resistance propulsion power plant propeller water jet propeller vane propeller steerage helm steering gear course stability manoeuvrability motions rolling pitching
heaving axis (pl. axes) crest angular head (oncoming) wave emersion slamming trough oblique wave surging swaying yawing green water
Read the text and find the answers to the following questions.
1. What forces is the moving ship affected by? 2. What does steerage imply? 3. What are three main motions of a ship? 4. Are there any measures against motions?
Fundamentally the external shape of ships is governed by the known laws of resistance. All objects are affected by them when moving through water, whether self-propeller or towed. These laws concern the underwater portion of the hull, out-of-water portion is subjected to resistance from the air. Both the underwater and abovewater portions are additionally and respectively affected by currents and tides on the one hand, and by storms and winds on the other. Man has always been improving the hull shapes to counteract the laws of resistance so that the ship will attain high speeds at a low expenditure of energy.
Propulsion is ship's ability to move through water at a specified speed and effective work of ship's power plant. The moving ship is affected by two forces - resistance to motion and propulsive (propeller) force. The propulsive device transforms the energy generated by the power plant into the work of propeller force. The propulsive devices are: screw, water-jet and vane propellers, the former being widely used. It consists of a boss or hub carrying radial blades. The screw propeller is generally placed at the after end of the vessel. The speed of ships is measured in knots, one knot being equal to one nautical mile (1852 metres) per hour.
Steerage of a ship is closely connected with her propulsion. It is the effect of the helm on a ship in motion. Helm means the rudder and the gear for turning it. Steering gear (steerer) is the steering wheel, steering engine and fittings by which the rudder is turned. Rudder consists of a rudder plate (blade) and a rudder stock (spindle). The ship's steerage is determined by two diametrically opposite seaworthy qualities - steady course ability and manoeuvrability. Steady course ability is the ship's quality to keep the preset direction of motion, and manoeuvrability is the ship's ability to alter the direction of motion in a proper way for rather short period of time.
Motions imply oscillating movements of a ship relating to her state of equilibrium. Anyone who observes the motion of a ship when floating among long smooth waves can notice that the ship is not carried away as if by a current, but that its motion is confined to rising and falling with a slight swaying to and fro within quite definite limits. A large body like a ship is affected by the depth of immersion, the configuration of the wave and the rotary action of the particles of water composing the waves, all of which produce an inclining effect on the ship. There are three main motions of a ship on waving - rolling, pitching and heaving. They can exist on still water too.
Rolling is the transverse oscillating rotation of a vessel about a longitudinal axis, which results when it meets waves, with crests approximately parallel to the length of the ship. It is rhythmic inclination of a vessel from side to side. This increases ship's resistance to propulsion, and requires a corresponding increase in the propulsive power necessary for a given speed.
Pitching - the angular motion which a ship makes about a transverse axis through her center of gravity in a seaway. It is downward falling of a vessel's bow and stern alternatively caused by head, oncoming waves. Heavy pitching is accompanied by flooding of the deck (coming green water), emersion of the ship's bottom and subsequent slamming, i.e. impact of the ship's hull upon the surface of the water. Pitching can result in shifting cargoes, slowing the speed, sea-sickness of the crew and passengers.
Heaving is the vertical motion given to the ship as a whole, especially noticeable when broadside - on to the waves. The whole ship rises on the crest and then sinks into a trough of a wave; it is periodical rise and fall of a vessel as a whole. In a sea way due to the periodical disturbances the oscillations in other directions will emerge, i.e. additional kinds of motions. They are surging, swaying and yawing.
Surging is oscillations of a ship as a whole along the longitudinal axis due to head or oblique waves.
Swaying is movement of the whole ship to port and then to starboard.
Yawing is angular oscillation of a ship as a whole about a vertical axis approximately through her center of gravity. It is deviations of the bow to port and to starboard alternatively.
In operating the ship it is necessary to avoid heavy sudden rolling. This is achieved by applying special stabilizers - bilge keels, stabilizing tanks, fin stabilizers, etc.
b) oscillating movements of a ship under the action of external disturbances;
c) rate of motion or moving;
d) ship's quality of performing turnings;
e) deviations of the bow to port or starboard;
f) downward falling of a vessel's bow and stern alternately;
g) measure of speed for ships;
h) devise with blades which turns to move a ship;
i) the act of directing a ship on her course;
j) opposing force.
Ex. 6. Expand the table matching the terms (the figures) with their definitions (the capital letters) and indicate for improving of which quality the following measures should be taken (the small letters).
A - oscillating movements performed by a ship freely floating at the water surface;
B - ship's ability to develop the pre-set speed at the certain output of the main engine;
C - ship's ability to keep the steady course or to alter the direction of motion at the navigator's command;
D - ship's ability to preserve her buoyancy and stability with one or more compartments flooded;
E - ship's ability to float in the certain position in relation to water;
F - ship's ability to return to her original upright position when inclined.
a - increase of the watertight hull above the load waterline;
b - arrangement of longitudinal bulkheads;
c - arrangement of transverse bulkheads;
d - bulbous bow, foils under the hull;
e - thrusters, active rudders, vane and water jet propellers;
f - bilge keels, active side rudders.
