_A._--It is difficult to a.s.sign the precise reason, but it appears to be a consequence of the largeness of the vacant s.p.a.ce between the valve plates.

When the piston of the air pump is drawn back, the air contained in this large collection of water will cause it to boil up like soda water; and when the piston of the pump is forced forward, this air, instead of being expelled, will be again driven into the water. There will consequently be a quant.i.ty of air in the pump which cannot be got rid of at all, and which will impair the vacuum as a matter of course.

468. _Q._--What expedient did you adopt to improve the vacuum in the engine to which you have referred?

_A._--I put blocks of wood on the air pump piston, which at the end of its stroke projected between the valve plates and forced the water out. I also introduced a c.o.c.k of water at each end of the pump between the valve plates, to insure the presence of water at each end of the pump to force the air out. With these ameliorations the pump worked steadily, and the vacuum obtained became as good as in the old pump. I had previously introduced an injection c.o.c.k into each end of the air pump in steam vessels, from which I had obtained advantageous results; and in all horizontal air pumps I would recommend the piston and valve plates to be so constructed that the whole of the water will be expressed by the piston. I would also recommend an injection c.o.c.k to be introduced at each end of the pump.

PUMPS, c.o.c.kS, AND PIPES.

469. _Q._--Will you explain the arrangement of the feed pump?

_A._--In steam vessels, the feed pump plunger is generally of bra.s.s, and the barrel of the pump is sometimes of bra.s.s, but generally of cast iron.

There should be a considerable clearance between the bottom of the plunger and the bottom of the barrel, as otherwise the bottom of the barrel may be knocked out, should coal dust or any other foreign substance gain admission, as it probably would do if the injection water were drawn at any time from the bilge of the vessel, as is usually done if the vessel springs a leak. The valves of the feed pump in marine engines are generally of the spindle kind, and are most conveniently arranged in a chest, which may be attached in any accessible position to the side of the hot well. There are two nozzles upon this chest, of which the lower one leads to the pump, and the upper one to the boiler. The pipe leading to the pump is a suction pipe when the plunger ascends, and a forcing pipe when the plunger descends. The plunger in ascending draws the water out of the hot well through the lowest of the valves, and in descending forces it through the centre valve into the s.p.a.ce above it, which communicates with the feed pipe. Should the feed c.o.c.k be shut so as to prevent any feed water from pa.s.sing through it, the water will raise the topmost valve, which is loaded to a pressure considerably above the pressure of the steam, and escape into the hot well.

This arrangement is neater and less expensive than that of having a separate loaded valve on the feed pipe with an overflow through the ship's side, as is the more usual practice.

470. _Q._--Will you describe what precautions are to be observed in the construction of the c.o.c.ks used in engines?

_A._--All the c.o.c.ks about an engine should be provided with bottoms and stuffing boxes, and reliance should never be placed upon a single bolt pa.s.sing through a bottom washer for keeping the plug in its place, in the case of any c.o.c.k communicating with the boiler; for a great strain is thrown upon that bolt if the pressure of the steam be high, and if the plug be made with much taper; and should the bolt break, or the threads strip, the plug will fly out, and persons standing near may be scalded to death.

In large c.o.c.ks, it appears the preferable plan to cast the bottoms in; and the metal of which all the c.o.c.ks about a marine engine are made, should be of the same quality as that used in the composition of the bra.s.ses, and should be without lead, or other deteriorating material. In some cases the bottoms of c.o.c.ks are burnt in with hard solder, but this method cannot be depended upon, as the solder is softened and wasted away by the hot salt water, and in time the bottom leaks, or is forced out. The stuffing box of c.o.c.ks should be made of adequate depth, and the gland should be secured by means of four strong copper bolts. The taper of blow-off c.o.c.ks is an important element in their construction; as, if the taper be too great, the plugs will have a continual tendency to rise, which, if the packing be slack, will enable grit to get between the faces, while, if the taper be too little, the plug will be liable to jam, and a few times grinding will sink it so far through the sh.e.l.l that the waterways will no longer correspond. One eighth of an inch deviation from the perpendicular for every inch in height, is a common angle for the side of the c.o.c.k, which corresponds with one quarter of an inch difference of diameter in an inch of height; but perhaps a somewhat greater taper than this, or one third of an inch difference in diameter for every inch of height, is a preferable proportion. The bottom of the plug must be always kept a small distance above the bottom of the sh.e.l.l, and an adequate surface must be left above and below the waterway to prevent leakage. c.o.c.ks formed according to these directions will be found to operate satisfactorily in practice, while they will occasion perpetual trouble if there be any malformation.

