- Power to Weight ratio is the ratio of horsepower produced to the weight of the engine itself.
- With high-speed or high-performance vessels, it is the most important criterion to use when comparing engines between two different makers since weight on these vessels is often critical, with vessel speed being the most important aspect of many military missions.
- Higher weight always equals higher displacement equals lower speed On commercial vessels, displacement is critical due to cost.
- Higher displacement requires more fuel to move the vessel.
- More fuel means higher cost of operation.
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.