Talk – Tuesday 10th January 2017 ‘Ship Propulsion – from paddles to jets’
Antony Tomkins – Hamilton Jet
According to IMO statistics over 90% of global trade is carried by sea. As a consequence, shipbuilders, designers and equipment manufacturers are constantly striving to find more efficient solutions to the century’s old problems associated with marine propulsion.
In the beginning marine propulsion was little more than men with paddles. The advent of the sail era improved efficiency somewhat, but in terms of powered propulsion, paddles remained the key principle.
Real developments started to take place around the 1830’s when John Ericson and Francis Pettit Smith started to work on the concept of screw propellers. These designs were based on the Archimedes screw concept, and used the rotational motion of the propeller to create a pressure difference between the forward and rear sections of the propeller. The water is then accelerated over this pressure differential imparting momentum and thus generating the forward thrust. The term screw propeller was adopted as the propeller “screws” the vessel through the water.
Pettit Smith and Ericson continued development work on the propeller system in the 30’s with Smith submitting the first patent for a screw propeller on 31st May 1835. Ericsson filed his own patent 6 weeks later. You will notice that this screw propeller had two complete ‘turns’.
Paddle steamers are relatively simple in operation and design, but their propulsive efficiency (PE) is low, and so for powered ships to become successful more efficient solutions were needed.
Pettit Smith began by building a model to demonstrate the concept to potential backers and the Admiralty. He was successful in finding a backer from a wealthy banker, and later that year built a 30” 6hp canal boat – the “Francis Smith”. In February 1837 the Francis Smith suffered a rather fortuitous accident when the vessel struck a submerged object and broke one complete turn of the wooden propeller. To Pettit Smith’s surprise the vessel speed rose from 4kts to 8tks! Pettit Smith immediately filled a new patent!!
Meanwhile Ericsson was developing his own concept. In 1837 he built his own 45” demonstrator the steam powered “Francis B. Ogden”. He showed this to the British Admiralty in the summer of that year, but despite impressive results, the Admiralty was underwhelmed, claiming that a screw vessel would never be as efficient in open seas as a paddle steamer!
Spurned by the British, Ericsson built the SS Robert F. Stockton and sailed her to the USA to demonstrate the possibilities of the screw propellers. The USA was so impressed that they commissioned Ericsson to design the first screw propeller warship, the USS Princeton.
Back in Britain, Pettit Smith had learnt of the Admiralty’s view on open water performance of the screw propeller. He decided to arrange a sea going voyage of his demonstrator vessel: on its return leg it was viewed in rough weather by senior officers of the Royal Navy and this reinvigorated their interest in the concept. With the renewed interest from the Admiralty, and having secured some financial backers, Pettit Smith set about building a much larger vessel, the SS Archimedes, a 125 ft steam powered schooner. SS Archimedes was demonstrated to the Admiralty at length, and on the whole impressed. Captain Edward Chappell, finally concluded that “Screw propulsion was equal, if not superior, to that of the ordinary paddle-wheel.”
This was a significant development and led to the adoption of screw propulsion on all future RN build and to the famous tug of war between HMS Rattler and HMS Alecto. The adoption of screw propellers in the Royal Navy was significant, but SS Archimedes would make a much more significant impact on the marine world.
Following her successful trials with the Royal Navy, Pettit Smith loaned SS Archimedes to the Great Western Steamship Company who were in the process of constructing the worlds’ largest steamship – the SS Great Britain. The principal engineer, I.K . Brunel, took advantage of the loan of the vessel to demonstrate the advantages of screw propulsion for the SS Great Britain.
The loan period allowed Brunel to conclude that screw propulsion was lighter in weight, thus improving fuel economy, could be kept lower in the hull reducing the ship’s centre of gravity, and making it more stable in heavy seas. By taking up less room, the propeller’s engines would allow more cargo to be carried. Elimination of bulky paddle-boxes would lessen resistance through the water, and also allow the ship to manoeuver more easily in confined waterways. The depth of a paddlewheel is constantly changing, depending on the ship’s cargo and the movement of waves, whilst a propeller stays fully submerged at full efficiency at all times. Screw propulsion machinery was cheaper. Consequently SS Great Britain was built with screw propulsion.
Brunel, however, chose not to adopt Pettit Smiths’ propeller design but instead adopted his own 6 blade design.
This proved to be a costly mistake as the design was far less efficient that Smith’s own 4 bladed-design, and contributed to the SS Great Britain’s troubled history.
However, the concept of screw propellers were now widely accepted as the future in both the military and commercial sectors
By 1855, 20 years after Pettit Smith’s first patent, 174 ships of the Royal Navy had been fitted with screw propulsion, and the vast majority of new vessels being built for commercial owners were exclusively propeller, or mixed propeller/paddle. The maritime world had now accepted propellers, and for the next 160 years navies, industry and owners focused their attention on improving the efficiency from screw propellers.
The work done by those early pioneers led to the developments and refinements of what is now commonly referred to as Fixed Pitch Propellers (FPP). The term ”fixed pitch” relates to the ‘distance’ that the propeller will travel in one rotation. The higher the pitch, the greater the distance. For an FPP propeller this pitch is a fixed dimension. It is important for the propeller designers to know the ships’ resistance (force required to push the vessel through the water) at all speeds, but especially at the desired top speed of the vessel, and the power required to maintain that speed. A FPP propeller must be designed to perform bestl at this design condition. This is fine for a vessel design where the operational state is fairly constant, but most vessels are not like this.
