Friday, May 8, 2009

Resistance and Propulsion Calculation in Ship Design

Naval architect when designing a ship has to perform resistance and propulsion calculation. This is done using statistical methods which are available from various standardized data released by a number of ship model basins around the world. While I was still a university student in the Department of Naval Architecture of Pattimura university of Ambon city, the resistance calculation was based on such methods as Holtrop Mennen, Taylor, Guldhammer and Yamagata. There are various power prediction methods available but naval architects have to determine or choose one of them based on the similarities in hull forms of the designed ship and the model data.  Power obtained from this calculation is called effective power. This is the power that is required to tow a ship or boat at certain speed. To determine the brake horse power of a marine engine, this effective power has to be added with a number of losses at the stern, shaft bearings and reduction gears. When all of these losses have been added, the brake power obtained from the calculation can be used to find a marine engine that is suitable for the designed ship. Most often the rated power of the marine engine that we need is not exactly the same as the brake power that we have calculated. To solve this problem, we just need to round the rate up to match it with the available engine on the market.
Please, watch the following Youtube video of how a resistance test is carried out using model in a towing tank.

For large ships, the power prediction method may not be enough. So, to better predict the resistance and propulsion characteristics of the designed ship, model test has to be carried out.
After performing the resistance calculation, the next step is adding the losses from the effective horse power obtained to predict the delivered horse power, the shaft horsepower and the brake horse power of the main engine of the designed ship. This is also known as forward calculation. The bhp obtained in this calculation might slightly be different from the available engine on the market as I have explained above. Therefore, after selecting the main engine, usually based on the brochure data from the marine diesel engine manufacturer, the propulsion calculation will be continued at afterward direction to the propelling device or the marine propeller.
Here, the brake horse power of the real engine will be reduced with the frictional losses along the shaft bearings and hull forms to obtain what is called effective horse power curve of the resistance calculation and the reduction of losses from the real main engine. When naval architects delineate these curves, they will be able to check the resistance and power of the designed ship.
I want to remind you here, that if you are designing a propeller, the rated power of the main propulsion machinery that is used for "afterward" calculation must not be the 100% MCR (Maximum Continuous Rating). The rate that naval architects or propeller designers must choose is the normal continuous rating which is around 80% to 85% of the MCR.  The easiest way to find it is by reading the engine's performance graph which is the work of the engine at the most efficient fuel consumption. This is chosen to prevent the engine from broken down. Naval architects must based the propeller design, on the engine brochure supplied by the manufacturer, on the most efficient rating of the curve on power - speed and specific fuel oil consumption of the marine diesel engine.
After determining the main engine and the propulsive efficiency of the designed ship, the next calculation is determining the QPC or Quasi Propulsive Coefficient which can be obtained by using Emerson formula. The design of the propeller can then be done if speed of advance of the ship VA and the value of Bp has been obtained.
I used Bp delta diagrams of Troost B4 series when designing the propeller of an Open Hatch Bulk Carrier in 2000. The ship was being constructed by PT PAL shipyard at that time.
The propeller designer must also perform cavitation calculation usually using Burril Cavitation Chart, and propeller blade strength calculation usually using D.W. Taylor method to ensure that the propeller is save and reliable in performing its duties during the operation of the ship.
The last step in the design of propeller is drawing. Generally, the drawing method used is Holst dated in 1924 as explained by Prof. W.P.A. van Lammeren in his book Resistance, Propulsion and Steering of Ships.
Resistance and propulsion calculation of a ship is now easier to be performed due to the availability of various software on the market. But it is advisable for naval architects and propeller designers to understand the whole process of manual design procedures which is the concept or philosophy of ship design that has supported the art and science of naval architecture for hundreds of years. by Charles Roring in Manokwari of West Papua