471. _Q._--What is the best arrangement and configuration of the blow-off c.o.c.ks?

_A._--The blow-off c.o.c.ks of a boiler are generally placed some distance from the boiler; but it appears preferable that they should be placed quite close to it, as there are no means of shutting off the water from the pipe between the blow-off c.o.c.k and the boiler, should fracture or leakage there arise. Every boiler must be furnished with a blow-off c.o.c.k of its own, independently of the main blow-off c.o.c.ks on the ship's sides, so that the boilers may be blown off separately, and may be shut off from one another.

The preferable arrangement appears to be, to cast upon each blow-off c.o.c.k a bend for attaching the c.o.c.k to the bottom of the boiler, and the plug should stand about an inch in advance of the front of the boiler, so that it may be removed, or re-ground, with facility. The general arrangement of the blow-off pipes is to run a main blow-off pipe beneath the floor plates, across the ship, at the end of the engines, and into this pipe to lead a separate pipe, furnished with a c.o.c.k, from each boiler. The main blow-off pipe, where it penetrates the ship's side, is furnished with a c.o.c.k: and in modern steam vessels Kingston's valves are also used, which consist of a spindle or plate valve, fitted to the exterior of the ship, so that if the internal pipe or c.o.c.k breaks, the external valve will still be operative.

Some expedient of this kind is almost necessary, as the blow-off c.o.c.ks require occasional regrinding, and the sea c.o.c.ks cannot be re-ground without putting the vessel into dock, except by the use of Kingston's valves, or some equivalent expedient.

472. Q.--What is the proper construction and situation of the injection c.o.c.ks, and waste water valves?

A.--The sea injection c.o.c.ks are usually made in the same fashion as the sea blow-off c.o.c.ks, and of about the same size, or rather larger. The injection water is generally admitted to the condenser by means of a slide valve, but a c.o.c.k appears to be preferable, as it is more easily opened, and has not any disposition to shut of its own accord. In paddle vessels the sea injection pipes should be put through the ship's sides in advance of the paddles, so that the water drawn in may not be injuriously charged with air. The waste water pipe pa.s.sing from the hot well through the vessel's side is provided with a stop valve, called the discharge valve, which is usually made of the spindle kind, so as to open when the water coming from the air pump presses against it. In some cases this valve is a sluice valve, but the hot well is then almost sure to be split, if the engine be set on without the valve having been opened. The opening of the waste water pipe should always be above the load water line, as it will otherwise be difficult to prevent leakage through the engine into the ship when the vessel is lying in harbor.

473. Q.--What is the best arrangement of gauge c.o.c.ks and gla.s.s gauges?

A.--Gauge c.o.c.ks are generally very inartificially made, and occasion needless annoyance. They are rarely made with bottoms, or with stuffing boxes, and are consequently, for the most part, adorned with stalact.i.tes of salt after a short period of service. The water discharged from them, too, from the want of a proper conduit, disfigures the front of the boiler, and adds to the corrosion in the ash pits. It would be preferable to combine the gauge c.o.c.ks appertaining to each boiler into a single upright tube, connected suitably with the boiler, and the water flowing from them could be directed downward into a funnel tube communicating with the bilge. The c.o.c.ks of the gla.s.s tubes, as well as of the gauge c.o.c.ks, should be furnished with stuffing boxes and with bottoms, unless the water enters through the bottom of the plug, which in gauge c.o.c.ks is sometimes the case.

The gla.s.s gauge tubes should always be fitted with a c.o.c.k at each neck communicating with the boiler, so that the water and steam may be shut off if the tube breaks; and the c.o.c.ks should be so made as to admit of the tubes being blown through with steam to clear them, as in muddy water they will become so soiled that the water cannot be seen. The gauge c.o.c.ks frequently have pipes running up within the boiler, to the end that a high water level may be made consistent with an easily accessible position of the gauge c.o.c.ks themselves. With the gla.s.s tubes, however, this species of arrangement is not possible, and the gla.s.s tubes must always be placed in the position of the water level.

474. Q.--What is the proper material of the pipes in steam vessels?

A.--Most of the pipes of marine engines should be made of copper. The steam pipes may be of cast iron, if made very strong, but the waste water pipes should be of copper. Cast iron blow-off pipes have in some cases been employed, but they are liable to fracture, and are dangerous. The blow-off and feed pipes should be of copper, but the waste steam pipe may be of galvanized iron. Every pipe pa.s.sing through the ship's side, and every pipe fixed at both ends, and liable to be heated and cooled, should be furnished with a faucet or expansive joint; and in the case of the cast iron pipes, the part of the pipe fitting into the faucet should be turned. In the distribution of the faucets of the pipes exposed to pressure, care must be taken that they be so placed that the parts of the pipe cannot be forced asunder, or turned round by the strain, as serious accidents have occurred from the neglect of this precaution.

