## TerminologyIn this section I'll try and define some of the main terms that you may come across in discussion of a ship design.
- The waterline at which a vessel floats at in
its Load
Waterline condition (as defined below under Full Load). It is measured from the point where the
water surface intersects the bow to where the water surface intersects
aft most point of the stern or transom. Weight
Categories
- The waterline at which a vessel floats at in its design condition.
Typically a design condition is stated as either Design Waterline,
Full Load, or some other typical operating
condition. Right now for design purposes I have assumed all
calculations (such as powering estimates and fuel consumption) are to
be based on a ship's Half Load, and as such
a vessel's Full Load Displacement and Design Waterline
are currently assumed to be the same. As I understand it,
however, in some navies things like powering and fuel consumption may
instead be determined at a half load or part load condition. As
such, eventually I may revise the calculations to allow designing ships
for other loading conditions.
Load Waterline
There are a number of coefficients and
other parameters that can be calculated for a vessel to help give an
simple defintion to its hull shape and form, or other
characteristics. These coefficients and parameters are often used
in early stage resistance and powering estimates, seakeeping estimates,
and stability calcs. Some of these more common coefficients and
parameters are defined below.Basic
Coefficients - - A
vessel's Block Coefficient or Block Coefficient
is one of the most basic coefficients that is used to help define and
categorize a vessel's hullform shape. A vessel's Cb is defined as the ratio of the volume contained within
hull below the waterline as )shown in Red in the figure below) compared to the volume of a block whose
length is equal to the vessel's Length between perpendiculars, and
whose width is equal to the vessel's waterline beam, and whose height
is equal to the vessel's mean hull draft (shown below as a Blue Box. Block
Coefficient is a handy term for relating the relative
fullness of a ship's underwater hullform and is sometimes used in
equations for estimating resistance, seakeeping and stability,
etc. Very full ships, like bulk carriers or large tankers can
have block coefficients approaching 0.90 whereas faster vessels,
including modern naval surface combatants can have Block
Coefficient as low as 0.45, or so (as shown in a Figure on the
Block
Coefficients page).Initial Hullform DefinitionIn accordnace with Archimedes Principle, if you assume a standard density for salt water of 35 cubic feet per Long Ton (or 1 cubic meter per 1.025 metric tonnes) then: = Displacement * Density / (Lbp * Bwl * Tm)*Cb*
However, when doing this calculation you need to make
sure that you are using Lbp, Bwl, and Tm, and not Loa, Boa, or Tnav
otherwise you can get incorrect results. or Midships Area Coefficient is another basic coefficient, however
whereas a ship's Cm is a three
dimensional term (based on length, beam, and draft) a ship's Block Coefficient is two dimensional, being related only to
a vessel's beam and mean draft. In general if you were
to cut a ship in half at its mid-length, as shown in the figure below then its Midships
Area Coefficient would be equal to the ratio of the cross sectional
area of the hull below the waterline at that section (shown in the
Figure below in Midships Area
CoefficientRed) divided by a
box whose width is equal to the vessel's waterline beam (Bwl) and whose
height is equal to the vessel's mean draft (Tm) (shown in the Figure below in Light Blue).
it is not uncommon for the hull bottom to have a greater slope to it
(as shown for section (C) in the Figure below) which is typically
described of as having a "high deadrise".Block Coefficients
are
used sort of interchangeably, though in reality they can be a little
different from each other. For our purposes, a ship's Maximum Sectional Area Coefficient is really of more importance
to us, as it is more handy for use in defining hull shape, etc.
As such that is what I intend to use in my calculations, though for
alot of the ships I have collected data on its not fully clear if what
they have reported is Maximum
Sectional Area Coefficient or Midships Area Coefficientand occassionally I know I will
inadvertantly refer to Maximum
Sectional Area Coefficient when I
really mean Midships Area Coefficient. As
I noted above though, for the most part for conventional type vessels
the two terms should be relatively similar, and hopefully it won't
cause too much confusion.Maximim Sectional Area Coefficient
- A vessel'sPrismatic Coefficient or Prismatic Coefficient is coefficient related to both a ship's Cp and its Block
Coefficient.
If you were to take a ship's Maximum Sectional Coefficient
and stretch it along the length of the hull, as shown in Maximum Sectional Coefficient in the Figure below Light Blue[Add Figure] you would have a prism whose
volume is equal to Lbp * Bwl * Tm * Cm. The vessel's Red) divided by the volume of this prism
(as shown below in ).
Mathematically, a vessel's Light Blue
is equal to its Prismatic Coefficientdivided by its Block Coefficient or in other words;Maximum
Sectional Coefficient**Cp = Cb/Cm**
page.Initial Hullform Definition
- Waterplane Area Coefficient
- Displacement Length Ratio
- Waterplane Inertia Coefficient
- Vertical Prismatic CoefficientOther Hullform Parameters - Half Entrance Angle - Longitudinal Center of Buoyancy
Location - Wetted Surface Area
- Transom Area Coefficient
- Transom Beam Coefficient
- Transom Draft Coefficient - Cubic Number |