Chapter XLII: Marine Engines (2)

A common paddle wheel has a cast iron centre into which the wrought iron arms are set and secured by wrought iron bolts and nuts.

The bolts have hook heads to grip the back of the arm, and receive a nut and plate to secure the paddles.

Paddle wheels are sometimes provided with cast iron floats to act as counterweights to some unbalanced part of the engine. They are mostly required on side lever engines having a single crank; they are placed nearly opposite to the crank, but not quite, so that they may prevent it from stopping on the centre, and be difficult to start again.

Paddle wheels for engines having a single crank sometimes have their floats of varying breadths, so as to keep the speed of revolution as uniform as possible. This is accomplished by making some of the floats wider than the others. The broadest floats are in action when the crank is at its points of greatest power, and the narrowest at the time the engine is on a dead centre, hence there are four general graduations of breadth in the circumference of the wheel.

A radial paddle wheel is one in which the floats are fixed to the paddle arms, and their ends are in a line radiating from the centre of the paddle shaft.

A feathering paddle float is pivoted at the centre of its ends, and so arranged that by a mechanical movement it will remain vertical when in the water, notwithstanding the circular path it revolves in.

The object of feathering is to cause the thrust of the float to be as nearly as possible in a horizontal line, and therefore more nearly parallel to the line of the ship's motion, and thus utilize more of the paddle power to drive the ship.

The eccentric for feathering the floats is fixed to the ship's side, and sometimes carries a plummer block or pillow block for the paddle shaft bearing. The centre of the eccentric sheave or wheel is placed ahead of and level with the paddle shaft axis. The working surfaces of a feathering wheel are of brass, and the bushes of the paddle arms of lignum vitæ.

The surfaces are lubricated by the water, but sometimes oil lubrication is provided for the eccentric sheave.

A disconnecting paddle engine is one in which the paddles may be driven separately or together. This is effected at the inner port bearing by a clutch wheel, which slides endways on the shaft and is driven by feathers seated in the shaft. This clutch wheel is operated by a lever so as to engage or disengage with the crank pin, which is fast in the outer crank.

Disconnecting paddle engines are always fitted with loose eccentrics, such engines being used for steam tugs and ferry boats, where quickness of turning and of reversing is of great importance.

The thread of a screw propeller is its length measured along the outer edge of the blade.

The angle of the thread is its angle to the axial line of the propeller shaft.

The length of the thread is the length of the outer or circumferential edge of the blade.

The area is the surface of one side of the blade.

The diameter is the distance apart of the two points on the edges that are diametrically opposite and furthest apart.

The pitch of a propeller is its degree of spirality, and is represented by the distance it would move forward if the water was a solid. It is measured by drawing a line representing the axis of the propeller shaft, and at a right angle to it a line representing in its length the circumference of the circle described by the tips of the blades; from the point of intersection of these two right angle lines a diagonal line is drawn representing the angle the blade at its outer edge stands at the propeller shaft axis. The greatest distance between the diagonal line and the line representing the propeller circumference is the pitch of the propeller.

A left handed propeller has a left hand thread or spiral, and revolves from left to right to move the ship ahead.

A right hand propeller has its blades inclined in the opposite direction, and of course revolves in the opposite direction to a left hand one.

The slip of a propeller is the difference between the distance the ship is moved by the propeller and the distance it would move if the water was solid. Slip is usually expressed in the percentage that the distance the ship actually travels bears to the distance she would have travelled if there had been no slip. From 10 to 20 per cent. is lost in slip.

A screw of increasing pitch is one in which the angle of the face of the propeller blade to the axis of the shaft increases as the thread recedes from the shaft, or from the centre to the circumference of the blade, or in both directions.

In a uniform pitch the angle of the blade to the propeller axis is the same at all distances from the axis.

An example of a screw of uniform pitch would be a piece of angle iron wound around a parallel shaft. If wound on a tape shaft, the largest diameter being nearest to the ship's stern, it would have an increasing pitch. If wound around a parabola, the pitch would vary at every point in its diameter and thread.

