## Stability At Large Angles

How is a ship's stability at large angles measured Why are cross curves used What do you understand by an angle of loll Question 2. What effects do liquid free surfaces in a ship have on its stability Can other cargoes present similar effects Question 3. Why does a ship need stability What factors are usually considered in setting standards Question 4. A ship with vertical sides in way of the waterline is said to be wall sided. Show for such a vessel that for inclinations, p, within...

## Ittc 1957

109 0.00153 0.001531 1010 0.00117 0.001172 In a non-viscous fluid the lines of flow past a body close in behind it creating pressures which balance out those acting on the forward part of the body. With viscosity, this does not happen completely and the pressure forces on the after body are less than those on the fore body. Also where there are rapid changes of section the flow breaks away from the hull and eddies are created. The effects can be minimized by streamlining the body shape so that...

## Launching

Large ships are quite often nowadays built in docks and in this case the 'launch' is like an undocking except that the ship is only partially complete and the weights built in must be carefully assessed to establish the displacement and centre of gravity position including its transverse location. Some weight adjustments may be necessary to ensure the ship does not have excessive trim or heel on floating off. In the more general case the ship is launched down inclined ways and one end, usually...

## Interior Area Of Waterplane

Approaches the lost buoyancy and added weight methods. These give different GM values but the same righting moment. First the volume of the damaged compartment, Figure 7.13, up to the original waterplane, and the area of waterplane lost, are calculated making allowance for the permeability. Suppose the area of original waterplane is A and the area lost is a, where 1 is the permeability. Let the lost volume of buoyancy be v. A first approximation to the parallel sinkage suffered is given by

## Types Of Resistance

When a moving body is near or on the free surface of the fluid, the pressure variations around it are manifested as waves on the surface. Energy is needed to maintain these waves and this leads to a resistance. Also all practical fluids are viscous and movement through them causes tangential forces opposing the motion. Because of the way in which they arise the two resistances are known as the wave-making resistance and the viscous or fictional resistance. The viscosity modifies the flow around...

## Grounding

With few exceptions ships are not intended to ground and when they do, the hull and underwater fittings may be damaged. The extent of damage will depend upon a number of factors, including (2) the speed and angle of impact (3) the sea state at the time of grounding and up until the ship can be refloated (4) the area of ship's hull which impacts the seabed. If the area of hull in contact with a smooth seabed is relatively small the stability can be calculated in a manner similar to that...

## O

The coamings of the upper or weather deck hatches are raised above the deck to reduce the risk of flooding in heavy seas. They are liable to distort a little due to movement of the structure during loading and unloading of the ship. This must be allowed for in the design of the securing arrangements. Coamings can provide some compensation for the loss of hull strength due to the deck opening. A double bottom is fitted along the ship's length, divided into various tanks. These may be used for...

## Dynamic Stability And Control Of Submarines

Modern submarines can travel at high speed but sometimes their mission requires them to move very slowly. These two speed regimes pose quite different situations as regards their dynamic stability and control in the vertical plane. The submarine's static stability dominates the low speed performance but has negligible influence at high speed. For motions in the horizontal plane the submarine's problems are similar to those of a surface ship except that the submarine, when deep, experiences no...

## Modifying The Manoeuvring Performance

As with other aspects of ship performance it is difficult, and sometimes dangerous, to generalize on the effect of design changes on a ship's manoeuvring qualities. This is because so many factors interact and what is true for one form may not be true for another. Broadly however it can be expected that (1) Stern trim improves directional stability and increases turning diameter. (2) A larger rudder can improve directional stability and give better turning. (3) Decrease in draught can increase...

## The Inclining Experiment

As the position of the centre of gravity is so important for initial stability it is necessary to establish it accurately. It is determined initially by calculation by considering all weights making up the ship - steel, outfit, fittings, machinery and systems - and assessing their individual centres of gravity. From these data can be calculated the displacement and centre of gravity of the light ship. For particular conditions of loading the weights of all items to be carried must then be added...

