Ship's propulsion layout

More than one way to get the job done

Authored by: Martin Leduc, December 2001

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All commercial ships have a common thread. Within the ship, there is a power plant and, generally, a propeller which converts mechanical energy into motion in turn propelling the ship through the water.  Most casual observers of ships just see the outside, but like everything else, things on the inside are pretty much different from one ship to another. On this page we will graphically see the different types of propulsion packages commonly used on trading vessels.


Much like a chef in his kitchen, the Naval Architect has several base ingredient which make up most of his "meals". Let us start by identifying the various propulsion "ingredients" of a "dish", huh, ship. The two most common ingredients are the prime mover and the propeller. Then, things get creative in between. BTW These illustrations are not to scale, also click the links to learn more or view picture examples.


The Diesel principal engine is currently the preferred choice as prime mover. In the past steam turbines and boilers (not shown) previously held a dominant role in ship propulsion and perhaps gas turbines will in the future. 


The clutch allows the engine to be operated independently of the shafting and gear - an on / off switch.


The marine reduction gear allows the propeller to turn at it's most efficient speed regardless of the prime mover's speed. Multiple input gears accept many forms of input but unites them into a common output. 


The thrust block directs the forward propulsive energy of the shafting into the ship's hull and not into the machinery. 


The OD box is an essential component of the controllable pitch propeller (CPP) system. The CPP allows the engine to turn at it's most efficient speed while giving the ability to the crew to control the motion of the vessel. i.e. The engine turns clockwise at 500 rpm at all times, during that time, the ships can go forward ten knots, or 20, and or reverse.


The electric motor and "podded" drives are currently the new developments in the shipping world. They increase the efficiency and reliability of the propulsion package by simplifying it (by getting rid of the shafting). 


Of course, the most common "ingredient" is still the fixed pitch propeller. Solid design which has proven itself over time.  


Deep sea vessels

Deep se ship set upProbably the most common propulsion set up out there. Look into any harbours, you will most likely see ten ships (over 10,000 grt), and perhaps nine, if not all of them, will have this set up. Bulkers, container ships, OBO, large fishing vessels, tankers and some cruise ships are some vessels that use this setup. 

The engine is generally a slow speed two stroke engine, such as a B&W or Sulzer. It is directly connected to the propeller shaft. Forward thrust of the fixed pitch propeller is channeled to the frames of the ship by the thrust block. Reverse is obtained by running the engine in reverse, they are designed with a variable cam which allows this to happen. This set up is simple, efficient and "easy" to operate and maintain.


Specialized vessels

Two clutched engines into a common gearbox feeding power to a CPPThis setup is very common on medium size specialized vessels, such as Stand By Vessels (AHST) Research, cable ships and such. The CCGS Gordon Reid is a search and rescue vessel, it features two of this of type of set up. The four engines and two shaft with CPPs offer a high degree of redundancy and versatility. In the case of the Reid, the outside engines also drive fire pumps feeding water to big fire monitors above the wheel house; as you would find on a oil rig "stand by" vessel.  

Two four stroke, medium speed engines are used here for their low size to power ratio. Having two engines also offers a level of redundancy. They can be operated simultaneously or individually because of the clutch. The gear box will compensate for the high speed of the engines and allow the propeller to turn a more efficient slower speed. In the power range of typical four stroke Diesels, the thrust bearing is usually fitted within the gear box, but occasionally it is an independent fixture. The CPP system provide the quick response these type of vessels require. 


Single four stroke driving a CPP through a gear boxCoastal vessels

This set up is common on larger fishing boats, coastal freighter, some medium size tugs (~4000 hp). It is without mystery; the parts are easily serviceable because there usually "off the shelf".  

One four stroke medium speed Diesel engines which is geared and can be run without turning the shafting because of the clutch. The CPP offers a responsive level of control and reverse! The set up is straight forward and easy to maintain. 


Small boats

Small boat set up.This is typical set up of small vessels. Yacht, tugs, fishing boats, small ferries etc. It is very common to see this in vessels having less than 2000 hp. It is very simple and the whole package is pretty much "off the shelf". 

The Diesel engine can be a two stroke (i.e. Detroit Diesel) or a four stroke (i.e. Cummins, Caterpillar). It is almost always a high speed engine which requires the marine gear (i.e. Twin Disc). The gear has a built in clutch, actually two clutches, one for forward and one to obtain reverse. The gearbox also has the thrust bearing built in. The fix pitch propeller is attached to the end of the shaft. This set up is very manageable and reliable. 


Cruise ships

Podded drive - newer cruise ship designThe height of practical modern ship engineering, the Diesel electric cruise ship. Most of the cruise ships are relatively new and most of them feature a propulsion set up much like this one. It offers more room for paying passengers, it is versatile and has a high degree of reliability. It is an efficient system, as you can adapt supply power to your demand. It is technologically complex but relatively easy to operate.

