Propulsion

Abstract

The efficiency and acoustic performance of any propulsor will be affected by the flow into it. This is determined by: the hull shape, particularly the aft body and the tail cone angle; the casing ; the sail ; and the aft appendages. There will be an uneven wake field into the propulsor which will depend on the sail design and aft control surface configuration. This will result in fluctuating forces , causing vibration and noise. The quality of the flow into the propulsor can be assessed quantitatively using either the Distortion Coefficient , or the Wake Objective Function , and these are both explained. Results are presented to estimate the Taylor wake fraction, and the thrust deduction fraction as functions of the tail cone angle and the ratio of propeller diameter to hull diameter. The hull efficiency , which is the ratio of effective power to thrust power , can be estimated. The relative rotative efficiency is the ratio of the open water propulsive efficiency to the efficiency of the propulsor when operating in the wake. The Quasi Propulsive Coefficient (QPC) is the ratio of useful power to the power delivered to the propeller . Submarines are often propelled by a large optimum diameter single propeller . It is important to avoid cavitation , and the Cavitation Inception Speed depends on depth of submergence. Blade number is important, and this is discussed. Many submarines use pumpjets, which comprise two or more blade rows within a duct. The principles of pumpjets are discussed, along with some design guidance. The diameter of a pumpjet is usually smaller than that of a propeller , resulting in a lower rotor tip speed. Contra-rotating propulsion ; twin propellers; podded propulsion ; and rim driven propulsion are also discussed. Propulsor performance can be assessed using either the thrust identity or torque identity method, and both are described.