Boat and Ship Propellers

In general the working principles of boat propellers and ship propellers are the same. They convert the rotational movements of the blades into thrust. Boat propellers have sizes between 30 centimeters to around one meter in diameter depending on the size of the boat and its stern draft whereas for ship's propeller, a open hatch bulk carrier can have a diameter of  up to seven meters.

Because of its large diameter, the rpm of a ship's propeller will be much lower compared to the rpm of a boat's propeller. In addition, the position of the boat propeller is located near the sea surface, this is the reason its efficiency is lower. Propeller that is closer to water surface can easily attract air into its rotational motion. Its high rpm also makes the boat's propeller always works in cavitation condition.

For ship's propeller that is operating in fully loaded condition, i.e. at its summer load water line, the blades will receive huge loads or pressure. From this operating condition, the propeller material used for a small boat is usually aluminum. Propellers for large ships are manufactured  from such materials as Ni-Al-Bronze; or Cu-Ni-Al. Propeller designer knows that the design of a marine propeller will always be done for service speed of the ship. It is the speed that main engine usually runs at its CSR. CSR stands for Continues Service Rating. There is also similar other term called NCR or Normal Continues Rating. When designing a marine screw propeller, I usually use the Bp delta charts of B-Series propellers from Wageningen.

Well, I will discuss more about marine propellers and their design characteristics in my later posts.

As an illustration of how a large ship's propeller will look like, please watch the following video of how divers polish its blades under water.

Efficient propulsion system in ships and boats

There are many factors that influence the fuel consumption of ships or boats. Naval architects know that the interaction between the propeller, the hull and the main engine greatly influences the fuel consumption of ships whether they are big or small. So, in order to obtain ships that have optimum propulsion efficiency, naval architects must design the ships with the propulsion system that are fully integrated.

This is not an easy thing to do because every propulsion unit is contradicting one to another. For example an efficient hull form must have lower block coefficient with streamlined shape that in the contrary reduce the carrying capacity of the ships or boats that are being designed. In addition, the use of diesel engine is seen as a highly efficient main engine that is now used in many ships but we must not forget that diesel engines have vibrations and exhaust gases that will not be comfortable for crews and passengers.

Besides the hull form and the main engine, the propeller plays very important role in the design of the propulsion system of a ship. Marine propeller is usually located under water at the stern of a ship or boat. If the propeller is well designed and manufactured by experts in ship's propulsion, the fuel and financial savings will be greatly significant throughout the operational years of the ships or boats. by Charles Roring. Also read: How to estimate ship's length for a new design

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

Propeller and ship’s hull

Ship propeller and boat propeller is still considered as the most economical propelling devices for ships and boats. Other propelling devices such as paddle wheel, magnetohydrodynamic propulsion system have efficiencies that are lower than screw propeller.

Propeller designer and naval architect must understand how propeller, ship's hull, rudder and main engine interact from one to another.

Ship's Hull - The hull form of a ship influences the flow of water from the forward end of the ship to the stern where the propeller is mostly installed. The finer the hull form the faster will be the ship's speed in water. The fullness of ship's hull form is reflected by its block coefficient or C_B. Ships with higher block coefficient (0.70 to 0.98) will tend to be slow in the water. Even if the block coefficient is small (0.50-0.70), the speed of ship moving in the water can be slower due to the fouling of hull surface. Hull fouling is mostly caused by barnacles, and tube worms. If the fouling is worse, it can increase the frictional resistance of ship up to 40%.

Another way which naval architects do to reduce the resistance of ship thus increasing the speed or improving the propulsive efficiency is by adding bulbous bow to the ship. By doing this, the naval architect and propeller designer can reduce the wave making resistance of the designed ship.

Ship, hull and rudder are integral propulsion system of ships. But they will not work without a main engine inside the hull. Today, marine diesel engine is used as the prime mover of ships. The use of steam and gas turbines are only used in small number of ships due to the complexity of the design although they relatively have low vibration.

Marine diesel engine for large vessels is especially designed to have low RPM. Theoretically, the lower the RPM of a propeller, the better will be its efficiency. Modern propellers are designed and manufactured to have high skew to minimize the vibration and maximize the efficiency.

In the last few decades, the development of multi-hull ships has led to the building of catamaran, trimaran and SWATH (small water plane area twin hull) ships. These new hull types are mostly used on passenger ship, ferries, fast patrol boats and ships or boats that need high speed. by Charles Roring in Manokwari of West Papua - Indonesia. Also read: Propeller design methods; Propeller Strength Calculation; Cavitation of Marine Propeller

Propeller design methods

There are two methods available for the design of marine screw propeller (both ship propeller and boat propeller) at this time. They are hydrodynamics and aerodynamics theories; and the test results of systematic propeller models carried out by towing tanks and cavitation tunnels.

The hydrodynamics and aerodynamics theories - The propulsion phenomenon of ship's or boat's propeller is still difficult to be explained. The existing theories are divided into two main theories, i.e.:

• Theory of momentum - this is based on the assumption that the propeller acts as the accelerator of water that passes through it. The resulting reaction creates thrust force which propels a ship.

• Propeller blade element theory - this is based on the assumption that each propeller blade consists of a large number of concentric strips relative to the boss of the propeller. Every strip acts as a small aerofoil which has resultant velocity as the result of the combination of axial and rotational speeds. These speeds react against the aerofoil and generate elementary lifting forces and drag forces on the blade surface. If they are integrated throughout the whole area of propeller blade, they will provide thrust force that propels the ship, and also torque to the propeller.

In practice, most naval architects and propeller designers use the second design method, i.e. the test results of systematic propeller models. To enable ship and propeller designers do their jobs, a number of research institutions have made propeller model standards whose characteristics are presented in a journal under the title "A New Usable Propeller Series," on page 174, vol. 26 No. 3, July 1989 of Marine Technology tabulated by S.B. Denny et al.

The propeller models which I usually use are Troost/ Wageningen B-Series. Before using the models, a naval architect or a propeller designer must understand the theories of resistance and propulsion of ships. This subject is extensively explained in such books as Resistance and Propulsion of Ships - Sv. Aa Harvald, Resistance; Propulsion and Steering of Ships - Prof. W.P.A. van Lammeren, Troost L, Koning J.G.; The Design of Marine Screw Propellers - T.P.O'Brien.

When designing the propellers of ships, I usually use or read the latter two books as my main references in deciding the parameters of ship resistance and propulsion. The books may not be available on the market. If you are serious about learning this subject, I suggest that you read similar books published by Elsevier publisher. By Charles Roring in Manokwari of West Papua - Indonesia. Also read: Propeller Strength Calculation and Cavitation of Marine Propeller