475. _Q._--What is the best mode of making pipes tight where they penetrate the ship's side?

_A._--In wooden vessels the pipes where they pierce the ship's side, should be made tight, as follows:--the hole being cut, a short piece of lead pipe, with a broad f.l.a.n.g.e at one end, should be fitted into it, the place having been previously smeared with white lead, and the pipe should then be beaten on the inside, until it comes into close contact all around with the wood.

A loose f.l.a.n.g.e should next be slipped over the projecting end of the lead pipe, to which it should be soldered, and the f.l.a.n.g.es should both be nailed to the timber with scupper nails, white lead having been previously spread underneath. This method of procedure, it is clear, prevents the possibility of leakage down through the timbers; and all, therefore, that has to be guarded against after this precaution, is to prevent leakage into the ship.

To accomplish this object, let the pipe which it is desired to attach be put through the leaden hause, and let the s.p.a.ce between the pipe and the lead be packed with gasket and white lead, to which a little olive oil has been added. The pipe must have a f.l.a.n.g.e upon it to close the hole in the ship's side; the packing must then be driven in from the outside, and be kept in by means of a gland secured with bolts pa.s.sing through the ship's side. If the pipe is below the water line the gland must be of bra.s.s, but for the waste water pipe a cast iron gland will answer. This method of securing pipes penetrating the side, however, though the best for wooden vessels, will, it is clear, fail to apply to iron ones. In the case of iron vessels, it appears to be the best practice to attach a short iron nozzle, projecting inward from the skin, for the attachment of every pipe below the water line, as the copper or bra.s.s would waste the iron of the skin if the attachment were made in the usual way.

DETAILS OF THE SCREW AND SCREW SHAFT.

476. _Q._--What is the best method of fixing the screw upon the shaft?

_A._--The best way is to cut two large grooves in the shaft coming up to a square end, and two corresponding grooves or key seats in the screw boss opposite the arms. Fit into the grooves on the shaft keys with heads, the length of which is equal to half the depth of the boss, and with the ends of the keys bearing against the ends of the grooves in the shaft. Then ship on the propeller, and drive other keys of an equal length from the other side of the boss, so that the points of the keys will nearly meet in the middle; next burr up the edge of the grooves upon the heads of the keys, to prevent them from working back; and finally tap a bolt into the side of the boss to penetrate the shaft. Propellers so fitted will never get slack.

477. _Q._--What is the best way of fitting in the screw pipe at the stern?

_A._--It should have projecting rings, which should be turned; and cast iron pieces with holes in them, bored out to the sizes of these rings, should be secured to the stern frames, and the pipe be then shipped through all. Before this is done, however, the stern post must be bored out by a template to fit the pipe, and the pipe is to be secured at the end to the stern post either by a great external nut of cast iron, or by bolts pa.s.sing through the stern post and through lugs on the pipe. The pipe should be bored throughout its entire length, and the shaft should be turned so as to afford a very long bearing which will prevent rapid wear.

478. _Q._--How is the hole formed in the deadwood of the ship in which the screw works?

_A._--A great frame of malleable iron, the size of the hole, is first set up, and the plating of the ship is brought to the edge of this hole, and is riveted through the frame. It is important to secure this frame very firmly to the rest of the ship, with which view it is advisable to form a great palm, like the palm of a vice, on its inner superior corner, which, projecting into the ship, may be secured by breast-hook plates to the sides, whereby the strain which the screw causes will be distributed over the stern, instead of being concentrated on the rivets of the frame.

479. _Q._--Are there several lengths of screw shaft?

_A._--There are.

480. _Q._--How then are these secured to one another?

_A._--The best mode of securing the several lengths of shaft together is by forging the shafts with f.l.a.n.g.es at the ends, which are connected together by bolts, say six strong bolts in each, accurately fitted to the holes.

[Ill.u.s.tration: Fig 44. End of the Screw Shaft of Correo, showing the mode of receiving the Thrust. A, discs; B, tightening wedge.]

481. _Q._--How is the thrust of the shaft usually received?

