Military

Propellers

Propulsors include propellers, pumpjets and waterjets. There are several screw-type propellers, which are distinguished by their abilities to accommodate the effects of cavitation, which generates noise and reduces propeller efficiency. The waterjet is a different type of propulsor. As an alternative for countering propeller cavitation problems for high-speed craft and special-purpose craft, the waterjet, which is driven by a gas turbine or high speed diesel, provide a jet-reactive thrust of high-velocity water expelled through a nozzle. With a speed range above 45 knots, waterjets, whose principal advantage is improvement of vehicle maneuverability, are typically applied to patrol boats, surface effect ships, hydrofoils, motor yachts, and fast ferries.

In the 1800s a new form of propulsion, the steam engine, began to change the role of the ship. The first steam-powered naval ships were produced in the 1820s, but the need for side paddlewheels and huge engines still limited the ship's role as a gun platform. In 1835, Richard Gatling [of Gatling Gun fame] invented, but missed a previously patented ship's screw propeller by only a few months. By 1850, the first screw propeller made the side-wheeler obsolete, and freed deckspace necessary to carry more guns.

Propellers date back to antiquity -- 130 BC. However, it was not until the steam engine was invented that propellers became widely used. About 100 years ago, propellers replaced sails for ship propulsion. The most common means now employed to move vessels through the water is the longitudinally short screw propeller.

The theory of the action of the screw propeller has been the subject of consideration by many investigators. One theory is that the propeller impresses change of motion upon the water without change of pressure except such as is caused by the rotation of the screw. Another theory is that the thrust is obtained by change of pressure, the only changes of motion being the circumferential velocity due to the rotation of the propeller. Still others are referred to as the momentum theory and the vortex theory. The confused nature of the water at the stern of a moving boat makes the application of theory to marine screw propeller operation extremely complicated and not definitively known.

The Actuator Disc Theory was the first practical theory of propellers. Developed by Rankine in 1865, it was extended by R. E. Froude in 1886. The theory stated that force (thrust) equals the mass times acceleration of the fluid. It gives no information on geometry but gives the ideal efficiency of a propeller. The Blade Element Theory states that blade element equals radial cross-section of a blade. The total force (thrust and torque) on a propeller blade is determined by analyzing the forces on each blade element (W. Froude, 1878 and Taylor 1893). Unfortunately this leads to erroneous results that ideal efficiency of a propeller is unity.

There was considerable controversy between the two proponent groups over the Actuator Disc Theory versus Blade Element Theory. There was a lot of discussion, but actually both theories were wrong because a propeller could not be designed using either theory. Debate continued until after development of circulation theory by Lancaster in 1907. Later, the relationship between momentum changes in fluid to forces on a blade element was shown by Betz and Prandtl in 1927, by applying the Kutta-Joukowski Circulation Theorum (Aerodynamics).

David W. Taylor was in the forefront of developing a propeller series to predict performance of propellers from model data. At this time, there was no way of designing propellers or predicting forces. A series needed to be developed. One of the first propeller series originated with David Taylor. According to Taylor's 1933 book, he tested 177, 16-inch, bronze propellers to devise the series. If those propellers were bought today, they would cost between $4 and $5 million. There was a lot of money spent in the early days of the DTMB. The series covered blade shape, blade sections, blade rake, hub size and pitch to blade used. Taylor's series served as the basis for propeller design for many years, mostly for circular arc sections.

A number of other series were developed; the most common in use today is the Troost series, which uses more modern propeller blade forms and sections. Both are series-dimensional coefficients. The Troost series is a wake adapted propeller, where the pitch is reduced toward the tip, called pitch distribution. The Troost series has 120 propellers.

Originally blade sections were flat face circular arc sections sometimes with rounded leading edges and were easy to build and measure. Original airfoil type sections did not show much improvement in performance (efficiency and cavitation). The development of NACA series, particularly 6 series with "a" series meanlines, led to improvement in cavitation.

Work of Prandtl-Betz-Helmhold in Germany led to development of circulation theory of propellers -- Lerbs (DTMB-1952) - Lifting Line Theory using Induction Factors. Induction factors are an equation or calculation for a singular integral equation. It is a very complicated equation, and almost not one did a propeller design by hand. Fortunately, at this time, designers started using computers such as the UNIVAC I and II. This was before Fortran and computations were performed on the computer using binary systems--zeros and ones. Back then, the engineer performed the analysis and the programmer transferred the information into zeros and ones.

In 1955 propellers began to be designed on the computer, which enabled propellers to be designed using induction factors. These factors were a big advance because they made wake-adapted propellers possible. Prior to the UNIVAC, computations existed for uniform flow, which was sufficient for aircraft propellers but insufficient for ship and submarine propellers. Also during the pre-UNIVAC days, computations required about six weeks to perform by hand -- and that is without making a mistake. The same calculation ran in 24 minutes on the UNIVAC I, and that was run several times through. Before the UNIVAC, slide rules and curves served as calculators.

Several design methods developed around circulation theory in late 1930s to 1950s, all required some empirical factors. Computerized lifting line/lifting surface theory and use of the NACA a=0.8 meanline was a break-through. Theoretical lift of the NACA a=0.8 meanline is achieved in viscous flow, i.e. theoretical = experimental lift. Successful theoretical design developments in early 1960s were dependent on development of high-speed digital computers and their application to lifting line and lifting surface theory, most of which were at or sponsored by the David Taylor Model Basin. Developments include Original Computational Fluid Dynamics and use of the NACA a = 0.8 meanline as proposed by Lerbs in early 1950s.

Empirical diagrams developed for selecting blade area for cavitation; later cavitation diagrams were developed from pressure distributin of NACA sections in steady flow. Propellers had thin blade sections. Brocket (DTMB) developed diagrams for propellers operating in non-uniform flow. This resulted in thicker blades.

A major shipboard noise source comes from the ship's propeller excitation of the ship structure. The excited structure then re-radiates as airborne noise.The AO-177, first of a new class of Naval Auxiliary Oilers, experienced high levels of inboard airborne noise and initial-stage erosion damage on its skewed, seven-bladed propeller during builder's trails. To evaluate the problem, extensive model experiments were conducted, including flow visualization, wake survey, powering experiments, and a crucial series of cavitation experiments including propeller-induced hull pressure measurements in a large water tunnel. Experiments with two fin designs showed the superiority of a flow-accelerating configuration. Other experiments showed some benefits of altering the propeller blade shape. Propeller analyses were undertaken to provide design alternatives for retrofitting the ship with a new propeller. A full-scale trail with the final fin design provided evidence or reduction of the highest levels of airborne noise, reduction in the initial-stage erosion damage, and minimal effect on ship speed. The result is that the AO-177 was accepted by the fleet for normal service, with AO-177 commissioning 10 January 1981.

A new and improved design in aircraft carrier fleet propellers reduces the propeller excitation of the ship structure. The propellers installed are approximately 21 feet in diameter and weigh approximately 65,000 pounds each. They are very similar in size, weight, and material to the propellers on previous ships of the Nimitz class, but the blades are shaped differently to reduce wear and erosion. The propellers have been outfitted with a protective covering that will be removed later in the construction process.

The Paddlewheel Effect is the tendency for the ship propeller to develop a sideways force (propeller side force or bias). With reverse propeller and a right-handed screw, the ship will back with a tendency of the stern to go to port; this will cause the ship bow to turn to starboard.




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