Bulletin 70

September, 2007

Propulsors Revisited

The Seawolf (SSGN-21) and the subsequent Virginia class submarines are propelled by something other than a screw. In a previous SRC bulletin the propulsor was discussed in a general sense. The propulsor which drives the latest of American submarines is super-secret and therefore not open to specific description. Despite this restriction, SRC continues to receive many questions regarding how a propulsor is different from a traditional propeller or screw. This bulletin attempts to answer some of these questions using the most current information available.

A submarine screw provides thrust by accelerating a rearward water flow at the stern of the hull. Because it is a rotational thrust the exit flow is both outward and to the rear. That portion of the thrust that is other than rearward is wasted. The problem confronting the designer of submarine propellers is one of maximizing that portion of thrust that drive's the submarine forward while eliminating, or at least reducing the adverse effects of rotational outward flow. Progress has been made in producing single, large, hollow, multibladed and complex-curved screws. These are much more efficient than those of the mid-twentieth century. They produce less cavitation and are therefore quieter. Yet, even these have their limits. Exit flow continues to be non-linear and entrance flow is made turbulent by planes and rudder vortices. This often sets up a "beat" and presents a problem in sound emission and signature identification. Parenthetically, the Type 21 German submarine of the Second World War solved the problem by placing the rudder and stern planes at the rear of the screw, but structural considerations normally outweigh the advantage of such an arrangement.

The propulsor solves many of the problems inherent in the traditional screw. It multiplies the number of blades into something akin to a jet engine's impellers, but its greatest departure is the inclusion of a circular ring at the extremity of the blades. In most propulsors this ring or duct is fixed to the hull with impeller blades spinning within it. In such case the tolerance between blade outer edge and inner duct liner must be as close as possible in order to eliminate outward thrust called blade edge vortex. In some ducted propulsors the ring is fixed to the blades, spinning with them. This system introduces a flywheel problem which slows response time to speed changes and reversing. To overcome this problem impeller blade adjustable pitch is possible, but this requires intricate engineering much like an adjustable aircraft propeller pitch.

The circular duct's interior shape can be designed to allow a straight flow or can be contoured to accelerate flow before striking the impellers. Once again, a similarity can be drawn between a jet engine and a ducted impeller. It is also similar to the upper contour of an aircraft wing.

It is important that water entering the duct and interfacing with impeller blades be as uniform as possible. Likewise, the accelerated exit flow must be as free from outward thrust spiraling as possible. A stator made of multiple static blades is placed at the forward end of the duct. The blades are mounted to introduce a spiral current in the opposite direction of the impeller blade thrust. This serves to straighten the exit flow. If insufficient, a second stator may be located to the rear of the impeller. To further complicate the design, stator blade pitch may be adjustable according to the rotating speed of the impeller. Stator assembly assumes a rigid duct as opposed to the spinning type.

The final consideration is the option of vectored thrust. Small submarines use trainable thrusters to give them finite control of close-quartered movement. It is possible to eliminate the rudder and stern planes by introducing a universal in the drive shaft with directional control rods to the propulsor. While the complexity of such a design seems overwhelming, the engineering is well established in helicopter rotor control. Vectored thrust is a common reality in most current jet fighter aircraft.

One significant disadvantage of the ducted propulsor is its weight. Located at the extreme after end of the submarine its negative moment is large. Hollow, light-weight material is used in the construction of the assembly. Obviously, much technical design information came from the aerospace industry.

SRC hopes that the above information is helpful in understanding the basic elements of propulsor theory. Further questions will be answered as more information on the subject becomes available.