A thrust bearing is a journal bearing provided with a number of corrugations or collars fitting with corresponding corrugations or recesses in the thrust block, the area thus provided serving to resist the end thrust placed by the propeller upon the shaft.

It must be freely lubricated by ways leading to each collar or corrugation, and so situated that it is accessible for examination. It is sometimes at the end of the first length of shaft aft of the engine.

A stern tube is a sleeve enveloping the aft end of the propeller shaft to protect it from the sea water, which would corrode it. At the aft end of the stern tube is a gland and stuffing box. At the inner end, which extends to the aft bulkhead, it has a flange which is bolted to the bulkhead.

The bearing area of the shaft and stern tube are lined with brass (about half an inch thick) to prevent their oxidation from the action of the sea water.

A lignum vitæ bearing is a wooden bearing generally fitted to the outer end of the stern tube in propeller engines, or to the outer ends of the paddle shaft of paddle engines. It consists of strips of lignum vitæ dovetailed into the bearing or bush, and running lengthways of it. These strips are prevented from working out by a check plate at each end of the bearing.

Screw propellers may be fastened to their shafts in several ways, as by a key or feather sunk in the shaft, and projecting into a keyway in the propeller bore, and a nut on the end of the shaft with a safety pin outside the nut, or by a key passing through the boss of the propeller, and a safety pin or plate upon the key.

The principal pipes of a marine engine and boiler, and the parts they connect, are, the main steam pipe, connecting the stop valve on the superheater to the steam chest of the engine cylinders; the waste steam pipe from the safety valve to the open air; the blow-off pipe, connecting the blow-off cocks on the bottom of the boiler with the blow-off Kingston cock on the ship's side; cylinder jacket pipe from the stop cock on the boiler to the steam jacket.

The circulating suction pipe, connecting the main Kingston valve with the bottom of the circulating pump; the circulating delivery pipe, connecting the discharge compartment of the condenser with the main delivery valve on the ship's skin; the air pump suction, connecting the body of the condenser with the suction side or bottom of the air pump; the main exhaust pipe, connecting the exhaust passage of the low pressure cylinder with the condenser; the feed water suction pipe, connecting the donkey feed pipe with the hot well; the feed water delivery pipe, connecting the donkey feed pump with the check valve on the boiler; the bilge suction pipe, connecting a strum box in the bilge with the bilge pump; a suction pipe from the strum in the bilge to the donkey pump; the bilge pump delivery pipe, connecting the bilge pumps with bilge delivery valves on the ship's side.

A mud box is a rectangular box usually placed in the engine room, and serving to clear the bilge water from foreign substances, as small pieces of wood, coal, etc.; the construction is as follows: It is on the suction side of the bilge pumps, and is provided with a hinged lid that affords access to clean it out, and that must obviously close air tight, or the bilge pumps will not draw. The box is divided into two compartments by a loose division plate that stands vertical, and is perforated so as to act as a strainer.

The steam from the boiler passes through the superheater, main stop cock or valve, main steam pipe, separator, regulating and throttle valve, steam chest, steam port, steam passage into cylinder, returns through steam passage and port, exhaust cavity of valve into either the condenser or the low pressure cylinder, as the case may be, finally exhausting into condenser, whence the water of condensation is pumped by the air pump into the hot well. In the case of a jet condenser part only of the condensed steam goes back to the boiler, the rest going into the sea through the injection discharge pipe.

A steam jacket[61] is an outer casing to a steam cylinder, the space between it and the cylinder being filled with steam direct from the boiler, with the object of preventing condensation of the steam in the engine cylinder.

[61] See page 374 on steam jackets.

A drain cock is supplied to the bottom of the jacket to pass off condensed water. Steam jackets should be lagged or felted to prevent condensation.

The parts of an engine that require to be felted or lagged are the cylinders and the steam pipes; the boilers also should be felted or otherwise covered to prevent loss of heat by radiation, and the uptake protected by means of thin plates, kept, by means of distance pieces and bolts, at a distance of two or three inches from the plates of the uptake.

