems and the active" systems. In the passive systems energy is drained from the ship; in the active systems power is provided to produce a resistance couple.

Bilge Keels

The fitting of bilge keels was the earliest method and is the easiest way to introduce roll damping. It was about 1870 when the first ships were equipped with bilge keels. Their damping effect is greater as speed is increased. They are of the passive stabilizer type as the roll energy of the ship is transferred to the water particles in the vicinity of the ship’s hull.

Not all ships are suitable to be fitted with bilge keels. An example is an ice-breaker where the hull contours must be smooth so that she can drive herself upon the ice and collapse it.

Movement of Solid Weights

This system was first installed by Thorneycroft on the yacht Cecile in about 1891. The movement of the weight was controlled by a pendulum which actuated a hydraulic engine.

At about the same time Norden experimented with weights moved on rails across the deck and Cremieu had arrangements made on a channel steamer to move a truck on a curved track in a chamber filled with a viscous fluid.

These systems were not completely unsuccessful but the difficulty was that the time lag between the control signal and the movement of the weight was too large and no correct phasing was achieved. Were those systems to be tried nowadays a much greater measure of success might be expected owing to the existence of synchros and servomechanisms.

Movement of Water

The fitting of anti-rolling tanks dates back to about 1880. Water chambers or slosh tanks were installed in the upper part of a ship. The water movement always lags behind the roll of the vessel and its direction is always downhill thus removing potential energy from the ship. The free surface effect of these tanks reduces the moment of the ship’s stability couple and hence lengthens the rolling period.

In 1910 the German, Hermann Frahm of Hamburg, gave the roll stabilization problem his serious attention and developed the U-tube tank situated above the centre of gravity of the ship. In this manner the stabilizing moment created by the waterflow in the cross-connection assisted the stability couple. For efficient operation the period of transfer of the water in the tank should be approximately equal to the natural period of the vessel. In the early systems an airduct with a throttle valve connected the top of the tanks. The aim of the valve was to control the period of water transfer, though it is doubtful now if manipulation of this valve could have changed the period to an effective amount. In other systems, the tank tops were vented to the atmosphere (see Fig. 3.2) thus eliminating the need for an airduct.

In some installations the cross-connection was entirely removed and the bottom of the tanks were kept open to the sea. The behaviour of these sea-ducted tanks is somewhat less well understood.

Frahm’s passive tanks were installed in over 1,000,000 tons of German shipping. Their effectiveness is greatest near resonance, i.e. synchronism between the period of the waves and the natural roll period of the ship.

A further development from passive to active tanks was made by Minorsky in the U.S.A. in 1928. In this case an air compressor supplies air above atmospheric pressure to the upper part of the tanks and stabilization is achieved by varying the relative amounts of water in the tanks on the port and starboard sides of the ship. The control mechanism is a gyroscope which, by its precession, makes and breaks electrical contacts and so starts and stops the air compressor which thus regulates the flow of water in the tanks. The German cruiser Prinz Eugen had such a system using fuel instead of water. Until recently the Royal Rotterdam Lloyd motor ship Willem Ruys employed this system.

At the present time roll stabilization by means of passive tanks is being re-developed and regaining favour. This is the so-called "Diversified tank system" using two or more tanks with different parameters. One installation uses two tanks each side with cross-over ducts containing butterfly valves. Another installation, fitted on many ships nowadays, employs a three-tank system; one on the port side, one on starboard side and between them a centre buffer tank (flume tank). Vertical nozzle stanchions (see Figs 4.3 and 4.4), which act as constrictions, separate the tanks and regulate the period of water or fuel transfer. Ways are provided to alter the nozzle restriction, if required. Such tanks at the moment, provide the most suitable passive stabilizers.

Gyroscopes

This type of stabilizer uses large and high-speed gyroscopes to provide a resistance couple to the rolling motion of the vessel. The first work on it was carried out by Otto Schlicke in 1906. It was a passive stabilizer, only requiring power to keep the gyro spinning. Its movement was restricted in the athwartships plane, but it could swing freely like a pendulum in the fore and aft plane of the vessel. The rolling motion about the fore and aft axis causes the gyro to precess about the transverse axis (i.e. in the fore and aft plane). When this precession takes place a large opposing couple, proportional to the angular velocity of roll, is exerted on the ship. The couple can be varied by braking the pendulum motion.

In 1915 the activated gyro-stabilizer was devised by the Sperry Gyroscopic Company. This system possesses several advantages over the Schlicke installation. Motors, instead of brakes, are provided, which forcibly precess the gyro in such a direction so as to create a stabilizing couple opposing the external rolling couple. The motors run at a constant speed and are started, stopped and reversed by a small pilot gyro which senses the angular roll velocity. An illustration of the layout is given in Fig. 3.4.

