Tuesday, June 30, 2009

Controllable Pitch Propellers

Controllable Pitch Propellers are propelling device for ships or boats that provide more flexibility in the blade pitch variation which is important for ship maneuvering in restricted waters such as harbor or narrow bay areas. The use of controllable pitch propeller is increasing day by day. According to Llyods Register of Shipping, in 2000, the market share of this propulsion system reached 35 percent. The CP propellers are mostly installed in ships that need higher degree of maneuverability such as passenger ships, ferry, tug boat and trawler vessels. For military application, the installation of controllable pitch propellers in landing ship tanks greatly helps the ships to load and unload military personal and their logistic supplies easily in shallow waters.


Although controllable pitch propeller has higher degree of freedom for the operaton of ships or boats, its design, manufacturing, installation, and operation are more complex than the conventional fixed pitch propeller. In general, the controllable pitch or CP propeller has two operating mechanisms. They are pull-push rod and hub piston mechanisms which are depicted below:

cp-propellers.gif

The CP propeller can be controlled directly from the bridge or from the engine room using hydraulic or mechanical system. In the hydraulic system, the actuating force needed to change the pitch of the propeller is transmitted to the blades of propeller by piston in servo motor that pushes or pulls oil inside the shaft of the propeller. A servo motor assembly consists of control rod assembly, locknut, piston, and oil.
Controllable Pitch Propeller Schematic Operating Systems: (a) push-pull rod system and (b) hub piston system Source: page 22 of Marine Propellers and Propulsion second edition by John Carlton
Another advantage which controllable pitch propeller provides during the operation of a ship is that it can move the ship astern at full power. It means that the ship can move backward without the help of tugboat. Such capability cannot be found in conventional propelling system that uses fixed pitch propeller.

cp-propeller.jpg
Besides having better properties in maneuverability, CP-propellers, in general, has overall propulsion efficiency that is lower than fixed pitch propeller due to their higher boss diameter ratio (DB/D) which is around 0.24 to 0.32D. For fixed pitch propeller the boss diameter ratio (DB/D) is between 0.16 and 0.25. Marine screw propeller with higher boss diameter ratio tends to have lower efficiency caused by complex hydrodynamic problems such as cavitation and wake. by Charles Roring in Manokwari of West Papua Indonesia
Also read: Boat Propeller

Monday, June 29, 2009

Propulsion Systems Other Than Propeller

by Charles Roring

Propeller is now the most common form of propelling device for ships and boats। In the Book of Marine Propellers whose author is John Charlton, it is said that propeller is not the only form of marine propulsion system. Propeller predecesors such as paddle, paddle wheel and sail are still being used as marine propulsion systems although their percentage has been far lower then the time when propeller had not been invented.

bonanza6.jpg

Research and development in the field of naval architecture and marine hydrodynamics have led to the invention of newer propulsion system such as cycloidal or Voith Schneider propellers and magnetohydrodynamic propulsion system. Both systems are different from the conventional system.

Voith Schneider propellers was first invented in 1920s by Kirsten-Boeing. Then, it was developed an improved by Voith-Schneider. This kind of propulsion system works on vertical axis with some six to eight vertically mounted vanes rotate on a disc in horizontal plane. Voith Schneider propeller is mostly used in ships or boats that need highly manouvering capabilities. They are passenger boats, tug boat.

Magnetohydrodynamic propeller. The term propeller here should not be associated with the conventional marine screw propeller. Propeller in this propulsion system is simply the terminology for a propulsion device. Magnetohydrodynamic propeller does not have any moving parts in its main propulsor. Water that passes through a duct is accelerated by magnetic coil and electrodes which wraps the duct. Although it has not been a viable propulsion system for commerical uses, this propulsion system has a potential advantage of being able to operate in vibration-free environment. Such property is needed in warships and passenger ships. This propulsion sytem still needs more research before a commercially viable device can be manufactured. Also read: Material and Strength of Marine Screw Propeller and Pitch diameter ratio of a propeller and engine performance of a ship; Ship displacement calculation

