Tuesday, May 12, 2009
Controllable Pitch and Fixed Pitch Propeller
I remembered time in 1999 when I was designing a propeller for a large 45,000 DWT Open Hatch Bulk Carrier. At the time, I had to choose between CP Propeller and FP Propeller as the propelling device for the ship. I was not sure whether the ship will need high maneuverability during her service especially at narrow straight, ports and bays.
Naval architect and propeller designer often have to compromise all the design constrains to obtain the optimum dimensions of the ship and the propeller so that the required speed can be met and the fuel efficiency can be reached.
After reading a large number of books and publication on resistance, propulsion and steering of ships, I finally knew that CPP or Controllable Pitch Propeller has boss diameter which was larger than the conventional fixed one.
In addition, the CP-propeller was mostly used for ships such as passenger ships that need high maneuverability. In theory, if the propeller has larger boss diameter, its efficiency will be lower.
Now, we come to the discussion of propeller efficiency. There are many factors that affect the overall performance of a propeller operating under the sea water behind a ship. To obtain higher efficiency in CP-Propeller, experts in hydrodynamics and cavitation tunnels around the world develop what is now known as highly skewed propeller. This kind of propeller has more skew on its blades. Computer calculation and model test results have shown that the CP-Propeller with highly skewed blades have better efficiency and less vibration.
Just because the CP-Propeller can now be equipped with highly skew blades does not mean that the problem related to overall propulsion efficiency is solved. There are still many conflicting factors which naval architect, marine engineers and of course propeller designer must consider.
For operational point of view, CP-Propeller has more complicated mechanism than the traditional FP-Propeller. It means that if the propeller face mechanical failures during its service in the middle of the sea, it will not be easy to repair it, especially if they happen inside the boss.
Based on this consideration, I chose the fixed pitch propeller as the propelling device for the OHBC ship.
The standard model which I used was the Troost B-4 Series from Wageningen, the Netherlands. I didn't use highly skew blades because of the lack of data of such blades at my hand. Highly skewed propeller is quite a new blade type which was invented just several decades ago.
Years after my first propeller design work, I finally found that the highly skewed propeller was designed using special computer program that applies the surface panel method developed from lifting line theory. I wish I could understand this innovative theory.
As propeller designer, I need around two weeks to do the resistance calculation, propulsion calculation and propeller design calculation and additional one or two days for drawing the propeller of a ship. Now, with the availability of computer software, a propeller designer can do his or her job just within one or two days.
But for larger ships, naval architects, marine engineers, and propeller designers must perform model tests in towing tank and cavitation tunnel before finally delivering the final design of the ship and the propeller to the shipyard. The power of a newly designed ship has to be accurately predicted before the construction to ensure that the ship will have the required speed during her service at sea. In addition, the ship also needs to have high propulsion efficiency to ensure that fuel oil consumption will be as low as possible. I believe that fixed pitch propeller will still be used in the world of shipbuilding but the controllable pitch propeller is increasingly being used in many ships today due to the improvements in blade design, material and the availability of computer technology. by Charles Roring in Manokwari of West Papua Indonesia