## The Design Process

There are a number of recognized stages in developing a new design. Different authorities use different terms for the various design stages. For the present purposes the terms feasibility studies, contract design and full design will be used. In talking of documentation it should be appreciated that much of the information is nowadays in electronic form, emanating from the CAD and feeding into computerized manufacturing systems. The aim at the feasibility stage is to confirm that a design to...

## Stabilization

A ship's rolling motions can be reduced by fitting a stabilization system. In principle pitch motions can be improved in the same way but in practice this is very difficult. An exception is the fitting of some form of pitch stabilizer between the two hulls of a catamaran which is relatively shorter than a conventional displacement ship. In this section attention is focused on roll stabilization. The systems may be passive or active. Of the passive systems, bilge keels (Figure 12.4) are the most...

## The geometry

A ship's hull is three dimensional and, except in a very few cases, is symmetrical about a fore and aft plane. Throughout this book a symmetrical hull form is assumed. The hull shape is defined by its intersection with three sets of mutually orthogonal planes. The horizontal planes are known as waterplanes and the lines of intersection are known as waterlines. The planes parallel to the middle line plane cut the hull in buttock (or bow and buttock) lines, the middle line plane itself defining...

## Marine Pollution

As well as the effect of the environment on the ship, it is important to consider the effect of the ship on the environment. Over the years the pollution of the world's oceans has become a cause for increasing international concern. In 1972 the United Nations held a Conference on the Human Environment in Stockholm. This conference recommended that ocean dumping anywhere should be controlled. One outcome was the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other...

## Acquiring Seakeeping Data

Computations of performance criteria require good data input, including that for waves, response operators and limitations experienced in ship operations. The sources of wave data have been discussed. The designer must select that data which is applicable to the design under review. The data can then be aggregated depending upon where in the world the ship is to operate and in which seasons of the year. Obtaining the response amplitude operators The designer can call upon theory, model testing...

## Representing the hull form

The hull form is portrayed graphically by the lines plan or sheer plan (Figure 3.4). This shows the various curves of intersection between the hull and the three sets of orthogonal planes. Because the ship is symmetrical, by convention only one half is shown. The curves showing the intersections of the vertical fore and aft planes are grouped in the sheer profile the waterlines are grouped in the half breadth plan and the sections by transverse planes in the body plan. In merchant ships the...

## What Is A Ship Launch Fore Poppet

Made of the weight and centre of gravity position at the time of launch. To facilitate this a detailed record of all material built into the ship is kept, often backed up by actual weighing. In manual calculations the procedure adopted is to treat the launch as a quasi-static operation. That is it assumed that all forces and moments are in balance at every moment. A profile of the ship is moved progressively down a profile of the launch ways, taking account of the launching cradle. The moments...

## Classification societies

There are many classification societies which co-operate through the International Association of Classification Societies (IACS) (www.iacs.org.uk), including Russian Maritime Register of Shipping As with IMO, a lot of information on the classification societies can be gleaned from their web sites. The work of the classification societies is exemplified by Lloyd's Register (LR) of London which was founded in 1760 and is the oldest society. It classes some 6700 ships totalling about 96 million...

It will be appreciated that the type of calculations discussed above lend themselves to the use of computer spreadsheets and Microsoft Excel is very convenient here as it is in many engineering situations as presented in Liengme (2002). A spreadsheet can be produced for the calculations in Table 4.1. This has been done to create Table 4.4. The first four columns present the ordinate number and the values of x, y and Simpson's multiplier. Assuming the x values are in cells B3 to B11, the y...

## Appendix C Glossary of terms

In many cases the fuller definition of these terms, and the context in which they are used, will be found in the main text. They can be found by reference to the index. Where appropriate the usually accepted abbreviation is given. Terms shown in bold in the explanations are defined elsewhere in the glossary. Added mass. The effective increase in mass of a hull, due to the entrained water, when in motion. Added weight method. One method used in the calculation of a ship's damaged stability when...

