Long stroke: 3.2 - 4.0 Super long stroke:4.0 -4.7 Ultra long stroke :>4.7
Advantages of super long stroke:
Power to weight ratio is more.
Reduced fouling, smoke, NOx
Increased thermal efficiency: Simple
geometric relationships show that an engine cylinder with longer
stroke-to-bore ratio will have a smaller surface area exposed to the
combustion chamber gasses compared to a cylinder with shorter
stroke-to-bore ratio. The smaller area leads directly to reduced
in-cylinder heat transfer, increased energy transfer to the crankshaft
and, therefore, higher efficiency.
Low quality fuel can be burnt as more time is available for fuel combustion.
Slow rpm is possible so large diameter propeller can be used and hence propeller efficiency increases.
More time for heat dissipation to cooling water.
Better scavenging: Scavenging is the process in which the exhaust products in the cylinder are replaced by fresh air. As the stroke-to-bore ratio increases, so fresh air has to travel larger distance between the intake ports at one end of the cylinder and the exhaust ports at the other end. This increased distance results in higher scavenging efficiency and, as a result, lower pumping work because less fresh air is lost via charge short circuiting.
Higher compression ratio can be achieved and hence reduced exhaust temperature
Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole is lesser in size than the corresponding circular hole. More over it is prime concern to have a smoothed generous radius at the corners to eliminate stress concentration. Hence other geometrical shapes like rectangle and square are ruled out. To compensate for the loss of material in the shell due to opening, a doubler ring has to be provided around the opening. The thickness of the ring depends on the axis length along the dirrection in which the stresses are maximum and the thickness of the shell. It is important to align the minor axis along the length of the vessel, as the stress in this direction is maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t= thickness of the shell plating Circumferential stress: Pd/4t More over a considerable material and weight saving is achieved as minor is along the direction of maximum stress.
It is a two dimensional drawing of a three dimensional surface of the ship’s hull form. This plan is very useful for the following information:It is used for marking the location of a hull Damage on this plan by identifying the strake number , letter and frame number so that the exact location of the damage and also suggested repairs are marked in a localised copy. The shell expansion can be used for finding areas of painting surfaces such as topside, boot topping and bottom areas by applying Simpsons rules directly. In the shell expansion the vertical scale used is different from the horizontal scale and a suitable adjustment has to be made when calculating areas. This becomes useful in solving disputes concerning areas of preparation and painting. It gives information on the thickness of the original strake which is indicated by the number in the circle shown in the strake. The quality of steel used is also shown by letters A,B,D E and AH, BH,DH, EH.