Various non conducting substances are employed to prevent radiation, as, for example, felt, mineral wool, asbestos, and various kinds of cement.

The pieces of the engine through which the steam pressure is received and transmitted are as follows:

The piston, piston rod, cross head, cross head gudgeon, connecting rod, crank pin, crank shaft and couplings to the propeller shaft.

Trunk engines are generally used in war vessels where it is required to have the engines below the water line. The trunk passes through the cylinder and the piston is upon the trunk, the connecting rod passes down into the trunk and connects direct to the piston. A stuffing box and gland in each cylinder cover keeps the trunk steam tight. The trunk forms a guide to the piston in place of the ordinary cross head and guides, and thus saves the room required by those parts.

The cylinders for a right handed propeller should be on the starboard side of the vessel, so that the pressure on the piston, when the engine is going ahead, shall be in a direction to lift the trunk in the cylinder, and thus act to relieve the gland and cylinder bore of the weight of the trunk and piston.

An oscillating engine is one in which the cylinder is mounted on bearings called trunnions, so that the cylinder can swing and keep its bore and the piston pointing to the crank at all parts of the engine revolution. This enables the connecting rod and slide bars to be dispensed with. The trunnions are hollow, one containing the steam and the other the exhaust passage.

Oscillating engines are used for paddle steamers, because their construction permits of a good length of piston stroke, while still keeping the engine low down in the vessel.

The valve motion for an oscillating engine consists of an ordinary eccentric gear or motion, with the addition of various mechanical arrangements to accommodate the valve gear to the vibrating motion of the valve chest.

The stuffing box of an oscillating engine is made deeper than usual because the gland bore has more strain on it, and extra wearing surface is therefore required to prevent its wearing oval.

Geared engines are those with gear wheels to increase the revolutions of the shaft above those of the engine, and thus obtain a high propeller speed without a high piston speed.

The pressure that propels a vessel is taken by the thrust block in a screw propeller engine.

The pressure that drives a paddle steamer is applied to the hull at the shaft bearings and their holding beams, and to the bed plates. The amount of fuel required per horse power per hour, by modern compound engines, is from about 1-1/2 to 3 lbs., and by common condensing engines from 3 to 5 lbs. per horse power per hour.

The unit or measure of a horse power is the amount of power required to lift 33,000 lbs. one foot high in a minute.[62]

[62] See page 407, Vol. II.

Nominal horse power is a term used to represent the commercial rating or power of an engine, and is usually based upon the area of the piston. It gives no measure of the engine power, however, because it does not take the piston speed into account.[63]

[63] See page 374, Vol. II.

In a surface condensing engine the duty of the air pump is to merely pump the condensed steam and vapor from the condenser to the hot well, whereas in a jet condensing engine it has to also take the condensing water from the condenser, hence an air pump for a surface condenser may be made smaller than that for a jet condenser. As the air pump works against the pressure of the atmosphere, therefore the smaller it is the less of the engine power is absorbed in working it.

The injection cocks are regulated for opening by rods having handles attached. If the injection cocks are not open wide enough, the condenser will get hot and impair the vacuum, while if opened too wide, the water in the hot well will be cold and the boiler feed will be cold. These cocks should be so regulated as to keep the temperature in the hot well at about 100° Fahrenheit.

The parts of a marine engine that are exposed to danger in a cold climate are all pipes through which cold water circulates, and are liable to freeze.

The precautions necessary to prevent freezing in cold climates are to cover all pipes liable to freeze, to keep the water circulating through them, or to let it out of them if necessary, as in the case of the engine standing.

A marine engine may fail to start, or may be prevented from starting by the following causes:

1st. The H. P. slide valve may be off, or away from its seat, thus admitting the steam to both sides of the piston at the same time.

2d. The engineer may have forgotten to disengage the hand turning gear from the crank shaft.

