Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGthe propeller. Within certain limits, the thrustthat can be developed increases as the diameterand the total blade area increase. Since it isimpracticable to increase propeller diameterbeyond a certain point, propeller blade area isusually made as great as possible by using asmany blades as are feasible under the circumstances.Three-bladed and four-bladed marinepropellers are commonly used.Thrust DeductionFigure 5-4.— Linear velocityand reactive thrust.147.47Because of the friction between the hull andthe water, water is carried forward with the hulland is given a forward velocity. This movementof adjacent water is called the wake . Since thepropeller revolves in this body of forward movingwater, the sternward velocity given to thepropeller is less than if there were no wake.Since the wake is traveling with the ship, thespeed of advance over the ground is greater thanthe speed through the wake.At the same time, a propeller draws waterfrom under the stern of the ship, thus creating asuction which tends to keep the ship from goingahead. The increase in resistance that occursbecause of this suction is known as thrust deduction.pressure on the water and so develop more reactivethrust than point A. The higher the linearvelocity of any part of a blade, the greater willbe the reactive thrust.Real propeller blades are not flat but are designedwith complex surfaces (approximatelyhelicoidal) to permit every infinitesimal area toproduce equal thrust. Since point Z has a higherlinear velocity than point A, the thrust at pointZ must be decreased by decreasing the pitchangle at point Z. Point M, lying between pointsZ and A, would have (on a flat blade) a linearvelocity less than Z but greater than A. In a realpropeller, then, point M must be set at a pitchangle which is greater than the pitch angle atpoint Z but less than the pitch angle at point A.Since the linear velocity of the parts of a bladevaries from root to tip, and since it is desiredto have every infinitesimal area of the blade produceequal thrust, it is apparent that a real propellermust vary the pitch angle from root to tip.Propeller SizeThe size of a propeller—that is, the size ofthe area swept by the blades— has a definite effecton the total thrust that can be developed onNumber and Locationof PropellersA single propeller is located on the ship'scenterline as far aft as possible to minimize thethrust deduction factor. Vertically, the propellermust be located deep enough so that instill waterthe blades do not draw in air but high enough sothat it can benefit from the wake. The propellermust not be located so high that it will be likelyto break the surface in rough weather, since thiswould lead to racing and perhaps a broken shaft.A twin-screw ship has the propellers locatedone on each side, well aft, with sufficient tipclearance to limit thrust deduction.A quadruple-screw ship has the outboardpropellers located forward of and above the inboardpropellers, to avoid propeller stream interference.Controllable Pitch PropellersAs previously noted, controllable pitch propellersare in use on some naval ships. Controllablepitch propellers give a ship excellentmaneuverability and allow the propellers to developmaximum thrust at any given engine rpm.90