## Wind

Unfortunately for the ship designer and operator the air and the sea are seldom still. Strong winds can add to the resistance a ship experiences and make manoeuvring difficult. Beam winds will make a ship heel and winds create waves. The wave characteristics depend upon the wind's strength, the time for which it acts, its duration and the distance over which it acts, its fetch. The term sea is applied to waves generated locally by a wind. When waves have travelled out of the generation area...

## Some General Design Attributes

It has been seen that a ship will need to possess certain characteristics, or attributes, to meet an owner's requirements. It is constructive to consider some general attributes of design which apply to all, or most, ship types. Different ship types are discussed in a later chapter. Usually there will be a certain volume of goods the ships of a fleet need to carry. This may have been established by a market survey. The 'goods' may be cargo, people or weaponry. How many ships are needed and the...

## Multihulled vessels

These include sailing catamarans, trimarans, offshore rigs, diving support vessels and ferries. Catamarans are not new as two twin hulled paddle steamers of about 90 m length were built in the 1870s for cross channel service. They were liked by passengers for their seakeeping qualities but were overtaken fairly soon by other developments. The upper decks of catamarans provide large areas for passenger facilities in ferries or for helicopter operations. Their greater wetted hull surface area...

## Directional Stability And Control

It was seen in an earlier chapter that when a ship, at rest in still, water is disturbed in the horizontal plane there are no hydrostatic forces to return it to its original position or to increase the movement. The ship is in neutral equilibrium. When a moving ship is disturbed in yaw it is acted upon by hydrodynamic forces which may be stabilizing or destabilizing. If stabilizing, the ship will take up a new steady line of advance but unless some corrective action is applied, by using the...

## The impact of technology and computers

Over the last half-century technology has had a tremendous impact upon how ships are designed, built, operated and maintained. One could mention a myriad of examples but the following will serve as illustrations (1) Satellites in space have made it possible for ships to locate their position to within a few tens of metres using global positioning systems. The satellites can also pick up distress signals and locate the casualty for rescue organizations. They can measure sea conditions over wide...

## Crude Oil Carriers

These carry the unrefined crude oil and they have significantly increased in size in order to obtain the economies of scale and to respond to the demands for more and more oil. Designations, such as Ultra Large Crude Carrier (ULCC) and Very Large Crude Carrier (VLCC), have been used for these huge vessels. The ULCC is a ship of 300 000 dwt or more the VLCC is 200 000-300 000 dwt. Crude oil tankers with deadweight tonnages in excess of half a million have been built although the current trend is...

## Equilibrium

Equilibrium of a body floating in still water A body floating freely in still water experiences a downward force acting on it due to gravity. If the body has a mass m, this force will be mg and is known as the weight. Since the body is in equilibrium there must be a force of the same magnitude and in the same line of action as the weight but opposing it. Otherwise the body would move. This opposing force is generated by the hydrostatic pressures which act on the body, Figure 5.1. These act...

## Ship Handling

Several aspects of the handling of a ship are not brought out by the various manoeuvres discussed above. At low speed any hydrodynamic forces on the hull and rudders are small since they vary as the square of the speed. The master must use other means to manoeuvre the ship, including 1 Using one shaft, in a twin shaft ship, to go ahead while the other goes astern. 2 When leaving, or arriving at, the dockside a stern or head rope can be used as a pivot while going ahead or astern on the...

## GZ curve See curve of statical stability

A term applied to bulk carriers of 40 000-65 000 DWT. Heave. The vertical movement of a ship, as a rigid body, in a seaway. Heel. The slow angular movement of a ship about a fore and aft axis. Angular movements as a result of waves are referred to as rolling. Hogging. A ship is said to hog when the hull is bent concave downwards by the forces acting on it. Hogging is the opposite of sagging. 418 APPENDIX C GLOSSARY OF TERMS Horizontal prismatic coefficient Cp . One of the...