482 



FOREST AND STREAM. 



|May 19, 189S. 



twitting. 



THE BALANCING OF MARINE ENGINES AND 

 THE VIBRATION OF VESSELS. 



THE recent issues of Engineering have been most interesting 

 from the very full reports which thev contain of the annual 

 meeting of the Institution of Naval Architects, which was held early 

 in April. Of several very valuable papers, the most important were 

 by Messrs. Yarrow and Tborneycroft, both famous as builders of 

 fast torpedo boats, the former on the subject of balancing of en- 

 gines and vibrations of hulls; the latter on steadying vessels at sea 

 by means of a movable weight and controlling mechanism. The 

 former paper we reprint entire from Engineering, as it cannot fail 

 to interest all who are connected with screw vessels, whether for 

 trade or pleasure. In addition to the accompanying diagrams, the 



lecture was illustrated by photographs on a large screen, showing 

 the effect of the vibrations on the water about the vessel. 

 Mr. Yarrow said: 



All who are acquainted with the working of steamers provided 

 with large power and of high speed, such as torpedo boats, torpedo- 

 boat catchers, and very fast passenger steamers, will be familiar with 

 the fact that they are subject to considerable vibration under some 

 conditions, especially since the adoption of steel for shipbuilding and 

 high piston speeri. To overcome it is daily becoming a matter of 

 increasing importance, as higher and higher speeds are being con- 

 tinually demanded. It is to a study of the laws which govern this 

 vibration and to the possibility of avoiding it that I invite your 

 attention this evening. 



I will first describe some experiments which we have carried out 

 during the last few years, with a view to throw light upon this sub- 

 ject. At the outset we felt the want of some means of accurately 

 indicating and recording the extent and character of vibrations, and 

 therefore devised an instrument for the purpose. This instrument 

 I have here this evening for your inspection. It has been in use for 

 the last six years, and is, I believe, reliable. It consists, as will be 

 seen, of a heavily weighted drum suspended by elastic connections. 

 This drum is provided with suitable clockwork to cause it to revolve, 

 and is regulated to mats one revolution par minute. Attached to an 

 upright fixed to the stand of the machine is a pencil, which presses 

 lightly against the drum, round which a sheet of paper is w r rapDed, 

 in the same manuer as in an ordinary indicator. It will be readily 

 understood that if the base of this instrument be placed upon a plat- 

 form, or the deck of a vessel subject to vertical vibration, this 

 movement will be transmitted to the whole apparatus, excep'ing 

 that portion which is suspended by the elastic connection, and this 

 being heavily weighted will not follow the vibratory motion of the 

 stand. No*- if we place our instrument on the stern or any part of a 

 steamer which is vibrating and start the dru-n revolving, 'the pencil 

 being pressed against the paper makes aline, indicating the relative 

 vertical positions of the pencil and the paper, or in other words, 

 makes a diagram which records the character and extent of the 

 movement of that pari of the ship upon which the instrument stands. 



This instrument we call a "vibrometer," and it has mainly been 

 devised by my friend Mr, Nesbitt. 



I believe the cause of vibration in screw vessels when running in 

 smooth water with tbeir propellers well immersed, to be mainly due 

 to the forces produced by the unbalanced moving parts of the 

 machinery, such as the pistons, piston-rods, valves, gear, &c, except- 

 ing when it is the result of bad workmauship or bad state of repair. 

 For example, as clearly pointed out by Mr. Barnaby in his treatise 

 on marine propellers, vibration may be set up by a screw, the centre 

 of gravity of which is out of the centre line of the shaft, or it may be 

 due to want of uniformity in the position, area, or shape of the blades; 

 these causes being avoidable by proper care, I do not propose to deal 

 with them. I would, however, take this opportunity to observe that 

 sufficient attention is not always paid to these points. 



