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.