The engines are usually four stroke medium speed engines, this is because of their bigger size to power ratio, so you don't need a big engine room space. In the past, you would have two big engines to drive the vessel and three others to supply the large electrical hotel load. This would underutilize the horse power in the vessel. In newer ships, all the engines are generators and they supply one common electrical bus. Engineers can then adapt power supply to the demand, making the whole process a little bit more efficient, much like a power utility on land.

The propulsion is done via electrical motors which are built into pods controlled by cyclo-converters (not shown). The pods hang below the ship hull increasing their efficiency. Essentially the propulsion becomes just another "load" on the "power grid". It is a big load that's why there is so many engines, sometimes of varying size. The QE2 has nine identical MAN medium speed Diesels generators. Having so many engines makes it easier to perform maintenance on the fly, reducing down time and increasing profitability. Another feature, is the ability to physically isolate the various engines therefore reduces the chances of a  catastrophic failure of the system in case of a fire or such. The engine room is several compartments as oppose to the one large compartment that it use to be. It is my opinion that we will see this type of propulsion system become more prevalent on many different types of ships as the price of technology decreases and the system simplifies.  



IcebreakerThe cruise ship did not really innovate the idea of Diesel electric propulsion. It has been around since the beginning of Diesel application in ships. Back then there was no way to get reverse save for a Diesel electric set up. Submarines were another big consumers of Diesel electric set ups. But the most accessible consumer was the icebreakers of the world. They require large horse power but often encounter shock loading from hitting big pieces of ice and such. Diesel engines do not appreciate shock loading. 

The Diesel electric propulsion allowed the shocks to be "filtered" by the electrical machinery and pass on to the engines in more manageable increments. The system illustrated here is from the CCGS Sir Wilfrid Laurier, a Canadian medium icebreaker. It features three 2200 kW medium speed V16 Alco 251. The AC generators feeds a common bus at 6600 VAC. The propulsion system consist of two electrical motors driving two fixed pitch propellers. Fixed pitch propellers and their simplicity are less likely to fail in ice conditions as oppose to the more delicate CPP ones. The motors are controlled by two cyclo-converter. Cyclo-converters are a web page unto themselves, but basically, they allow the electric motors to turn, forward or reverse, at any given power (speed) required.  


Naval vessels and tankersNaval type gas turbine / diesel set up

Naval vessels and other military ships have other priorities than fuel  efficiency. Therefore we often see high powered propulsion set up in these vessels. That amount of power comes from fuel thirsty gas turbines or steam turbines. The US navy is an avid consumer of steam plants - a great number of their vessel obtain steam from nuclear means. Traditional steam plant are still in operation but over the years the marine gas turbine has become more prevalent for its power to weigh ratio (no need for a big boiler and it's systems). In Canada, our Halifax class frigates are power with gas turbines, but of course, we are not a rich nation, so the propulsion plant also includes a more economical medium speed Diesel plant. In the merchant marine, steam propulsion plants are relatively rare, as the Diesel engine can do the same work more efficiently. Large tankers still use steam plants, this is due to the huge power requirements that the Diesel engine has not been able to practically deliver. 

Turbine plants are usually compact,steam turbine set up "lightweight" high powered, and turn at a high rate of speed. Their operation is simple but their maintenance and fuel consumption make sure it is not the most popular choice for ship's propulsion. Above right, we see the military type set up. One gas turbine and two cruising Diesels - medium speed four strokes. Each is capable of inputting power and that power is controlled by a CPP.  To the right we have a tanker steam turbine set up. As there is no CPP, the steam turbine will have a reversing turbine built into each. The power feeds into the gear box reducing the rpm, then into a fix pitch propeller.  


Double ender ferries

Double ender ferryThe "double ender" ferry is another unique animal. It is quite common to see this type of vessel in the North American northwest. BC and Washington state ferries use the double ender ferries extensively. They carry large quantities of vehicles and passengers on short runs in calm waters. To save the time of turning the vessel around at each berth, they just simply make sure it goes easily one way as it does the other way. 

This gives rise to an interesting design; one propeller and rudder at each end of the vessel. The actual propulsion equipment is not that complicated. Take for example the BC Ferries Queen of Cowichan. It has two MAK 550 medium speed four stroke Diesel engines each producing about 6000 hp. They are couple to a  gear box by two air operated clutches. The shafts are quite long, as they must go to both ends of the ship down the centerline; and they clutched as well, to allow both engines to drive one shafts or both. With a CPP system as well, and two rudders, this type of vessel is very responsive. The ship has a service speed of 21 knots.  



This is, by no means a definitive list of propulsion systems. There are several "set ups" common to smaller markets. The water jet, essentially a geared Diesel engine driving a "propeller" within a housing - impeller in a pump - shooting out water where directed. Water jets are used extensively for fast ferry applications as are Harnesson Drives (smaller applications). There is also tugs which use some specialize propulsion which allows them to be extremely maneuverable - Voight Sneider Propeller, Z drives and such. There is also exciting new technology still on the drawing board such as the Magneto Hydro Propulsion. But these will have their own web page. Constructive comments and corrections are always welcome, click here for that.