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464 present time, and while making all al- lowances for the fact that the aver- age “today ‘may’ give a- smaller ° ship than normal, due to the limited num- ber of large liners under construction, yet it is seen that the field already covered as regards power is consider- able. Such power as 5500 equivalent steam indicated horsepower still re- quires twin screws with the ‘diesel en- gine. - The largest cylinder at sea still re- mains 700 brake horsepower, but with the particular construction in this case of opposed pistons, the number of cylinders is limited to four. The time is not far distant when diesel en- gines of 400 to 500 brake horsepower per cylinder, of various types and in numbers, will be operating at sea with success. The tendency, with increas- ing sizes, will be to adopt fresh water cooling, both for the pistons, cylinders and cylinder heads, so completely to obviate the chances of local heating and, therefore of excessive stressing due to deposits of salt or sand. The extra complication of the coolers, circulated with salt water; for extract- ing the heat from the fresh water and of the extra pumps is a minor matter compared with the security so attained. Complicated Construction The deisel engine still remains a massive and somewhat complicated power plant, and no movement toward ‘simplification has yet definitely set in. Undoubtedly, when shipowners and their superintendents have come to ap- preciate the principles of operation of this prime mover, a number of so-called “sadgets” at present introduced as safeguards, and in order to make as- surance doubly sure, will be discarded. Only in this way does it seem pos- sible definitely to attain greater sim- plicity. As regards reducing the mass of the engine, there are only two ways in which this can be achieved, either by increasing the mean ef- fective pressure in the cylinders or the piston speed. The government factor in the design of all internal combustion engines is the heat flow factor. The greater the cylinder dimensions, the more vital is this consideration. This factor, ex- pressed in pounds of fuel consumed per square inch of combustion volume surface per unit of time, is directly dependent upon the piston speed and for a constant factor of heat flow, the lower the piston speed the higher the mean effective pressure possible. The converse is equally true. The tendency for some years past has been to reduce this heat flow factor MARINE REVIEW with increased size, but the gradual im- provements in materials and designs which have permitted of increasing size, now allow augmented heat flow by in- creasing the mean effective pressure in the cylinders and the piston speed. The piston speed with a single act- ing internal combustion engine can well be considerably higher than with steam practice because of two con- siderations—firstly that the maximum pressure for which bearing surfaces are designed is only maintained for ap- proximately 12% per cent in the case of 4-stroke engines and 25 per cent of the cycle for 2-stroke engines, and secondly because the pressures on the bearing and guide surfaces are not re- versed as with double acting steam en- gines. The inclination, therefore, es- pecially with 4-cycle machinery, is to increase piston speed. Single or Twin Screws One point that is not perhaps suf- ficiently emphasized is that, with diesel machinery, the higher the power per cylinder the greater. the weight per horsepower, so that for a given power of ship ‘there is a definite saving in machinery weights when twin-screw engines are adopted in comparison with single-screw machinery. This. saving in weight means a certain reduction in cost, although the lesser machinery cost is balanced by the increased cost of hull, due to the extra bossing of the stern and the two tunnels for the two lines of shafting. The chief ad- vantage of single-screw machinery lies in the reduction in personnel which is possible, as obviously an increased en- gine room complement is required to superintend and to maneuver two en- gines. Nevertheless, for powers above 2000 to 3000 shaft horsepower, twin- screw diesel engines will be the rule for some time to come and are to be advocated. The saving in fuel costs with diesel machinery must be considered in con- junction with the lubricating oil con- sumption, which is higher than with steam machinery. At first, this subject was not perhaps fully appreciated, but today it can confidently be stated that, with the latest internal combustion ma- chinery, the problems associated with the necessary lubrication of the piston rings and the forced feed to the main bearings have been most conscientious- ly attacked, and the consumption of lu- bricating oil has been reduced to a fig- ure of relatively small importance, 1.5 gallons of lubricating oil for all pur- poses per ton of fuel oil consumed should be the relation, and has been attained. How shall the auxiliaries be driven? November, 1922 So far “as steering gear is concerned, the electric- hydraulic system has proved itself’ @fficient and~ is finding’ ‘increas- ing favor even on steamships. For lights and fans, electricity is also re- quired. For the heating of a ship, steam is still the most convenient and the least costly method, although either exhaust raised steam at sea and some combination of electrically generated heat and hot water pipes will no doubt find increasing favor in the future. Therefore, for the remainder of the plant, such as winches and pumps, the choice lies between steam or electric drive. Where first cost is concerned, considerable advantage lies on the side of steam, but the more economical solution from the point of fuel con- sumption, is undoubtedly the electric drive. Electric current is generated by diesel-driven dynamos, which, except in special cases, should -not be less in num- ber than three. For reasons of inter- changeability these three should be of the same size. One should be suffi- cient for normal sailing at sea, two are required for maneuvering the ship or for working cargo fully in port, and one is always a standby. The con- sumption of fuel for working cargo and pumps in port with banked fires in a steamship is about ten times the amount of fuel required by the diesel electric system of auxiliary working on similar motorships. Will Gain Favor The foregoing refers particularly to 4-cycle engined ships. When 2-cycle machinery is adopted, the practice of driving the scavenging pumps_ separ- ately from the main engine to permit of rotary machines being used for this duty. will undoubtedly gain favor. In this case, the diesel-driven generators for supplying electric current for gen- eral duties, as well as for driving these blowers, become of very considerable size, approximately 25 per cent of the power of the main engine. Such fig- ures lead naturally to a consideration as to whether the correct angle from which to review and attack the prob- lem is not to regard the machinery of the ship as a central electric power, station delivering current to electric motors for propulsion and all the other numerous duties. The motorship has arrived today at the position where the shipowner has few remaining doubts as to the capaci- ty of the oil engine. The three factors of first cost, personnel and upkeep are still regarded, in- some quarters, as deterrents. The first cost for similar sized steamers and motorships is much in favor of the former. This is the gen- eral comparison made, although it is