Monday, June 22, 2009

Propeller Mean Pitch Calculation

In my previous article, I said that the pitch of propeller is determined from power coefficient Bp whose parameters are the RPM of the propelling machinery of the ship, the delivered power of the main engine, and the speed of advance. In this case, the speed of advance is the speed of water flowing toward the propeller. Also I wrote that the pitch ratio of propeller is obtained from the Bp-delta.jpg diagram. We must understand that the value which we read from the diagram is the pitch ratio at the tip of the propeller.
If we use B-series propeller as our standard for the design of our propeller, then we must calculate the mean pitch ratio of the designed propeller. The pitch of B-series or Troost propeller varies according to its radius. It means the surface of the propeller is spiral. If the generatrix or generator line of a propeller has curved form then the pitch distribution of the propeller is not linear.
The following table is an example of mean pitch ratio calculation for an Open Hatch Bulk Carrier of 45,000 DWT. Based on the data of the B-series provided by T.P. O'Brien in his book, The Design of Marine Screw Propeller on page 132. The tip pitch ratio is 0.793 and the diameter of the propeller is 6.8 meters.
After tabulating the calculation of the pitch distribution based on P/D diagram, the product of pitch and radius x will have to be devided with the sum of the radius fraction of the propeller. Mean Pitch Ratio: sigma.jpgxp/ sigma.jpg x =4.2075/ 5.4 = 0.779.
So, the mean pitch ratio (P/D)mean of the designed propeller is 0.779
Propeller is still the most common form of propulsion device that propells ships around the world. By studying the propulsion properties of propellers we can design high efficient propellers that can move ships at sea in higher speed, with much less vibration and low fuel oil consumption.
Related articles:
Controllable Pitch and Fixed Pitch Propeller
Propeller Pitch Ratio
Pitch Ratio of Ship Propeller
by Charles Roring in Manokwari of West Papua - Indonesia

Saturday, June 20, 2009

Propeller Pitch Ratio

Before we proceed to the mean pitch ratio calculation of a propeller, we must know, first, the definition of the terminology. Pitch is the distance a propeller travels along an x axis after one revolution. An ideal pitch can be seen if we imagine the propeller as a cork screw that moves forward through a solid material. So, the speed of the cork screw is V = pitch x revolution. But in reality a ship's propeller always works or moves through sea or fresh water. So, such principle of moving through solid material cannot be applied in the design of marine screw propeller.aremiti_3.jpg

cork-screw.jpg
Source: Basic Principles of Ship Propulsion
When a propeller revolves in the water, the fluid which is the sea water will be accelerated afterward. This happens because the water yields. Technically, ship designers or naval architects call this phenomenon as slip. Slip decreases the speed of a propeller. We will discuss about propeller slip in another article.
propeller-pitch-slip.jpg
Back to the propeller pitch ratio, before calculating this parameter, a propeller designer must determine the tip pitch ratio of the designed propeller at the service speed of the designed ship. The value of propeller pitch-diameter ratio (P/D) can be obtained from Bp- diagram. Bp is influenced by: the revolution of the engine at an optimum condition favorable for the propeller operation N; the delivered horse power PD; and speed of advance VA. Such parameters are also needed in the calculation of with the addition of another parameter, i.e. D which is the diameter of the propeller obtained from stern detail of the designed ship. The followings are the formulas for Bp and


and
After calculating the values of Bp and , the next step is reading them on the Bp- diagram. The diagram which I usually use is propeller Troost / Wageningen B-series. An example of the diagram is presented below:

bp-delta-propeller.jpg
Source of the figure: Marine Propellers and Propulsion written by John Charlton
From the above diagram, we can get the values of open water efficiency 0 and the pitch diameter ratio P/D of the propeller.
For the same delivered power and speed of ship, if we increase the engine revolution, the diameter and the pitch will be lower. To high pitch and diameter of propeller will cause the main engine to operate in over-loading condition. After all the principal dimensions of the ship's propeller have been determined or calculated, the next step which a propeller designer or naval architect has to perform is calculating the strength of the propeller and drawing the propeller. by Charles Roring in Manokwari of West Papua Indonesia.
Also read: Boat Propeller

Tuesday, June 9, 2009

Material and Strength of Marine Screw Propeller



by Charles Roring in Manokwari of West Papua - Indonesia

The strength and dimensions of a ship's propeller is influenced by various propulsion factors and the material choice. If the resistance and propulsion parameters of a ship has been determined, the next step is determining the propeller dimensions. One of them is the thickness of the blade.

