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August, 1910 TAE MARINE REVIEW $71 COMPARATIVE BACKING TRIALS The Result of a Series of Experiments Conducted by the Navy Department on the Scout Cruisers Birmingham, Chester and Salem 4N. the our beginning of experience with turbines, attention was naturally . cen- tered on their en- gineering character- istics, such as steam consumption, and de- tails of construction and operation. With the satisfactory development of these elements, we have been able to devote more attention to the tactical features mvowed,: as...affecting » thé ease: of handling turbine-engined vessels. With reciprocating engines a back- ing power about equal to the ahead power is afforded without any in- crease in weight, except that of the backing eccentrics, rods. and_ links. With turbines, however, backing pow- er requires additional turbines with a considerable increase in weight. It is necessary, therefore, to restrict the backing power of naval turbines to that actually required by tactical con- siderations. The practice has been to provide a backing power estimated at about 50 per cent of the ahead power. It has been found, however, 'in prac- tice that this backing power is rather less than 40 per cent of the ahead power. Trials have recently been held with the three scout cruisers to determine the sufficiency of the backing power provided respectively by Parsons and by Curtis turbines, and as compared with that obtained by reciprocating engines. The criterion has been taken as the distance headreached by the vessel during the interval required to bring her dead in the water from a given speed ahead. A_ general de- scription of these vessels follows. Their hulls are similar. PRINCIPAL HULL DATA. Length between perpendiculars, feet. .420 Length over all, feet and inches...... 423--2 Bie on 1. We. L,,: feet. ose ccna 420. Breadth, molded, feet and inches.... 46--8 Breadth, extreme, feet and inches.... 47--0% Ratio of length to beam.........-++s 8.97 Draught (official contract trial, 3,750 tons displacement), feet and inches. 16--9 4,000 tons displacement, feet and ATIOHOS os eo cee Snes Sabie erase 17--4% 4,710 tons displacement (about fully _ loaded), feet and inches........-- 19---2% isplacement per inch at mean draught fe 2 any fons. oe 31.07 Area of midship section (3,750 tons isplacement), square feet........+-- 566 Area of L. W. L. section (3,750 tons displacement), sqlare 'fedts 60,9) Sas 12,960 *tted surface section (3,750 tonsdis- - Placement), Sdiiare 'feet oc ese 19,900 cefficient (at 3,750 tons 'disp!acemert) a. 0.40 rere 88 ey 0.72 = 1. plane, .-3, EE Re ee 0.66 Machinery Installations, BIRMINGHAM, The engines, which turn the propel- lers outboard, are of the vertical in- verted four-cylinder, direct-acting, trip- le-expansion type, with unjacketed cylinders, placed in two water-tight compartments, and operating twin- screws. Each engine was designed for an indicated horsepower of 8,000 at 200 r.° p. m,° with a steamchest pressure of 250 lbs. per gage. Begin- ning forward, the order of the cylin- ders is, forward L. P., H. P., I. P. and after <. Po" The fotwatd Lio Panda, P. cranks are opposite, as are the I. P. and after L. P., the second pair be- ing at right angles with the first. All main valves are of the piston tyre, worked by double-bar Stevenson links. There is cne piston valve for the H. P. and two: each {oF the bP. tid ts: P. cylinders. Cylinders: > Number fer each envine scat see eke 4 HP. cdiameter mMuchea ae rei cee a's oe 28% iusPe: diameter, .anchess cas. saa sg ae 45 Ryo. oP. diameter, 1mehess.asiees wae pee 62 AG ee Pos diameter; inches i144 65 0s #46 oe 62 Diameter of piston rods, inches........ 6 Stroke: of 'allo pistons, imches. 3 ....... 54% 36 Propellers: Material: a... Mag ines eee Manganese bronze Nuniber "or "blades sys ia oe ee ee 3 Diameter, feet and inches............ 12--6 Pitch (as set) mean, feet and inches. 15--3 Pitch, adjustable, from feet and : INCHES, ssi oe meen et 14--6 to 16--6 Ratio of diameter to pitch3. 26... 1.22 Area, projected, square feets....:...4 40.8 Area, helicoidal, square' feet......-.. 49.4 Area, disk," square ' feet... %. 200... 12217 CHESTER. "Phe propelling machinery of the Chester, designed for a working steam pressure at the turbines of 250 Ibs. (gage) and about 507 r. p. m. for the contract speed of 24 knots, consists of Parsons marine steam turbines, driving four independent shafts, each shaft being fitted with one propeller. The installation comprises six ahead turbines in all, two of which--the H. P. and I. P. cruising--are for use at low powers in order to secure econ- omy. - There are also two. backing turbines, located in each of the ©. £. casings, and it is to be observed that this arrangement permits reversal only of the two inboard shafts in maneu- vering. The outboard shafts are operated en- tirely by main H. P. turbines; the starboard inboard by the i. P. (star- board) turbine and H. P. cruising tur- itp? bine, and the port inboard by the i. P. (port) turbine, andthe I.) P. eruis- ing. Propellers: Mateniaa e058 Manganese bronze, cast solid Number: of propellers: .. 530.4... ee Number of blades (each propeller)..... 3 Diainéter,. feet' ones a 6 Pitch feet ca. ioe ee ae 6 Ratio of diameter to pitch......... ee Area,' projected, square feet...<... 06... 17.02 Area, heliccidal, square feet...... eee oe. Area, "disk, 'square. feét, s20.45 1 28.27 SALEM. The Salem is driven by outboard turning twin screws, the propelling machinery consisting of two Curtis marine reversible impulse turbines, one on each shaft, designea to develop 8,000 b. bh. p.,,at-350: ©. p, a with 250 Ibs. (gage) steam-chest pressure. The machinery is arranged in two. water- tight compartments, separated by an athwartship. bulkhead, the starboard screw being operated by the forward turbine. Each turbine has a pitch diameter of rotating wheels of about 120 (cen- ter to center of buckets); and for the ahead motion consists of seven stages: each wheel is fitted with three rows of buckets, except in the first stage, which contains four rows. For backing, there are two stages, the wheels or rotors being mounted on the shaft in the same casing as the go-ahead stages. Propellers: Number: of blades 0.3, 0 ta 3 Diameter, feet and inches..:.......% 9--6 Pitch, feet and inchessys sss a se 8--8Y% Ratio, diameter to 'pitch:..¢...72.... 1.09 Area, projected, square feet........ 36.8. Area, helicoidal, square feet........ 43.7 Area: disk; square teeta oe i ee ee 70.9 These trials were conducted with great care and thoroughness, the con- ditions of displacement, sea, speed, and number of boilers in use, were practically identical for all, and the re- sults may be taken as an authoritative indication of the relative backing pow- ers of the three types of machinery as now installed. The speeds selected for the trials were 10, 16, 22 and 24 knots. Three runs were made by each vessel at each of the two lower speeds, two runs at 22 knots, and one run at the high speed. At each speed only the boilers required for a sustained run at this speed were used. There was no bottling up of steam preparatory to backing, and no speed- ing up of blowers during the backing