into her construction aft than forwards, the boat may float with her stern up, or vice versa, according to distribution of weights. Designers have to calculate these weights and so distribute the weight of the motors, gasoline tanks, water tanks and everything else to be carried, as to bring the boat down just level with her L. W. L. When floating at that line the center of buoyancy will be where the calculations showed it came.

69. Construction and Accommodation Plans.-Besides the lines shown in Fig. 60, a set of plans of a boat includes the construction plans and the interior layout, or accommodation

plan, also a tabulated list of all measurements necessary for reproducing the lines known as a Table of Offsets, and a complete list of specifications.

70. Laying Down a Boat.-After the drawings and plans are completed, the boat is laid down on the mold-loft floor in full size as a whole or in parts. The laying down consists in cutting lines representing those that appear on the plans. From the mold-loft plans, or directly by measurements from the drawings, the scrive board, molds, and templets are prepared. The scrive board is, in effect, a full-sized drawing, showing the shape of every frame of the boat; but it is not always used, templets being sometimes built up from the smaller drawings and carried directly to the bending slab where the frames are shaped. Detail drawings are made of all parts of the boat, showing, on a convenient scale, their exact shape and size.

MARINE GASOLINE ENGINES

(PART 1)

PRINCIPLES OF OPERATION

TWO-CYCLE PRINCIPLE

DEFINITIONS AND NAMES OF PARTS

1. Marine gasoline engines belong to the class of engines, or motors, known as internal-combustion engines, so called because power is obtained by burning the fuel within the engine cylinder. This fuel consists of a mixture of air and vaporized gasoline, kerosene, or some other liquid. The burning of the fuel, which is so rapid as to be practically instantaneous and is called the explosion, produces gases of high pressure that act against a movable piston, driving it outwards and doing the required work through the proper mechanism. Provision is made by means of ports and valves in the cylinder walls for admitting the fuel and for allowing the burned gases to escape.

2. Some of the essential parts of one of the simplest forms of marine gasoline engines are shown in Fig. 1, which is a crosssectional view of a vertical, single-cylinder, two-port, two-cycle engine. In this illustration the cylinder is considered as being cut in half crosswise, and the front half removed, exposing to view the various parts. The cylinder proper a is surrounded by the jacket space b, through which water is passed for the

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purpose of cooling the cylinder walls, which become hot from the repeated explosions; the cylinder is supported by the airtight crank-case c. The movable piston d is connected to the crank e by the connecting-rod f, which is attached at the top by the piston pin g and at the bottom by the crankpin h; as the piston moves up and down it causes the crank to rotate, thereby turning the crank-shaft, which is rigidly attached to the crank. The crank-shaft cannot be shown, as it is concealed by the crank e. The piston is provided with piston rings i, which form an air-tight joint between the piston and cylinder walls. It is made hollow, so as to receive one end of the connectingrod f, being of the socalled trunk type.

d

P

FIG. 1

m

3. At j, Fig. 1, is shown a device, called a carbureter, into which the gasoline, or other liquid fuel, is led, vaporized, and mixed with air. This mixture, known as the combustible mixture, or charge, is taken into the crank-case through

the inlet port k. It is prevented from flowing back into the carbureter by the valve l, which is held to its seat by a spring and will only open inwards. The charge is admitted to the cylinder through the transfer port m and the burned gases are allowed to escape through the exhaust port n. Both of these ports are opened and closed by the piston on its up-and-down movement. The projection o on the piston face is called the baffle plate, or deflector, and directs the charge into the upper

part p of the cylinder, known as the compression space, or combustion chamber. At q is a device, called the igniter, by means of which an electric spark is produced in the cylinder to fire, or ignite, the charge.

The burned gases resulting from the explosion in the cylinder of an engine are known as the exhaust gases or simply the exhaust. These gases are waste, and are allowed to pass out into the atmosphere.

4. Gasoline engines may be either vertical or horizontal, depending on the arrangement of their cylinders. Engines having cylinders placed vertically like that shown in Fig. 1, are classified as vertical engines, while those with their cylinders placed horizontally, that is, supported on one side, are horizontal engines. Practically all gasoline engines used on motor boats are of the vertical type.

5. The end of the cylinder attached to the crank-case in any engine is called the crank-end, while the other end is called the head end; that part of the cylinder wall enclosing, or covering, the head end is known as the cylinder head. The move ment of the piston from the head end to the crank-end is called the forward, or outward, stroke and the movement in the opposite direction is called the return, or inward, stroke.

The engine is said to be on dead center when the crank and connecting-rod form a straight line. Two dead-center positions are possible: When the piston is at the end of its inward stroke, the crank is on its upper, or inner, dead center; and when the piston is at the end of its outward stroke, the crank is on its lower, or outer, dead center.

GASOLINE-ENGINE CYCLE

6. A cycle is any chain, or series, of events, or happenings, occurring over and over again in the same order. As applied to a gasoline engine, the term cycle refers to the operations, or .events, that take place within the cylinder from one explosion to the next, and by means of which the fresh charge is drawn into the combustion chamber and exploded and the exhaust

gases are expelled. These events always occur in the same order and are repeated after each explosion.

The cycle on which an internal-combustion engine operates is one of the distinguishing features of the different types. Gasoline engines are classed as two-cycle engines and four-cycle engines, depending on the method of expelling the burned gases from the cylinder after the explosion and of taking in a fresh charge of combustible mixture; or, in other words, on the number of strokes of the piston required to complete a cycle, or for each explosion.

7. A two-cycle engine is one in which only two strokes of the piston, an inward and an outward stroke, are required to complete the cycle. The combustible mixture is admitted to the cylinder and the exhaust gases are expelled at the end of the downward stroke. The fresh charge is then compressed in the compression space or combustion chamber on the upward movement and exploded and allowed to expand and do work during the next downward stroke. In this cycle, the fresh charge is drawn into the air-tight crank-case and slightly precompressed so that it will readily flow into the cylinder against any pressure existing therein.

8. A four-cycle engine is one in which four complete strokes of the piston are required to complete the cycle; or, in other words, an explosion occurs once for each four strokes of the piston or two revolutions of the crank-shaft. In this type of engine, the fresh charge is drawn into the cylinder through the inlet port by a separate outward, or downward, stroke, and the burned gases are expelled for the most part by a separate upward movement of the piston. Generally speaking, one event occurs during each of the four strokes of this cycle; that is, considering the stroke upon which the charge is drawn into the cylinder as the first stroke, the mixture is compressed during the second stroke, burned during the third stroke, and the exhaust gases are expelled during the fourth and last stroke, after which the conditions at the beginning of the first stroke again exist and the cycle is completed.