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BILL OF MATERIAL MARK NAME MATERIAL CAST STEEL CAST SEMI- STEEL CAST STEEL CAST _IRON MILD STEEL Al PORTABLE PLATES RUDDER COVER PLATES VERTICAL BLADE WELDED TO POST PORTABLE PLATES TIGHTEN SET SCREWS H \ TO PROPELLER NUT AND ! \ PERMIT DRAWING OF \ TAILSHAFT-WITHOUT DISTURBING CONTRA- | PROPELLER. ' \ \ ' HUB CAP, IN HALVES, TO PROVIDE ACCESS | | OLD RUDDER PLATE PORTABLE PLATES TO PROVIDE ACCESS TO PINTLES C— LOOKING TO PORT (RUDDER , UNSHIPPED ) the Contrapropeller, a curved pro- longation of the sternpost, intercepts the propeller discharge and deflects it aft in a solid stream. There re- mains, however, the turbulent water conditions abaft the sternpost and abaft the rudder stock. In Fig. 3, the Contrapropeller is designed to line up with the outside ‘diameter of the rudder stock, and is carried around and aft of the sternpost so as to fill the space be- tween the sternpost and rudder stock. This has proved -a very efficient de- sign and can be fitted to vessels in service at very small cost. The Contrarudder Principle Fig. 4 is typical of the latest de- sign, showing the Contrapropeller combined with streamline rudder, the combination being known as the Con- trarudder. The resemblance of the Contrarudder section to that of an airplane wing is not. more marked than is intended, as the principles employed in designing the airplane wing to give maximum lift with min- imum resistance in flight are followed with equally good results in design- ing the Contrarudder. It is usual when talking of stream- line rudders to conceive the stream- line body as being towed behind the vessel through a mass of water moy- ing in the opposite direction and parallel to the centerline of the ship. In the case of twin and quadruple screw ships, where the rudder is not located in the propeller race, this is of course, correct. However, it is COVER PLATE WELDED TO RUDDER ARM COVER PLATE RIVETED TO R NEO RS 2h OG CHANNEL WELDED To ARM SECTION AT A-A INTERMITTENT WELDING PLATE LINER COVER PLATE RIVETED TO CHANNEL VIEW AT C-C Fig. T—Design of Contrarudder fitted to a ship in service obviously wrong in the case of single serew ships where the rudder is di- rectly behind the propeller and where the water does not flow in a direct rearward stream but is thrown athwartships by the propeller at a considerable angle to the cen- terline of vessel, that is, to starboard ‘above the centerline of shaft and to port below the centerline. In this case, a symmetrical shaped rudder impedes the flow from the propeller. This condition does not exist when a Contrarudder is used, since the Contrapropeller portion, be- SO COVER PLATE TAPPED OR WELDED TO ANGLES SS UM ae p | aya 7 all pe VI Y % 4 Lip Cen (A | er i aN sa OLD RUDDER PLATE SECTION AT B-B CONTRARUDDER AS FITTED TO A SHIP IN SERVICE ing offset to port above the centerline of shaft and to ‘starboard below, properly guides the water from the propeller. Thus by assisting the wa- ter to get away from the propeller instead of retarding it, the Contra- rudder increases the velocity of the propeller discharge, resulting in in- creased thrust for the same horse- power, or if speed is to remain con- stant, decreased horsepower for the same speed. At the same time, due to the tur- bine blade-like shape of the Contra- rudder, the reaction of the propeller Table I Comparison Between Plate, Streamline and Contrarudders Principal Dimensions of Ship Length between perpendiculars............. 250 feet BOS" eisscsvctssascastestnassicens paubeveneungeewesbusehuanaed 42 feet POPUL CS TavsssacsuSoscactdgascartuyspsstescressvactacstesscstercsas 16 feet Displacement = c.cacscccsseseressssusescsvessseescuouscsaas 1975 tons SPEED SHAFT HORSEPOWER PER CENT REDUCTION IN S. H. P. in Knots Plate Rudder Streamline Rudder Contrarudder Streamline Rudder Contrarudder 9 400 Sa heb 20.0 10 560 520 440 er 21.4 12 960 900 790 6.25 PUT 14 1580 1480 1820 6.3 16.5 16 2640 2400 2200 9.1 16.7 17 8580 3200 3000 . 10.6 16.2 Table II Comparison Between Double Plate, Streamline and Contrarudder Principal Dimensions of Ships Length between perpendiculars.............. 290 feet CRIN Vics csabsvonnecaceorsonaecs sekdeseasbensssbabieanauetcaane 45.5 feet PL os sacsascocecsuutiohuveccicastaskissenieusommacseus ener 17.4 feet Displacement + iccsiisescoosxsceasesoeovacssaceastesseavanse 5000 tons SPEED SHAFT HORSEPOWER PER CENT REDUCTION IN S. H. P. In Knots Double Plate Rudder Streamline Rudder Contrarudder Streamline Rudder Contrarudder 8 467 : 449 439 3.86 6.00 9 685 648 620 5.40 9.50 10 940 880 830 6.38 11.70 11 1268 1210 1119 4.75 11.75 12 1800 1748 1610 2.90 10.60 MARINE REvieEw—February, 1932 31