3d. The propeller may be fouled with a piece of timber, or by a chain or rope (these causes sometimes occurring when the ship is in port), or there may be something wrong with the outer bearing of the propeller shaft.

4th. In the case of a propeller fitted with a banjo frame (for the purpose of raising the propeller) the propeller may be locked.

5th. An obstruction, as a block of wood, in the crank pit may prevent the crank from turning.

6th. The slide valve nut may have slackened back, thus loosening the slide valve.

7th. The slide valve spindle may have broken.

8th. When an engine has no auxiliary or starting, but an _impulse_ valve that merely lets a puff of steam into the receiver, this impulse valve may leak, and if the escape or relief valve on the receiver is too much loaded, it may gag the H. P. piston by giving it high pressure steam on both sides, and this may throw the valve off its seat. Similarly, if the engine has an auxiliary or starting valve, and it leaks, high pressure steam may be admitted to both sides of the L. P. piston, thus gagging it and causing its slide valve to throw back and away from its seat.

9th. The cylinders may be choked with water, and the drain cocks choked up.

10th. The crank shaft bearings may be screwed up too tightly.

11th. The air or the circulating pump may be choked with water, either the air pump overflow valve or the circulating discharge valve being secured down.[64]

[64] The air pump overflow valve should never be permanently fastened
down. More engines have been broken down from this than from almost
any other neglectful cause, because, from great leaks in the condenser
tubes and engines standing for a length of time, a larger quantity of
water may require to be got rid of during the first few strokes of the
pump than can pass through the small air or vapor pipe, which is
usually fitted from the hot well either into the bilge or else
overboard. Unless the valve in this overflow pipe is heavy enough of
itself (which is very rarely the case), it should be loaded by a
spring or weight, so that when the puff of the air pump causes it to
lift, and the vessel is rolling, sea water may not pass into the hot
well. To avoid this, some engineers erroneously fasten this valve
down. An experienced engineer states that in his experience five
engines have been broken down from this cause alone.

12th. From the engines being allowed to stand a long time in one position, and the glands being too tightly packed. An engine should be turned a little daily when not in use.

13th. From the piston rings being set out too tight to the cylinder bore.

14th. From the throttle or stop valve being shut, as from its spindle being broken.

15th. From the eccentric sheave, or wheel, having shifted on the shaft, some eccentrics having a key that is not sunk in the sheave, which is done so that the eccentric may shift rather than break if it should seize in its strap.

16th. From the H. P. piston leaking badly, or its ring being broken, which will permit the cylinder to fill with steam and the slide valve to unseat.

17th. If the engine has been overhauled, the forward eccentric may have been connected to the wrong end of the link, thus giving an improper motion to the slide valve.

18th. The expansion may be set to cut off too early in the stroke.

19th. From the air pump rod, or from the circulating pump rod being broken, or from the valves being broken.

20th. From the cylinder casing or the receiver being cracked so as to admit steam to both sides of the piston at the same time.

A defective vacuum, or loss of vacuum, may occur from the following causes:

1st. From the glands of the low pressure cylinder leaking.

2d. From the pet cock of the air pump being left open.

3d. From the joints of the connections about the condenser leaking.[65]

[65] To discover a leak about a condenser, pass an exposed light, as a
candle, about the joints, etc., and where there is a leak the flame
will be drawn in towards the condenser.

4th. From the condenser being cracked, and therefore leaky.

5th. From the injection cock or valve being closed.

6th. From the condenser tubes being foul for lack of being cleaned. From the L. P. cylinder escape valves or cylinder cocks being leaky, and therefore letting in air.

7th. From the slide valve and piston of the L. P. cylinder leaking.

8th. From the air pump valve being leaky or broken. From the circulating pump being defective, as from having leaky valves.

9th. From the Kingston injection valve not being properly opened, or from its outside orifice being choked.

10th. The bilge injection may be so connected with the air pump or condenser as to impair the vacuum when its valve is accidentally stuck and its stop cock is left open.[66]

[66] It is obvious that a defective vacuum may or may not prevent an
engine from starting, according to the degree of defectiveness.

