A DIRECTIONAL SCREEN SYSTEM FOR REVERSIBLE MARINE THRUSTERS – by Calvin A. Gongwer
Introduction
Marine vehicles, from large ships to umbilically controlled underwater robots (ROV’s) and small submarines, use ducted propeller thrusters to control their position and attitude and, except for the ships and some submarines, to provide their main propulsion. These thrusters have problems such as thrust-limiting cavitation at and near the surface, particularly in the case of ships, hazard to marine life, divers and equipment, interruption of operations from ingestion of foreign objects, etc.. This disclosure shows a screen system that can solve all the above problems. vehicles, from large ships to umbilically controlled underwater robots (ROV’s) and small submarines, use ducted propeller thrusters to control their position and attitude and, except for the ships and some submarines, to provide their main propulsion. These thrusters have problems such as thrust-limiting cavitation at and near the surface, particularly in the case of ships, hazard to marine life, divers and equipment, interruption of operations from ingestion of foreign objects, etc.. This disclosure shows a screen system that can solve all the above problems.
Description
A propeller, blade (A), rotates reversibly in a duct between two preferably hexagonal rigid screens (B) and (C) (Fig. 1). The screens protect divers and marine life from being drawn into the propeller and destroyed. Also, the screens may act as structural support for the propeller shaft and/or drive motor. Further, it can be shown that since the screens are streamlined for flow in one direction and unstreamlined for flow in the other direction they can be made to provide a hydrodynamic advantage to the thruster operation, tending to suppress loss of thrust from propeller cavitation and increase propeller efficiency in both directions, notwithstanding the screen’s resistance to flow. The hexagon is the preferable basic building block for the screen due to the large angle ( 120 degrees ) between intersecting legs. This reduces the hydrodynamic interference between the legs at the intersections. A screen with square or triangular openings may be preferable in some cases. In a blade (A), rotates reversibly in a duct between two preferably hexagonal rigid screens (B) and (C) (Fig. 1). The screens protect divers and marine life from being drawn into the propeller and destroyed. Also, the screens may act as structural support for the propeller shaft and/or drive motor. Further, it can be shown that since the screens are streamlined for flow in one direction and unstreamlined for flow in the other direction they can be made to provide a hydrodynamic advantage to the thruster operation, tending to suppress loss of thrust from propeller cavitation and increase propeller efficiency in both directions, notwithstanding the screen’s resistance to flow. The hexagon is the preferable basic building block for the screen due to the large angle ( 120 degrees ) between intersecting legs. This reduces the hydrodynamic interference between the legs at the intersections. A screen with square or triangular openings may be preferable in some cases. In a tunnel thruster located athwartships in a large ship, the screens would be located across both openings, port and starboard with the blunt edges outboard. Figure 1 shows the cross section of the screen elements which are blunt on one side and tapered on the other.
The performance improvement results from:
1) The downstream screen acts as a nozzle, accelerating the flow to the higher velocity of the exit jets and reducing the velocity inside the duct and around the motor, etc.. This is because the screen’s cross section area that is clear to the flow is preferably about 70% of the total area of the screen. The eddies behind the bluff screen parts are indicated in Figure 1. The incoming flow to the propeller is only slightly restricted since the screen parts are streamlined in this direction. The benefits of this effect are explained below.
2) The flow exiting the propeller has a large whirl corresponding to the torque on the propeller. A large portion of this energy of whirl is reclaimed in the exit screen due to the collimating effect of the screen with its bluff side downstream. The pressure drop across the screen urges the flow toward the axial direction. Due to the square exponent relation between flow velocity and head (meaning the transverse component of the velocity), if the transverse velocity component is reduced by only 50%, 75% of the whirl energy is recovered. This effect helps compensate for the drag of the screens.
3) The reduced flow rate thru the prop causes the pressure on the suction side of the prop blades to increase and thus suppress the cavitation as explained below. The physical picture at breakdown cavitation is shown in the Fig. 2 where the static pressure on the suction side of the prop blades is essentially zero. This can be expressed by the Equation I. which gives the static pressure on the suction side of the prop blades.
| 33 ft (atmospheric pressure) + d (depth in ft) – (Vp2 / 2g)( 1/S) = 0 |
I. |
where VP is the axial velocity thru the prop disc and S is the solidity of the prop (the projected blade area as a fraction of the swept disc area). Equation I. is obtained by applying Bernoulli’s theorem to the flow through the thruster inlet from the ambient sea. The slight drop in head thru the inlet screen is ignored since the screen is streamlined in this direction. VP is related to the exit velocity out the exit screen by the following:
| VP = Ve(Ae/AP) from continuity |
II. |
where Ae and AP are the flow cross section areas at the exit and prop disc respectively.
Substituting from II. into I. :
| 33′ + d’ – (1 / S)( Ve2 / 2g)(Ae2 / AP2) = 0 |
III. |
Since the static thrust T is given by the expression:
| T = r Ve2 Ae |
IV. |
where r (rho) is the mass density of sea water, IV can be substituted into III to give the expression for max thrust at incipient cavitation breakdown (sometimes called “super cavitation”).
Since from IV:
| Ve2 = T / rAe |
V. |
Then at the incipient cavitation breakdown condition:
| Tc = (33 + d)S2gr(Ap2 / Ae) |
VI. |
Thus, other things being equal, Equation VI shows that the thrust limit set by cavitation increases as Ae decreases. Reference is made to the writer’s note on breakdown thrust from cavitation which correlates actual experimental values of thrust breakdown with the equations above. This note is available on request.
4) The resulting alleviation of the cavitation problem at or near the surface allows the propeller to be designed for maximum efficiency, i.e., higher blade lift coefficients resulting in smaller area and skin friction and higher ratios of pitch to diameter.
Discussion and Conclusions:
The above has led to the design of a simple thruster with high efficiency and the ability to operate at shallow depths and at the surface with little or no reduction in thrust due to cavitation.
The directionally streamlined screens ( preferably hexagonal ) applied on both sides of the prop with their blunt edges out away from the prop have produced this improvement.
The screens when made in large scale can be applied to large ship transverse thrusters at each end of the tunnel, again with the blunt edges outward from the prop, with the same advantages.
The screens can be applied to general purpose propulsion such as tugboats where the large prop blades necessary to resist cavitation provide low efficiencies due to their large wetted areas subject to hydrodynamic skin drag.
Due to the strength and stiffness of screens of this design, at least one or both of them can be used to support the propeller and its drive motor. This eliminates the struts normally required.
An additional benefit is the increase in the velocity of the exit jet Ve, although the mass rate is less. This increase in Ve decreases the ratio of VF/ Ve where VF is an assumed forward speed of the ship. It is known that when at a certain critical value of VF/ Vethe thrust becomes nearly zero because the jet is deflected 90° and attaches to the hull ( Figure 4).