The maximum speed of any vessel is described as the point at which the energy being put into the water by the engines equals the energy lost to the environment by friction or ‘Drag’. The only way that speed can be increased at this point is to add more power, or to lose less power through drag, in other words to make the vessel more efficient at moving the water out of its way.
Drag is basically a measure of the hulls resistance to being driven through the water and is essentially comprised of two elements, Wave Making Resistance and Wetted Area. Planing hulls as we have seen are able to reduce their wetted area and wave making resistance by rising onto the surface of the water. Displacement hulls however are beholden to both of these forces, although wetted surface area is not a large factor at anything more than idling speeds.
Of far greater importance for a displacement craft at speeds from 6 knots to about 30 knots is the wave making resistance. Unlike a planing hull, the displacement hull is essentially stuck in the water rather than skimming on top of the surface and so hull shape and the way in which the hull interacts with its wake is of great interest.
As a displacement vessel passes through the water, it must push the water out of the way. In doing this, energy is lost due to the creation of waves. The quicker the water is moved the bigger the waves will be and the more energy is used in creating them. Larger waves are created by greater speeds and also by blunt bow shapes. A more ‘bluff’ bow needs to move the water more rapidly than a finer one, and although ultimately the same amount of water is moved, because the fine bow moves the water more gradually, the resistance is less.
In general a hull will experience high water pressure at the bow, where the water is being forced away from the hull, creating a wave, low pressure at or around amidships where the surface level is lower due to some of the water having been deflected away by the bow, and high pressure at the stern where a typical displacement hull is narrowing and so the water is pulled back in again thus creating another wave. The hull when viewed from the side essentially appears to be riding on a bow and stern wave, with a trough in the middle. At this point the vessel is said to be moving at its ‘hull speed’.
Wave making resistance is reliant on two factors, the length to beam ratio (the primary factor) and the beam to draught ratio. A vessel which is very long will be faster than one which is very short if both have the same beam at the waterline. Fundamentally, long thin things are easier to push through the water than short fat ones. Those of you who have experience of kayaking will know that the longer general purpose boats are much faster for the same number of paddle strokes than the short white-water designs.
As we have seen, a displacement vessel when travelling at its hull speed can be seen to be moving along in effect trapped between two waves. The longer a ship is, the longer the distance between these waves, known in Physics as the ‘Wavelength’. Since the speed of waves in deep water is proportional to the square root of their wavelength, and the wavelength of a vessels wake is based on its waterline length, in a simplistic sense, the longer the vessel the faster it is able to travel.
It has also been proven that deepening the draught of a ship reduces resistance. These attributes were perhaps best proven by the Japanese with their Naval ships towards the end of the Second World War, which became very long and very deep in order to gain the edge in terms of performance.
It is however not just the overall length and maximum draught of a particular hull which is important, but also the rate at which these factors change along the vessels length, or to look at it another way, the rate of change of the underwater cross sectional area of the hull.