In calculating the thickness of the blade, a propeller designer usually has to perform strength calculation so that he can determine the minimum thickness of the blade at radius 0.2 R of the ship's propeller. The calculation is usually based on Taylor's method which is well explained on pages 288 to 301 of The Design of Marine Screw Propellers written by T.P.O. Brien.

Propeller Materials – The Taylor formulas used in the propeller strength calculation are important in assessing the designed working stress and the safe thickness of propeller blade at 0.2 R. The Classification Societes have provided information about propeller materials and their properties which a naval architect or propeller designer can use to design the required propeler. The following table is the requirements provided by Det Norske Veritas for propeller materials

Propeller Material

Minimum ultimate tensile stress

(kg/mm2)

Minimum Elongation

(%)

Cast steel

41

20

Special propeller bronze

45

20

Ni-Al-bronze

60

16

Nodular cast iron, heat treated

Not heat treated

40

15

3

Special cast iron

55

-

Ordinary cast iron

24

-

Gun metal

14

8

The above information is presented on page 285 of The Design of Marine Screw Propellers by T.P.O. Brien. Besides the minimum tensile stress, other propulsion parameters which we need are delivered horse power PD, blade number, RPM, propeller diameter, chord diameter ratio at 0.2 R, material density, and rake of propeller.

The average designed working stress and material density for marine screw propeller is provided below

Material

Density

Design Stress (lbs/inch2)



Single Screw

Twin Screws


(lb/ft3)

Reciprocating engines

Turbine or diesel electric

Reciprocating engines

Turbine or diesel electric

Manganese bronze

525

6000

6250

6250

6500

Nickel-Al-Bronze

480

6750

7000

7000

7250

Cast iron

450

2500

2600

2600

2700

With the development of research and technology in ship's propulsion new materials have been introduced for marine screw propeller. Students and practicing propeller designers must refer to the latest data provided by various classification societies.

Propeller Strength Calculation - For calculating the compressive stress, the following Taylor's formula is usually used:

Then, for calculating the tensile stress of the propeller, the following Taylor's formula should be used

ST = SC (0.666 + S4 t.2/c)

For further explanation of the application of above Taylor's formulas for propeller strength calculation, I suggest that you read T.P.O. Brien's book, The Design of Marine Screw Propellers. The above formulas cannot be used independently. They have to be used with a graph depicting the Strength Criteria of Propeller formulated by Taylor which is given on page 296 of the book.

After performing the strength calculation, the thickness of the propeller is safe for the operation of the ship at the designed speed.


Friday, June 5, 2009

Pitch diameter ratio of a propeller and engine performance of a ship

by Charles Roring in Manokwari of West Papua


The performance of marine propulsion machinery is much influenced by the propeller installed behind a ship or a boat's hull. If a propeller is well designed, the engine will work well without suffering overloading or underloading. Propeller designer knows that the selection of pitch diameter ratio plays a very important factor.



lub1_10_large.jpg


If the pitch diameter ratio is too high, the main engine will suffer overloading. When the ship is moving on its service speed at sea, the sign of overloading can be seen from its exhaust pipe. A lot of black smoke will be released by the engine whose propeller's pitch diameter ratio is too high. Further inspection to the inside parts of the engine shows that the valves are burnt. Sometimes we can find cracks in the cylinder heads.