Active gyro-stabilizers were installed in about forty ships, the majority being yachts. A very well-known installation was in the Conte di Savoya (41,000 tons displacement). This latter installation comprised three large gyroscopes, each rotor weighing 110 tons and revolving at 910 rev/min. Maximum anti-roll couple was 5400 ton-ft and the weight of the complete plant was about 600 tons. A smaller installation was put in the British destroyer Vivien in 1924. There is a risk, however, in fitting this system to warships, of the gyroscope breaking up if struck by a projectile.

The roll-reducing action of gyro-stabilizers does not depend on the speed of the ship— as also is the case with the tank systems— and this type of stabilizer is therefore very useful for employment in vessels which are often stationary, e.g. pilot boats, weather ships, survey ships, etc.

This type of stabilizer is quite effective and though some ships are still equipped with it, production of it has stopped. Its disadvantages are its weight, cost and the amount of space it occupies.

External Fin Movement

The fins protrude from the ship’s hull and are operated in such a manner that the ahead motion of the ship produces a lift in one direction on one fin and in the opposite direction on the other fin. The couple so produced opposes the rolling couple of the vessel. This system is almost useless at low speeds but is extremely effective for high and constant speeds.

Dr. Motora of the Mitsubishi Nagasaki Shipyard introduced this type of stabilizer in 1925. It was initially operated by manual control (similar to steering a ship) but later on the shafts were rotated by motors actuated by a pilot gyroscope similar to that used on the Sperry Gyroscopic Stabilizer.

However, we have to thank the Denny-Brown Company for the perfection of this system. Their first installation was on the cross-channel steamer Isle of Sark in 1936 and since then this type of stabilization is unexcelled where stabilization is wanted at speed as on passenger ships and warships. These stabilizers have been fitted on ships of all sizes, from yachts to some of the largest liners, including the Queen Elizabeth. It took many years of experimental research— interrupted by the Second World War— before the design of the fin was perfected to obtain maximum stabilizing effect. The development of the control system (Admiralty Research Laboratory and Muirhead Ltd.) also followed the road of painstaking investigation and gradual improvement. At the present time the control system comprises three sensing elements which detect roll, roll velocity and roll acceleration.

Lately more companies (Sperry, Lidgerwood, Vosper, Siemens) have manufactured similar designs with variations in control and feedback devices.

The fins can be of either the non-retractable or the retractable and the hinged type. In the latter type the fins are housed by swinging them inside the hull in the horizontal or nearly horizontal plane.

Comparison between Activated and Passive Stabilizer Systems

The activated systems produce finer roll control but they are expensive. The passive systems cannot develop sufficient stabilizer moments unless associated with a few degrees of roll. For ships which operate with low or medium speeds and where cost is more important than the degree of stabilization, passive systems provide effective stabilization. The choice, in general, depends on the type of employment of the vessel.

This book will discuss subsequently the rolling of ships, the gyroscope— i.e. the brains of activated systems, anti-rolling devices in general and anti-rolling devices in particular.

CHAPTER I

Rolling of Ships

Publisher Summary

The rolling characteristics of a ship sailing in deep water in which a swell is running depend on various factors concerned with the vessel. A tender ship rolls slowly and easily in contrast with a stiff ship, which rolls with short and jerky motions. The factors involved relate to the size and draught of the ship, the height of the centre of gravity, the form of the body under water, and the distribution of weights in the vessel. The type of rolling is dependent upon the wave height and the effective period of the waves, that is, the time between the successive encounters of the ship with two wave crests. The first cause would introduce a uniform type of rolling depending on the characteristics of the ship and the oscillation produced is known as the free oscillation; the second cause would add a forced oscillation to the motion that is non-uniform as the wave heights of successive waves might differ; the wave series do not form a completely regular pattern. The combined result is that the rolling motion passes through cycles with maximum angles of roll separated by one or more minimum angles of roll. When a ship is made to yaw by waves overtaking it on the quarter, a heel might be produced for an appreciable time. This heel is dependent on the rolling action, but would add to the non-uniformity of the motion.

As most sailors know, the rolling characteristics of a ship sailing in deep water in which a swell is running depend first of all on various factors concerned with the vessel herself. They speak of a tender ship which rolls slowly and easily in contrast with a stiff ship which rolls with short and jerky motions. The factors involved relate to the size and draught of the ship, the height of the centre of gravity, the form of the body under water and the distribution of weights in the