A good example is an RNLI Lifeboat. The Tamar class lifeboat shown here has 2,000 hp and is designed to run at 25kts at its service weight of 32t. However, she is also capable of rescuing 112 casualties. This is a significant increase of approximately 9t, nearly 28% increase in displacement!
Obviously this means the boat will run slower, but it causes bigger problems for FPP systems. As the propeller is designed to move the volume of water required to maintain a fixed speed at a given power and displacement, if you change one of these variables then the propeller is no longer optimum for this state. The use of FPP propellers can lead to inefficiency, high fuel burn, decreased time between overhauls and eventually overloading. This is a problem for many applications such as cargo vessels, ferries, oil and bulk tankers, work boats and offshore supply boats.
Recognition of this problem lead to the developments of more sophisticated Controllable Pitch Propellers (CPP). The concept for these had been around for many years, and well used in aircraft since before WW2. But in the marine world it was not until electronically actuated pitch control became possible that the system started to take off. CPP’s allow the master of the vessel to change the pitch of the propeller to suit the requirements of the vessel at that time. By allowing the pitch to change CPP’s overcome the overloading and efficiency problems usually experienced with FPPs. They also can be reversed giving much better astern performance.
The disadvantages of CPPs are that they are mechanically complex, very expensive and rely heavily on a good control system. In some cases they cost more than the whole driveline for an equivalent FPP system. CPPs have become relatively efficient for speeds between 15-30kts, operating with PE figures of between 0.6-0.65, with some CPP systems claiming over 0.7.
For some applications where high thrust and efficiency is required below 20 kts, other solutions are needed.
This is where Cyclorotor propellers or Voith Schnieder Propellers (VSP) come in.
VSP’s are very different to conventional propellers. Rather than rotating in the horizontal plane like almost all other propulsors, VSP’s rotate in the vertical plane. VSP’s work by rotating wing- like foils through the water.
The foil generates lift which produces the propulsive force.
As the foils rotate around a central main shaft the angle of attack of each foil is altered so that the component thrust force is fully controllable. VSP’s have very high levels of PE at sub 20kts performance –typically in the region of 0.7-0.75 This allows very high bollard pull values.
Being able to change the angle of attack of the foil also allows VSP’s to provide full 360o with no loss in propulsive thrust. This makes them perfect for tugs and offshore supply vessels (OSV’s & PSV’s).
However VSP’s excellent PE number drops rapidly as vessel speed increases, they are mechanically complex requiring skilled maintenance (cost!) and increase the draft of the vessel. Operation in shallow harbours can be difficult.
For vessels requiring operation at both high speed and with excellent maneuverability, there are a couple of options. For large vessels operating at speeds up to 30kts, Azimuth thrusters (AZT’s) are often used. AZT’s are essentially conventional propellers mounted in a rotating ‘pod’, and come in various forms: single screw aft facing, single screw forward facing, duo screw, forward and aft prop.
On the left are two duo screw AZT’s.
On the right is an AZT housed inside a Kort Nozzle (propeller shroud) to improve low speed efficiency and high speed course keeping.
Powering an AZT can be mechanical, hydraulic or electrical. Mechanical/hydraulic units are shaft driven through a Z gear arrangement from the main propulsors in the engine room of the ship.
Electrical units have their own electric motors mounted inside the pod, so all that is transmitted through the leg is the electrical power and drive signal.
Regardless of what type of AZT’s are used, the results can be impressive for big ships. Azimuths have been very successful in recent years especially in the cruise industry. The AZT’s excellent control at zero speed has allowed large vessels to go places where they could not go before without tug assistance.
For vessels needing to run at over 30kts, there is an alternative to propellers: the waterjet.
Jets are a good solution for any vessel from 20ft to 350ft that needs to travel at high speed yet still have good maneuverability and high PE numbers. Vessels with high gross tonnages are generally precluded from going fast, as to do so is extremely expensive and technically demanding.
Water jets were originally pioneered in New Zealand by the Hamilton family as a result of their desire to drive small boats quickly up shallow rivers in and around the rivers of the Canterbury plains.
However as the concept developed the additional benefits, other than shallow draft, became clear to commercial, professional and military operators.
A waterjet is best thought of as a pump. The propulsive force is generated by the change in fluid momentum though the system. As the “pump” will be pumping the same volume of water at any given speed regardless of the vessel state and loading the PE of the unit is constant across any loading state.
This means that means that whilst a vessel will be slower during heavily laden states the fuel burn and loading on mechanical components will be constant.
This has made water jets very popular for fast offshore supply vessels, fast ferries, rescue boats as well as many other applications.
Water jets also benefit from the advantage of having no appendages under the water. This means that the vessel can run in very shallow waters, and the actual propulsive efficiency of the vessel is higher than the pure propulsor efficiency number as no appendage drag needs to be factored in – no rudders or propeller supports etc.
Like VSP’s and AZT’s water jets can also vector their thrust in 360 degrees, although the thrust values are not as high. Overall maneuverability is very high.
If we remember back to the RNLI example earlier, the advantages of waterjets are the main reason why the RNLI have chosen them for their new Shannon class lifeboat.
All the propulsion systems discussed here have pros and cons, and must be matched to the vessel and its intended usage, which is the task of the Engineers and Naval Architects.