From our experiments we have overwhelming proof that the vibra- 

 tion in a torpedo boat is precisely the same in extent and character 

 when the screw is on and the vessel driven by it through the water, 

 as it is when the boat is stationary and the engines simply revolving 

 without doing work, the propeller being removed. To prove that this 

 statement is correct I would beg your reference to Fig. 2, upon which 

 are shown enlarged vibrometer diagrams, which have been obtained 

 from torpedo boats when running and when stationary. These dia- 

 grams represent a fair average of over a hundred results. It will be 

 seen that diagrams A and A 1 are practically alike; also diagrams B 

 and B 1 , and diagrams C and C 1 , A, B, and C having been obtained 

 when the boat was under weigh with propeller on, and A 1 , B 1 , and 

 C 1 , when the vessel was stationary, being without propeller. The 

 engines were making exactly the same number of revolutions in each 

 corresponding pair of diagrams. Not only do these diagrams prove 

 that the screw had nothing to do with the vibration, and that it was 

 owing to the working of the machinery, but it will be seen how 

 greatly our investigation is facilitated by these facts, because experi- 

 ments can be carried out with a boat at rest, and we know that 

 the same results as regards vibration will be found under ordinary 

 working conditions. 



It is a well-known fact that engines will impart their vibration to a 

 boat at certain speeds much more readily than at other speeds, and 

 it often happens that at full speed a boat may be practically steady, 

 while at a slower speed the vibration is excessive. This is dependent 

 upon the extent to wmich the movements of the reciprocating parts 

 of the engine correspond with the period of vibration of the hull, 

 which may be considered in this investigation as an elastic body! 

 This is illustrated by the well-known fact that when soldiers are 

 Grossing a suspension bridge it is often found necessary to avoid 

 their marching in step. Some years since we had a boat in which 

 severe vibration occurred at 200, 400, 600, and 800 revolutions per 

 minute, but there was none at the intermediate speeds of 300, 500, and 

 700. The diagrams in Fig. 3 show very clearly how the vibration 

 varies at different speeds and the variation that takes place in the 

 extent of the vibration when passing from one speed to another. 



In a vessel, such as a fast Atlantic liner, which is intended to run 

 continuously at a nearly uniform speed (unless special means be 

 taken to balance the machinery), it is of the utmost importance to 

 carefully avoid the number of revolutions of the engines per minute, 

 synchronizing with the normal vibration of the hull. This cannot 

 be carried out in the case of warships and others which are 

 intended to run at varying speeds; for if the speed of the engines be 

 proportioned so as not to set up vibration at full speed they will 

 probably do so at intermediate or cruising speeds," and if 

 they set up no vibration at cruising speeds they will 

 probably vibrate at full speed. I believe it would be by no means a 

 difficult matter to determine in the original design, with fair accuracy, 

 what speed of engine would be suitable to avoid vibration being set 

 up in any given design of hull. We frequently hear of propellers 

 being changed in order to reduce vibration, and in many cases the 

 change is made with advantage, but it often happens that the 

 improvement is not directly due to the altered shape of the pro- 

 peller, but indirectly to the change causing an alteration in the 

 number of revolutions of the engine, and thereby preventing their 

 synchronizing with the natural vibrations of the ship. We near of 

 hulls being strengthened or built stronger than would otherwise be 

 necessary with a view to reduce vibration. Doubtless this is done 

 w T ith more or less success, due, possibly, not so much to the greater 

 strength of hull, but rather to the period of vibration being modified 

 by this stiffening so as to avoid its harmonizing with the movements 

 of the machinery. Building vessels of greater strength than would 

 otherwise be necessary, with the object of avoiding vibration, can- 

 not be considered a scientific method of dealing with the difficulty, 

 if it can be proved that vibration can be avoided by other means and 

 without extra weight; because extra weight of material added to the 

 ship tends to handicap its spied. The true cause of vibration being 

 due to the machinery, I think it will be admitted that the correct 

 mode of dealing with it is to so design the engines that they may be 

 steady within themsolves &,nd free from tendency to cause the hull 

 to vibrate. As a further proof that the vibration is due to the 

 machinery, I may mention that two years ago I made a passage to 

 the United States in one of the very fast twin-screw steamers. I 

 selected a berth in the central portion of the vessel, thinking it a 

 good position for comfort, but the vibration was found to be so ex- 

 cessive that after five days it was scarcely bearable to those passengers 

 whose berths, like my own, were situated at the points of greatest 

 vibration. The vibration was found to vary periodically. When 

 the two low-pressure pistons were descending at the same time it was 

 excessive; but when one low-pressure piston was ascending and the 

 other descending it was entirely neutralized. So distinct was the 

 vibra ion in my cabin that it was quite easy to count the number of 

 revolutions of the port and starboard engines, and we rigged up a 

 temporary vibrometer on our cabin side, which gave us diagrams, 

 indicating clearly the movements we were subject to. I believe all 

 who have studied the subject of vibration in steamers will agree 

 with me that many vessels which vibrate considerably are in conse- 

 quence credited with weakness, while in reality they are of ample 

 strength, the fault resting entirely with the engines and not with 

 the hull. Not only is this vibration a source of discomfort to the 

 passengers, but it clearly adds considerably to the wear and tear of 

 the vessel. 