Ex. 7. Fill in the table matching the terms (the figures) with their definitions (the capital letters).
1) cargo carrying capacity, useful deadweight;
2) deadweight (tonnage);
4) cargo capacity;
6) sailing range;
A - a way covered by a ship per a time unit;
B - ship's possible voyage duration (days at sea) without replenishment of stores intended for a crew and passengers;
C - the number of tons of cargo which a vessel can carry;
D - total volume of all cargo spaces (m3);
E - the sum of light ship weight and deadweight;
F - the number of tons that the vessel can lift when loaded;
G - the distance (miles) which can be covered by a ship without replenishment of stores of fuel, oil, feed water needed for power plant operation.
Section 2. Lines plan
Words and Terms to be Remembered
lines drawing (plan) lines plan (body lines plan, sheer lines) profile plan (sheer plan) cross section body plan (frame lines) moulded surface projection
buttock bow line buttock line midship section diagonal equidistant fore body after body
Read the text and find the answers to the following questions.
1. What is a lines plan? 2. What plans does it consist of? 3. What are buttocks and diagonals? 4. What is a lines plan necessary for?
Hull shape can be completely represented by the lines (sheer lines) plan showing the moulded surface of a ship.
Lines plan is a set of drawings showing the form of the hull projected on three planes perpendicular to each other. It consists of three plans: 1) a side projection known asprofile; 2) a plan showing the form of the hull at several waterlines called half-breadth plan; 3) a plan showing the form of the hull at cross sections called body plan. Lines plan shows the moulded surface formed by outer edges of frames, floors and beams without the thickness of outer shell plating.
The profile plan is a projection of the ship's lines to her center line plane. It shows the general appearance of the ship, giving the contour of the stem and stern, the arrangement of superstructures, position of bulkheads, extent of double bottom and position of decks. The lines which are the result of intersecting the moulded surface of the ship by planes parallel to the centerline are buttocks. They are called bow lines when in the fore body and buttock lines when in the after body. Bow and buttock lines are spaced at convenient equal intervals from the ship's centre-line. Buttocks are shown in the profile drawing of the ship's lines. The half breadth plan is a projection of the vessel's lines on the horizontal plane. It shows the shapes of decks and waterlines. The body plan is a projection of the ship's lines on the midship section plane. It shows the shapes of equidistantly spaced vertical sections of the ship. Straight lines extending from the longitudinal middle-line plane to the frame sections of the body plan are calleddiagonals or diagonal lines on the half-breadth and sheer plans. In the body plan it is not necessary to draw both sides of the ship. The sections in the fore body are drawn on the right-hand side, the sections in the after body are drawn on the left.
The lines plan is necessary for making all calculations and experiments, connected with the determination of ship seaworthiness and for development of other drawings.
Exercises and assignments
Ex. 1. Arrange the given words and word combinations into the pairs of synonyms. Find in the text the sentences in which they are used and translate into Russian.
Ex. 2. Match the definition with a vocabulary item.
a) projection of the ship's lines to her centre-line plane
2. body plan
b) a set of drawing showing the from of the hull projected to three main planes
3. profile plan
c) a projection of the ship's lines on the midship section.
4. lines plan
d) a line of intersecting the moulded surface of the vessel and planes parallel to the centre-line plane
e) projection of waterlines on the horizontal plane
6. half-breadth plan
f) a line extending from the centre-line plane to the frame sections of the body plan
Ex. 3. The diagram indicates the planes and projections of lines plan with figures. Match the terms (under capital letters) with the figures on the diagram and with term explanations (under small letters). Complete the table below. Consult the vocabulary items.
A - half-breadth plan;
B - midship plane;
C - waterline plane;
D - center line plane;
E - profile;
F - fore body;
G - body plan;
H - after body.
a - a plane of longitudinal division of the hull into two symmetrical parts;
b - the left-hand part of the body plan;
c - vertical transverse plane which divides the ship hull into fore and after parts;
d - the right-hand side of the body plan;
e - the projection on the midship plane;
f - a horizontal plane parallel to the constructive water line;
g - a plan or top view of half of a ship shown on the water line plane;
h - lateral view or side projection of the ship's lines.
Ex. 4. Do the following task.
1. Give the meanings of the word "section" in relation to the ship hull structure.
2. Make the difference between two terms: "hull" (êîðïóñ) and "body plan" (êîðïóñ).
3. Look carefully at the three projections on the lines plan and say which of them shows the following lines of a ship hull: stem, sides, keel line, stern, flare of the bow, sheer, camber, deadrise, deck line, waterlines.
4. Study the lines drawing (fig. 5) of a cargo ship and speak on all three projections of it.