How Propulsion Works: Classifying Ship Propulsion

No doubt, the math of propulsion devices is complicated. But it is all the same physics. By dividing each of these devices into their essential tasks, we find new opportunities. New methods of propulsion from different combinations of old tricks. Insight into this continuum allows us to better classify marine propulsion...

1.0 Introduction

Readers frequently ask me to compare different methods of ship propulsion.  To explain how a pump jet is different from a waterjet.  These are excellent questions, but they miss the larger insight.  All propulsion is the same.  The world of physics does not change from one type of propulsion to the next.  All ship propulsion follows the same physics, and the different methods of propulsion just emphasize on one sector of physics over another.  Insight into this continuum allows us to better classify marine propulsion and dissect past successes to search for new combinations.

2.0 The Physics of Propulsion

Every marine propulsion device works by doing two things:  create a pressure difference, and then apply that pressure to a surface for forward thrust.  Optionally, we may also add in a thrust augmentation device that helps generate the pressure difference.  Table 2‑1 shows common propulsion devices and demonstrates how they achieve these two tasks.

Frequently, the same device achieves both tasks, but by separating propulsion into these two items, we identify the different methods to achieve each task.

3.0 Pressure Difference

The pressure difference is where we convert the power of a rotating shaft (engine power) into some type of fluid power that generates thrust.  Think of it like injecting energy into the flow stream.  The pressure difference normally comes from some type of rotating foil section.  One side of the foil generates a higher pressure, and the other side generates a lower pressure.  This is the fundamental physics behind your basic slow speed propeller.  But as the ship velocity increases, we need to increase propeller RPM to keep up. 

The higher RPM speed introduces new problems.  As the propeller foils rotate faster, centrifugal force dominates flow.  This centrifugal force pushes the water radially out from the center of the propeller, which is a problem.  Propeller foils don’t work when the water runs radially along the blade.  The water throws out sideways and generates no pressure difference. 

One solution is to enclose the propeller inside a long tube.  We call this an impeller.  The walls of the tube trap the water flow.  Now, we can use the centrifugal force to our advantage.  It becomes the second method of generating a pressure difference.

The final option for pressure difference is to change the density of the fluid, which tends more into the field of thermodynamics.  Combustion on rockets or jet engines creates rapidly expanding gases.  That rapid expansion generates a massive pressure difference.  Unfortunately, combustion does not work well underwater.  We don’t really see this method in the marine world.  Although some experimental methods have tried to inject pressurized air for propulsion.  Those methods work, but, so far, they were less efficient than conventional foil sections.

For marine propulsion, foil sections are the best option to generate a pressure difference.  The shape of those foils mostly depends on ship speed.  At lower speeds, straight propellers tend to be the most efficient.  At higher speeds, the impeller option is best.  Even the impeller largely depends on the foil section instead of centrifugal forces. 

4.0 Pressure Surface

The pressure surface is where we see the most creativity.  Imagine we have magic device that generates a zone of higher pressure in the middle of the water.  The problem is that water pressure pushes uniformly in all directions.  If we contained our magic zone in a hollow sphere, it would push the same amount in all directions.  We get no thrust.  The trick is to angle the surfaces and leave one side exposed to create a net thrust in the forward direction.  Pressure difference creates the energy for movement, but the pressure surface determines the direction and effectiveness of that energy.

What makes a good pressure surface?  The ideal surface would face mostly in the forward direction and have a huge surface area.  We don’t see this, because a large surface behind the ship also generates large amounts of resistance.  That effectively cancels out any benefit from added thrust.  All practical pressure surfaces act as a compromise.  Let’s examine some past successes.

The most common pressure surface is the foil of propeller blades.  One major reason that propeller blades are so large:  we want to create a large surface area for the pressure to push against. 

The propeller blades also offer another trick:  the pressure surface can be multiple surfaces.  The front face of the propeller blade gets pulled by a negative pressure (suction).  But the back of the propeller blade gets pushed by a higher pressure.  Both of these combine to generate thrust in the forward direction.  Pressure surfaces work best by taking advantage of the local pressure difference and by minimizing resistance in the water.

Even in waterjets, the impeller blades act as the main surface to generate thrust.  But the waterjet employs a few additional features.  Just behind the impeller blades, we find the stators and the diffuser.  A diffuser is just an expanding cone to slow down the water flow.  And the angle of the cone converts that pressure into more forward thrust.  This shows a great innovation.  It was possible to create a larger surface area if we trapped the area inside an internal pipe, removed from external flow.  Of course, we don’t get this trick completely free, but it still offers more benefit than hindrance.

5.0 Thrust Augmentation

The final feature is thrust augmentation.  These devices help create the pressure difference by altering the flow conditions slightly.  They don’t create any power themselves, just improve the efficiency of other devices.  Normally, this effect is mild, only 5-15% difference.  The trick is applying an effective device without excess drag through the water.

A great example of thrust augmentation is the propeller shroud.  These shrouds may look like simple cylinders, but the ring is formed from a wing profile.  Depending on the orientation, this shroud can either accelerate or decelerate the flow through the propeller.  (There are also some secondary effects, but I’m ignoring them for clearer presentation.)  For slow speed ships, we accelerate the flow to get slightly more thrust on the propeller.  This happens on pump jets and ducted propellers in tugs. 

For high speed ships, the shroud decelerates the flow, which helps the propeller.  The faster ships may be moving too fast for a normal propeller to be effective.  Decelerating the flow puts us back in the more efficient range for propeller operations.  This high speeds also require a higher RPM on the propeller, and the duct helps to contain the centrifugal forces and keep the flow moving straight through the propeller.

At higher speeds, the waterjet nozzle acts as another form of thrust augmentation.  The impeller on a waterjet normally generates a relatively mild pressure difference, at a high flow rate.  Most of the waterjet energy goes into simply moving the water through the pump.  The nozzle on the waterjet converts some of the kinetic energy in the flowing water back into a pressure difference.  And since the nozzle is directly after the pump diffuser, that extra pressure difference adds on the pressure surface, increasing thrust.

6.0 Conclusion

No doubt, the math of propulsion devices is complicated.  But it is all the same physics.  We suffer from the mistake of seeing these as separate methods of propulsion.  A waterjet achieves the same goal as a propeller.  By dividing each of these devices into their essential tasks, we find new opportunities.  New methods of propulsion from different combinations of old tricks.  Because it isn’t really new.  All propulsion is the same physics, just with a different emphasis.

7.0 References