_A._--In some cases it is received on a number of metal discs set in a box containing oil; and should one of these discs stick fast from friction, the others will be free to revolve. This arrangement, which is represented in fig. 44, is used pretty extensively and answers the purpose perfectly. It is of course necessary that the box in which the discs A are set, shall be strong enough to withstand the thrust which the screw occasions. Another arrangement still more generally used, is that represented in figs. 55 and 56, p. 331. It is a good practice to make the thrust plummer block with a very long sole in the direction of the shaft, so as to obviate any risk of canting or springing forward when the strain is applied, as such a circ.u.mstance, if occurring even to a slight extent, would be very likely to cause the bearing to heat.

482. _Q._--Are there not arrangements existing in some vessels for enabling the screw to be lifted out of the water while the vessel is at sea?

_A._--There are; but such arrangements are not usual in merchant vessels.

In one form of apparatus the screw is set on a short shaft in the middle of a sliding frame, which can be raised or lowered in grooves like a window and the screw shaft within the ship can be protruded or withdrawn by appropriate mechanism, so as to engage or leave free this short shaft as may be required. When the screw has to be lifted, the screw shaft is drawn into the vessel, leaving the short shaft free to be raised up by the sliding frame, and the frame is raised by long screws turned round by a winch purchase on deck. A chain or rope, however, is better for the purpose of raising this frame, than long screws; but the frame should in such case be provided with pall catches like those of a windla.s.s, which, if the rope should break, will prevent the screw from falling.

DETAILS OF THE PADDLES AND PADDLE SHAFT.

483. _Q._--What are the most important details of the construction of paddle wheels?

_A._--The structure of the feathering wheel will be hereafter described in connection with an account of the oscillating engine; and it will be expedient now to restrict any account of the details to the common radial paddle, as applied to ocean steamers. The best plan of making the paddle centres is with square eyes, and each centre should be secured in its place by means of eight thick keys. The shaft should be burred up against the head of these keys with a chisel, so as to prevent the keys from coming back of their own accord. If the keys are wanted to be driven back, this burr must be cut off, and if made thick, and of the right taper, they may then be started without difficulty. The shaft must of course be forged with square projections on it, so as to be suitable for the application of centres with square eyes. Messrs. Maudslay & Co. bore out their paddle centres, and turn a seat for them on the shaft, afterward fixing them on the shaft with a single key. This plan is objectionable for the two reasons, that it is insecure when new, and when old is irremovable. The general practice among the London engineers is to fix the paddle arms at the centre to a plate by means of bolts, a projection being placed upon the plates on each side of the arm, to prevent lateral motion; but this method is inferior in durability to that adopted in the Clyde, in which each arm is fitted into a socket by means of a cutter--a small hole being left opposite to the end of each arm, whereby the arm may be forced back by a drift.

484. _Q._--How are the arms attached to the outside rings?

_A._--Some engineers join the paddle arms to the outer ring by means of bolts; but unless very carefully fitted, those bolts after a time become slack sideways, and a constant working of the parts of the wheel goes on in consequence. Sometimes the part of the other ring opposite the arm is formed into a mortise, and the arms are wedged tight in these holes by wedges driven in on each side; but the plan is an expensive one, and not satisfactory, as the wedges work loose even though riveted over at the point. The best mode of making a secure attachment of the arms to the ring, consists in making the arms with long T heads, and riveting the cross piece to the outer ring with a number of rivets, not of the largest size, which would weaken the outer ring too much. The best way of securing the inner rings to the arms is by means of lugs welded on the arms, and to which the rings are riveted.

485. _Q._--What are the scantlings of the paddle floats?

_A._--The paddle floats are usually made either of elm or pine; if of the former, the common thickness for large sea-going vessels is about 2-1/2 inches; if of the latter, 3 inches. The floats should have plates on both sides, else the paddle arms will be very liable to cut into the wood, and the iron of the arms will be very rapidly wasted. When the floats have been fresh put on they must be screwed up several times before they come to a bearing. If this be not done, the bolts will be sure to get slack at sea, and all the floats on the weather side may be washed off. The bolts for holding on the paddle floats are made extra strong, on account of the corrosion to which they are subject; and the nuts should be made large, and should be square, so that they may be effectually tightened up, even though their corners be worn away by corrosion. It is a good plan to give the thread of the paddle bolts a nick with a chisel, after the nut has been screwed up, which will prevent the nut from turning back. Paddle floats, when consisting of more than one board, should be bolted together edgeways, by means of bolts running through their whole breadth. The floats should not be notched to allow of their projection beyond the outer ring, as, if the sides of the notch be in contact with the outer ring, the ring is soon eaten away in that part, and the projecting part of the float, being unsupported, is liable to be broken off.