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116 'and scows. In this way a width of channel up to 700 ft. can be made at one time. All of the operations of the A SECTION oF BUCKET CHAIN OF DENVER, dredge, including that of the winches, are under the control of one man through levers in the elevated control TRE MarINE REVIEW house, shown amidships on the delivery side. After a year's service, the buckets with their pins and bushings are report- ed in good condition and the general loss of time and cost of repairs 1s said to compare favorably with that of a dipper dredge on the same class of work, The material encountered at dif- ferent parts of the channel included hardpan, clay, stone and gravel. The ordinary yardage for a_ single day's work was about 8,000, and under favor- able conditions as much as 10,800 cu. yds. was dredged in one day. In a run of 55 minutes' duration, 1,479 cu. yds. were dredged. The Denver was built in Milwaukee in 1890, and is 224 ft. keel, 37 ft. beam and 21% ft.. molded depth. She is equipped with a compound engine, 23: stroke, ° and 42 in. diameter by 42-in. with one marine type fire-box boiler, i2 it..6 in. diameter, 15: it. long. The Electric Propulsion of Ships Ww are pleased to be able to present ' herewith a letter from William P. Durtnall, whose system of electric pro- pulsion has attracted much attention in Europe. It is of interest to note that Mr. Durtnall, has been awarded the Denny gold medal for his system of ship propulsion, which is attracting a great deal of attention in marine en- - gineering circles. Editor Marine Review: I have read with considerable interest the very able report in your issue of December last, and am pleased to see that American engineers, and above all shipbuilders and owners, are looking into the great possibilities of electrically driven boats. Although I suppose most of your readers have heard of my system, I dare say that there are some who have not, and I have pleasure in enclosing to you a description of my system in one form of layout, which any electrical engineer, who has experience in poly- phase alternating current work and de- sign, will agree is most efficient and light in weight, and the machines that compose the system are of very robust and well tried types. \ No wasteful resistances are used in this system, and energy is not therefore wasted in reversing and speed regula- tion and the heavy starting torque re- quired in starting and reversing is most efficiently got by means of the low frequency current. The control can be from the flying bridge, and the engineer will have a nice easy job on the vessel that I am fitting up at one of the north of Eng- land ports with 600 H. P. with a steam turbine running at 3,000) R. "P.M. as the prime mover. The propeller runs at 85 R. P. M. for top vessel speed, and while the turbine runs constantly at the above speed the propeller can be given three distinct speeds of 85, 56 and 28 R. P. M., thus permitting the use of a most efficient propeller. The ship referred to is a tramp type cargo boat, and will probably in time arrive at an American port, when I expect my clients will be willing to allow any well known American naval architect or marine engineer to inspect her. There will be a saving in this in- stance of not less than 20 per cent in coal as compared with the reciprocating engines that are being displaced, and I hope to let you have the trial results at a later date. I have another boat in hand, in which suction gas will be used to supply an internal combustion engine as_ prime- mover; this boat is for the British coal trade. A private yacht is also to be equipped this year with two 50-H. P. DeDion petrol engines running at 1,250 R. P. M., and the twin propellers will run at 150 R. P. M. with full control from the bridge. Yours faithfully, WittrAmM P, DurtNatt. London, Feb. 4, 1910. For many classes of vessels and parti- cularly for commercial work, there is a rapidly-growing demand for an installa- tion that will give its power at low speed, which will be flexible and which can be reversed as easily as a steam engine. The requirements are being met every day more closely by developments 7 March, 1919 of engines themselves, but to a certain -extent the advantage of lightness anq compactness of higher-speed motors are being lost, and it is in an endeavor to "combine the good qualities of the high- speed engine with the 'high propeller efficiency of the low-speed type that electric transmission has been intro. duced. Before explaining the details of the Durtnall system, of which connection diagrams are shown in Figs. 1, 2 and 3, it will be as well to say a word on three-phase generators and motors which, though really quite simple, seem rather formidable: to those not much acquainted with electrical engineering Briefly, a three-phase generator is an alternating current dynamo with three sets of windings on its armature. The armature is stationary, and the field magnet is rotated and is supplied with direct current from a small separate dynamo or exciter. A separate current is generated in each armature winding, but as the current in any two is always equal and opposite to the current in the remaining winding, it follows that one winding will always serve as a return connection for the other two. Thus, though in a two-phase machine four lines of connections are required, a three-phase alternator requires _ three wires only, and each must be considered to carry one phase of the current. Now, if such a three-phase generator is driven at a constant speed by any satisfactory prime mover such as a. turbine, gas engine, etc., a current of a certain periodicity dependent upon the - number of poles in the field magnet will be produced. We will assume, for our purposes, that we have a four-pole field magnet, and that the engine speed is 900 R.°P. M.. We will now turn to the motor. What is known as a squir- rel-cage motor is used. It is perhaps the simplest form of electric motor that it is possible to make. The armature or rotor consists simply of a number of bars of copper insulated from each other and short circuited; the stator or surrounding iron frame may be regard- ed as a field magnet, which is supplied with alternating current. There 'is no electrical connection whatever between the stator and the rotor, and it must suffice to say that what is known as a rotating magnetic field or flux is pro- duced when a three-phase current is supplied to the stator. This exercises a pull on the rotor, which runs at ex- actly the same speed as the flux (neg- lecting a very small slip, which need not be considered). The peculiar prop- erty of a squirrel-cage motor is that its speed is entirely independent of the load; it will take very much _ higher loads than it is designed for, and even if excessively overloaded its speed never