The principal causes of heating are:

1st. The bearing caps being screwed down too tight.

2d. The bearings being left uncovered, thus allowing the brick dust used for cleaning the machinery, the dirt from coaling the ship, or the sand used for cleaning the decks, to get into the bearing.

3d. The oil grooves in the brasses being worn out or too shallow, or the brasses not being cleared at the sides.

4th. Improper fitting of the distance pieces or fit strips between the brasses.

5th. Bad oil or too light an oil.

6th. If the brasses are too slack and thump or pound, the back of the brass may be stretched by pening, causing the sides of the brass to close in upon and bind the crank journal or crank pin, and this will cause heating.

For other information concerning the engine see as follows:

Page.
Angularity of connecting rod 375
The slide valve 376
Double ported and griddle valves 377
Balanced valves 377
Piston valves 378
Separate cut off valves 378
Reversing gears 383
Finding the working results of a slide valve 376
Condensing engines 442, 444
Calculations on the mechanical powers 405
The unit of power 407
Calculating horse power 407
Calculations of safety valves 409
Heat, water, and steam 410
The expansion of steam 411
The conversion of heat into work 411
The indicator 413
Indicator diagrams 414, 421
The barometer 415
Calculating the horse power from indicator diagrams 419
Finding the steam of water consumption from an indicator 421

Figs. 3405 and 3406 represent a triple expansion marine engine, the construction being as follows:

The high pressure cylinder has a piston valve and the intermediate and low pressure cylinders flat valves. Each cylinder has a link motion, and all three link motions are shifted from the same shaft, which is moved by a steam reversing gear. At _a_, Fig. 3405, are the eccentrics for the link B, for the high pressure cylinder; _b´_, _b´_ are those for link B´, for the intermediate cylinder; and _c´_ _c´_ are those for the link C´, for the low pressure cylinder. From each link are rods E, Fig. 3406, connected to arms on the shaft F _f_, to an arm on which is connected the rod G, from the worm wheel H, whose actuating worm I is on a crank shaft operated by the small steam cylinder J. The slide spindles D work in guides, and their cross heads C span the edges of the links, gibs being provided to take up the wear.

The gear for turning the engine when there is no steam in the main boilers is constructed as follows:

On the shaft of the wheel _m_, Fig. 3405, is a worm _n_ operating a worm wheel _p_, on whose shaft is a worm which operates the large worm wheel shown on the main crank shaft.

Figs. 3407 and 3408 represent the compound engines of the steamship _Poplar_, concerning which _The Engineer_ (from which the engravings are taken) says:

"Both the cylinders of these engines are fitted with piston valves, placed at the back of the cylinders and worked by the single eccentric valve gear, which has been so largely adopted and so successfully carried out by this firm in triple expansion as well as compound engines. It will be noticed that whilst this valve gear permits of the cylinders being close together, it allows of the crank shaft being made in two similar pieces, and affords exceptionally long main and crank pin bearings, of the former of which there are only three, instead of the usual four. In the case of the _Poplar_ the cylinders are 29 in. and 55 in. in diameter and 33 in. stroke, and the crank pins are 11 in. long, whilst the centre main bearing, which does duty for both the engines, is 23-3/4 in. in length, each of the outer bearings being 18 in. in length, the diameter of the crank shaft being 9-1/2 in. Another very interesting feature about these compact little engines is the design of the front framework. Instead of the ordinary upright columns in front of each engine there is an arrangement which gives exceptional stiffness to the whole structure whilst affording the fullest possible accessibility to the main working parts, and which has the appearance of an arch, from the shoulders of which there are branches worked up to receive the feet of the cylinders, thus accommodating the close centres and providing for the support of the reversing wheel without in the least obstructing the gear below. The condenser is divided horizontally through the centre on a plan strongly advocated by the builders, the whole of the base of the engines being cast in one piece and made level on the under side, so as to enable it to receive support from, and be bolted to, the engine seating immediately beneath the crank shaft, as well as round the margin."