Hex Screen Dissertation

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).