Well it has been a long time, more than a fortnight since the last post. Sorry for the delay, but was held up with other stuff. And now that things are fine, let us get to the basic ship terminologies that we shall frequently use. The below diagram should give you an idea of the major dimensions that are involved throughout the life of a ship.
Talking about ship there are many interesting details with regard to its dimensions. Let us have a look at them one by one.
LOA i.e Length Overall is the extreme length that is usually specified as the length when you look at the specs for a ship. The LOA typically comes into play when considering the Green Water effects, internal arrangements etc. LOA from a hydrodynamic (dynamic fluid forces i.e waves) perspective is not much of a concern.
LWL i.e length on the waterline is the length that is hydrodynamically significant since this length influences the resistance as well as the sea-keeping properties for the ship. Sea-keeping means how the ship keeps itself (handles itself) in the sea.
B i.e Beam is the extreme width of the ship which is specified in the specifications. The value of B typically has to do only with the layout of the ship especially with helipad considerations. It is not much significant when we consider the ship from a hydrodynamic perspective.
BWL i.e Beam on the waterline is the extreme width on the waterline. BWL along with LWL influences the resistance offered by the ship. It also places restrictions on the engine dimensions and configurations that can be incorporated after considering the requisite classification society rules. The BWL also influences the sea-keeping. Infact it is the ratio of LWL/BWL that matters.
T i.e draft or draught is the underwater depth of the ship measured from the lowest point on the ship. It is usually the keel unless the bulbous bow extends a bit downward. The draft values at the bow, stern and amidships could be same or different based on a lot of factors, which we shall look at eventually.
H i.e height of the ship is the height of the bow tip from the ship’s lowest point (again usually the keel). The difference of H and T is called freeboard (F). Freeboard considerations majorly deal with the green water loading considerations so that the deck stays dry, for most higher sea states, although it cannot be guaranteed especially when freak waves are encountered.
Having talked about some of the ship major dimensions, there are some ship form coefficients that helps a naval architect to commence the ship’s design as well as predict its performance at sea. So let us take a look at these form coefficients.
Block Coefficient (CB)
Consider the water plane dimensions LWL and BWL. They form a rectangle on the waterplane. Now you extrude this rectangle by draft T under the water. This volume of cuboid that you get is the maximum possible underwater volume for the LWL, BWL and T you have taken.
Now you place your ship’s actual underwater portion in this cuboid. The ratio of the actual underwater volume to the volume of this bounding cuboid is called as the block coefficient (CB).
When designing a new ship, selection of block coefficient is of paramount importance as it will help one fix the underwater volume for the ship. So once you are given a displacement requirement, you can find the required underwater volume simply by dividing displacement by the salt water density.
(Usually taken as ρ = 1.025 T/m3)
Once you have got your V fixed, you can now choose appropriate LWL, BWL and T by trying out various combinations. Is it that straightforward? Not really.
There are some more major coefficients to be considered.
Water-plane Area Coefficient (CW)
It is the ratio of the area on the water-plane that is occupied by the ship to the maximum possible LWL*BWL.
Mid-ship Area Coefficient (CM)
Simply said it is the ratio of the cross sectional area of the ship underwater portion at the mid-ship to the maximum possible (BWL*T).
This coefficient influences the form of the hull amidships as to what extent the bilge would be rounded or the extent of chine if the hull is hard-chined.
Prismatic Coefficient (CP)
There are 2 types of prismatic coefficient.
Vertical Prismatic Coefficient (CP)V
Consider the water-plane area of the ship and extrude it through draft T. Now take the ratio of actual underwater volume to the above volume mentioned.
This ratio is called the Vertical Prismatic Coefficient (CP)V
Horizontal Prismatic Coefficient (CP)H
Consider the midship-area of the ship and extrude it through LWL to get a volume. Now take the ratio of the actual underwater volume to the above volume mentioned.
This ratio is called the Horizontal Prismatic Coefficient (CP)H
The prismatic coefficient along with the block coefficient gives an idea about the hull, whether it is slender (corvettes, destroyers etc) or is it fuller (tankers, bulk carriers etc).
Selection of these coefficients give an overall nature of the hull form, but to develop a hull form such as one shown above involves many other parameters such as the half angle of entrance, half angle of exit, rise of floor etc as shown below.
The half angle of entrance deals with the wave making characteristics of the hull whereas the half angle of exit and rise of floor deals with the flow at the stern. The half angle of entrance is kept low when it comes to destroyers, corvettes, frigates etc. In-fact the latest addition into such design consideration is the wave piercing knife like bow of the USS Zumwalt.
Having talked about the basic ship dimensions, we shall in the next post take a look at the stability of the ship when it is floating.
Next post : Ship Stability