If the propeller's pitch diameter ratio is too small, then a sure sign for this is that slow speed of the ship, and cavitation. Cavitation is usuall solved by increasing the developed blade area ratio and diameter of the propeller. We may feel that the fuel consumption is small but in reality the speed of the ship or boat is lower compared to other similar ships.



sbp.jpg


The solution for too low or too high propeller pitch diameter ratio is repitching. Repitching has to be done by experienced propeller manufacturer with the help of an experienced propeller designer or a naval architect who understands the resistance and propulsion system of ships or boats. Also read: Ship displacement calculation, and The propulsion system of submarine

Wednesday, June 3, 2009

Ship displacement calculation

Theoretically, the calculation of ship or boat displacement is executed using Simpson's, and, Tzebisheff, or Trapezoidal rules. The most common rule that naval architects use is the Simpson's rule. To understand the basic foundation of the application of Simpson's rule of integration in calculating the areas under curves, please watch the following Youtube video provided by Jennifer Ryan from SNAME.




Naval architects when designing a ship always have to calculate the displacement. The displacement of a ship is the volume of water it displaces when floating. It is the same as the underwater form of the ship and it represents the weight of the ship itself. As the Archimedes law applies, the weight of the displaced water is the same as the weight of the ship.
In the past, the calculation of the ship displacement is a tedious task. Now with the help of a spreadsheet computer application such as Microsoft Excel, or Lotus 123, it can be carried easily provided that all the ordinates of water-planes or sections are readily available from the lines plan drawings.
When I was studying Naval Architecture at the University of Pattimura Ambon in the Maluku islands of Indonesia, the calculation of the ship displacement was done using Simpson's rules.
Simpson integration formulas are well explained in such books as Statics and dynamics of the ship, Basic Ship Theory I. Naval architects use Simpson's rules to calculate the areas of curves of sectional areas, the underwater volume of a ship or boat and many other curve forms. They are not exact rules but they are quite accurate in calculating curve areas. The waterplanes curves which we need to calculate, to obtain the areas and then the volume, are represented by curves defined by Simpson's mathematical equations. This integration calculation is carried out on each waterplane and reintegrated against the draft common intervals to obtain the underwater volume of the ship.
If the calculation approach is done for each sections then we will get what we call as Bonjean curve. Bonjean curve is used not only by naval architects and ship builders but also by ship crews in assessing the displacement of a ship as well as the its center of buoyancy. The vertical and longitudinal center of buoyancy of a ship is needed to assess the metacentric height GM and the righting lever GZ which are two important parameters in the study of ship's stability. by Charles Roring in Manokwari of West Papua.
Also read: Waterplane area calculation using Simpson's rule

Tuesday, June 2, 2009

The propulsion system of submarine

The propulsion system of a conventional submarine is designed to meet two different operating condition. The first one is when the submarine is moving on the surface of the water and the second one when it is moving under water.





If it is on the surface the resistances it has to overcome are the same as those of conventional surface ships. There will be water (frictional and wave making resistances) and some air resistance. If the submarine is operating under deep water it will not face wave making resistance. Due to greater wetted surface, the submarine will have greater frictional resistance.


Every submarine has been designed and constructed to operate in three dimensions. It can move forward, diving, and manouvering to the right and left sides both on the surface or under water. To have such abilities, a submarine will need not only rudders for moving on horizontal planes but hydroplanes for controlling depth.


When moving on submerged condition, a submarine needs to be fitted with Air Independent Propulsion (AIP). The diesel-electric scheme is the most common form of the submarine propulsion system. Diesel engine will be used to propel the ship if it is on the surface but electrical drive supplied by batteries will be needed to propel the submarine when it moves underwater.


Often submarines need to operate in longer period of time underwater especially when encountering enemies. This operating condition needs highly efficient batteries that can provide greater and longer endurance. In the past, naval architect decided to equip such submarines with nuclear power plant.


Now other alternatives such as closed cycle diesel engines, fuel cells and Stirling engines are being developed. Actually, diesel engine is not an ideal choice for a submarine due to its toxic fumes which can harm the ship crews. Therefore, efforts to improve the efficiency and size of the batteries are now being carried out to increase the overall performance of the propelling system of the submarine. by Charles Roring in Manokwari of West Papua