Traveling with ship

by Charles Roring in Manokwari of West Papua Indonesia

Traveling with ships or boats has been a good choice for people living near the sea for hundreds of years. Ships help people traveling from one island to another.

When passenger airplanes were introduced some one hundred years ago, the number of people traveling by ships began to decrease. In recent years, the increasing number of airplane crash makes people worried about their safety. We can see that many airline companies release reports which say that they are facing bankruptcy. The real reason for the decreasing number of passengers, which they say, is the economic downturn. However, honestly we cannot ignore the psychological effects the news about plane crash brings to TV audiences. Although most of the accidents are caused by bad weather, some people who have watched such bad news are now considering of traveling by other mode of transportations.

In recent years, travelers are now considering ships as safe mode of transportation compared to airplanes. Traveling with passenger ships can be a boring voyage especially if the journey takes more than one day.

Therefore, ship designers or naval architects must design passenger ships as comfortable and enjoyable as possible. Modern passenger ships now are identical with luxurious hotels. They have such facilities as swimming pools, sun-deck, restaurants, movies, library, fitness centers, internet café, gambling hall, shops and even conference rooms. These facilities are provided to help the passengers feel relax so that they will be able to enjoy the trip rather than feel bored or anxious when looking at the dark color of the deep sea.

Besides, providing various facilities similar to luxurious hotels, passenger ships have to be designed with the highest safety standard stipulated by classification societies such as LR, GL, NK, ABS, and DNV, to prevent accidents. Modern navigational equipment as well as inflatable rafts and boats are installed in passenger ships to protect the passengers in case if an accident happens.

Today's modern technology helps naval architects and shipyard owners to design and build ships that are fully safe for sea transportation. It is important to note that the number of ship accidents is quite low compared to plane crash in recent years.

We will see that with improved speed, and better safety measures, passenger ship will be a good alternative for traveling in short distances which only takes one or two days to reach a destination. Also read: Propeller Rake and Blade Thickness Diameter Ratio; Controllable Pitch Propellers; Material and Strength of Marine Screw Propeller

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 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.

Propeller Mean Pitch Calculation

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: sum of xp/  sum of x =4.2075/ 5.4 = 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.So, the mean pitch ratio (P/D)_mean of the designed propeller is 0.779

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

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.

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.

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 P_D; and speed of advance V_A. 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:

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

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 P_D, 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

S_T = S_C (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.

Cost Estimation of House Construction

Suppose that you are going to have your new house built by a home construction company, and you have just received from an architect or a house designer a number of drawings of your new house. These drawings are floor plan, general arrangement of the house with front and side views, detail drawings of foundation, windows and doors, ceiling and roof.

The next step is consulting the cost of construction with the architect or a home builder. If you appoint an architect to do the design, the cost estimation and the construction of the new house, you don't have to find a home construction company by yourself. All you have to do is pay the cost and receive the keys of your newly built house.

If you want to minimize the cost, after receiving the design drawings of your house, you will consult them with a home builder. He will make the cost estimation for the construction of your house. In general the cost estimation is devided into two main subjects. The cost of materials and the cost of building the house.

You might probably be confused with all these terms. To help you understand them, I will give you the scope and sequence of calculating the house construction. The followings are types of works which have to be performed for building a conventional house.

1. Preparation works

2. Foundation and concrete works

3. Wall works

4. Floor and ceramic works

5. Roof works

6. Roof of the garage works

7. Ceiling works

8. Windows and door works

9. Sanitair works

10. Clean water and dirty water works

11. Electrical installation works

12. Painting works

13. Cleaning works

There are many points which a cost estimator needs to calculate for each number or type of works. For instance, for preparatory works, the works which a home builder has to perform are the clearing of land, the measurement and installation of marking or profile boards, the excavation for foundation works, earth removing.

After calculating the volumes of each of the works of the above list, they have to be multiplied with the unit cost of material to obtain the cost of material. Then the amount of cost for each type of work have to be summed to obtain the total sum of the cost of the material of the new house.

Similarly, the volume of works for each type of works of the above list have to be multiplied with the man-hour factors to obtain the labor cost for the construction of the house.

The price of house material is different from one place or country to another. This also applies to the labor cost.

If you are serious about building your own house the cheaper way, I suggest that you read some books related to this subject. One of the titles which I recommend is
Fundamental Building Technology
written by Andrew Charlett. Also read: How to add column and beam to a house using Archicad and Wooden house of Minahasa an earthquake resistant construction

by Charles Roring in Manokwari of West Papua Indonesia