Let us consider exactly why an engine produces vibration. In an 



ordinary inverted engine the steam presses on the cylinder cover 

 and on the piston, and from the piston the stress is transmitted to 

 the bedplate. Now, during the first half of the down stroke the up- 

 ward pressure on the cylinder cover is greater than the downward 

 pressure on the bed plate to the extent of what is needed to set the 

 reciprocating parts in motion, and this excess of upward over down- 

 ward pressure lifts the engine bed and that pottion of the hull to 

 which it is attached. By a like train of reasoning it can be shown 

 that during the latter half of the down stroke and the first half of 

 the up stroke the tendency is to lower the engine bed; also that dur- 

 ing the second half of the up stroke the tendency is to raise the 

 engine bed. To sum this up in a few words, during the upper half 

 of the revolution the engine tends to lift the vessel and during the 

 lower half to depress it. The main principle which governs the 

 wbole matter may be summed up. As no internal force can move 

 the center of gravity of a body it follows that any momentum gen- 



Propeller on 



Propeller on 



at Rest no Propeller t 



erated by steam pressure in the moving parts such as the piston, etc. 

 must be attended by an exactly equal momentum in the rest of the 

 ship in the opposite direction. 



We will now pass on to consider how to design engines so that they 

 may be perfectly free from vibration. For this purpose please refer 

 to Fig. i, representing a single cylinder inverted engine, which for 

 the sake of simplicity we will assume has no valve gear, i he re- 

 volving parts, such as the crank, crank-pin, and a portion of the con- 

 necting rod can be balanced by means of rotary weights in the usual 

 way, and we then have only the vertical unbalanced parts, such as 

 the piston, piston rod, etc., left to deal with. Now if we have two 

 eccentrics set opposite to the crank, at equal distances from it and 

 of equal stroke to it, and these impart an up-and-down motion to 

 weights which we will call '-bob weights," each of which is half as 

 heavy as the parts to be balanced, that piece of mechanism will re- 

 volve free from vibration, excepting that which is due to the angle 

 of the connecting rods. If we wish to place these weights at unequal 

 distances from the crank ( see Fig. 5), they must be proportioned to 

 vary in weight inversely as their distances from the crank, that is, 

 if one weight be twice as far from the center of the crank as the 

 other it will have to be half the weight of the other, the sum of the 

 weights in this case being the same irrespeclive of their position. If 

 we desire to reduce the stroke of these weights so as to obtain a con- 

 venient length of stroke (see Fig. 6), we shall then have to increase 

 their weight inversely as the stroke, that is, if we quarter the stroke 

 the weights will have to be quadrupled, and so forth. It will thus be 

 seen to be a simple matter to proportion the bob weights, their stroke 

 and their position to suit what may best work in with any design of 

 engine. 



To sum up in a few words what must be done to avoid the effect of 

 the momentum generated by the working parts being felt by the 

 hull, an equal momentum in an opposite direction should be pro- 

 duced. 



If instead of using bob weights rotary weights of equal amount 

 had been employed, having their centers of gravity in the same po- 

 sition as the centers of the eccentrics which give motion to the bob 



Fig. 3 



BoAf 



UNDER WEIGH 



Propeller on 



no Propeller 



FAST 



300 LB 



REVOLUTIONS AT E.F. 

 ABOUT 270 PER MIN 



1^.7, 



TRAVEL 0FJY BOSWEICHT =.5 



nvf TRAVEL OF PISTONS = 16 <«s 

 VALVES = Si*l. 



BOewnCHT y , , bobweight^x 



! I ^ 



TRAVEL OF: Y'B0BVJEIG»T=iy 



