A vessel's principal dimensions are illustrated in Figure 2.1. The vessel's overall
length is designated LOA. The length along its waterline, which may vary
slightly with load condition, is designated LWL. The length between perpendiculars
(LBP) is the length between the forward and aft vertical structure of
the ship's hull where it intersects the designed waterline (DWL). The DWL is
the nominal waterline at which a vessel is designed to operate, usually the full
load condition. For all practical port design purposes, the LBP and LWL are
very nearly the same, and are sometimes used interchangeably. Many contemporary
vessels have bulbous bows that project forward at the forward perpendicular
below the waterline, in some cases for a considerable distance; so caution
should be exercised in allowing for such projections.
vessel's maximum width, which usually occurs at or near the vessel's transverse
centerline or midships section, designated by the symbol ~. The depth of a
vessel's hull (Ds ) usually is measured at midships from the bottom of the keel
to the top of the main deck (strength deck). The draft (D) is the distance from
the vessel's waterline to the bottom of its keel or baseline. ,.,1
Many vessels draw more water aft than forward; so one must distinguish
between draft forward, draft aft, and mean draft. For hydrostatic calculations
the mean draft must be used, and for navigation requirements the maximum
draft must be known. All U.S. vessels of 150 tons (GRT, defined below) and
over are given a load line assignment by the American Bureau of''Shipping
(ABS) as required by the U.S. Coast Guard (USCG). The load line is indicated
by a plimsol mark displayed on the side of the hull, which indicates various
maximum safe drafts which a vessel can be loaded for various ocean areas
and seasons. A vessel's draft will increase approximately 2% to 3% in fresh
water compared to seawater. Trim refers to the difference in drafts fore and aft,
and list refers to a difference in drafts side to side.
When a tanker or cargo ship has discharged its cargo, it will often take on
ballast by flooding internal tanks with seawater in order to reduce windage and
alter the vessel's trim. Maximum ballast capacities of most vessels are on the
order of one-half of their full load displacement, and a tanker "in ballast" will
ordinarily be at from 30% to 50% of its full load displacement, depending upon
weather conditions. Therefore, a vessel usually will be somewhere between
lightship weight (LWT) and full load condition for most berthing and mooring
situations. For dry-docking, however, there usually is an attempt to get the
vessel into LWT condition. Freeboard (FB) refers to the height of the vessel's
deck above the waterline; that is, FB = D, - D.
A vessel's total weight, termed its displacement (A), varies with its load
condition from fully loaded, which is the displacement or displacement tonnage
(DT) (the figure usually given, as in the tables that follow in this text), to the
weight at the lightship condition (LWT), which is the weight of the empty
....essel, sometimes including minimal stores and partial fuel. Vessel displacement
normally is given in long tons (l.t.) of 2240 pounds, each equivalent to
approximately 35 cubic feet (c. f.) of seawater (36 c.f. fresh water). As a vessel
~isplaces ~ volume of water equal to its own weight (Archimedes principle), it
1S con~eruent to use 35 c.f./l.t. for converting from volume to weight. A vessel's
displacement in cubic feet sometimes is designated by the inverted delta
V. Note that the metric ton (rn.t.), which is used for most foreign vessels, is
very nearly equal to a long ton (1 m.t. = 2205 pounds).
The sto~age factor refers to the relative density of the vessel's cargo, usually
expre.ssed m c. f. /l.!': Tankers, for example, carry relatively high-density cargoes
10 the range of ~9 to 46 c.t. /l.t. and float low in the water, with the ship's
draft to hull depth ~tIO (D/ Ds) usually around 0.75. Containerships and Ro / Ro
vess~ls, by companson, carry relatively low-density cargoes with storage factors
.m exc~ss of 100 c.f. /l.t. and D/ o, around 0.5, depending upon other
specific design characteristics. Merchant vessels usually are referred to in terms
of their cargo-carrying capacity, or dead weight (DWT), which is essentially
the vessel's loaded .oT minus its LWT. Note that the actual cargo capacity
equals the DWT nunus the weight of the ship's crew, stores, and fuel. For
purposes. of figuring port duties, shipping costs, and so on, registered tons or
~onnage 1S empl~yed. A registered ton is figured as 100 c.f. (2.83 c.m.) of
internalspace, with a gross registered ton (GRT) equal to the ship's total internal
volume and a net registered ton (NRT) being the total internal volume available
for cargo (i.e., GRT minus living, machinery, and fuel storage spaces,
etc.). For most contemporary tankers and cargo ships, the LWT is on the order
~f 25% to.35 % of the loaded displacement. The facility's designer is primarily
interested m the vessel's actual displacement, and often needs to know both the
LWT and the DT.
A.cc~rding to. reference (I), the following approximations can be applied for
preliminary design purposes when specific vessel data are lacking in order to
obtain vessel displacement tonnage, DT, from GRT or DWT: '
General cargo ships: DT = 2.0 x GRT, or DT = 1.4 to 1.6 x DWT.
Container ships: DT = 1.4 x DWT.
Bulk carriers (and tankers): DT = 1.2 to 1.3 x DW1'.
Passenger liners: DT = 1.1 GRT.
Fishing vessels:
Small: DT = 2.0 to 2.5 x GRT.
Large: DT = 1.5 to 2.0 x GRT.
The lower end of the ranges given above generally applies to the smaller vessels
of the given type. As a general nile, the ratio of DT /DWT decreases with
increasing vesselsize.4
The vessel form and dimensions are of obvious importance to port engineers,
and are discussed further in following sections of this chapter. Knowledge of a
vessel's hull structure is important in fender design applications and dry-dock
ing. Other vessel characteristics of interest are speed and maneuverability, which
are addressed in the general literature on port engineering, navigation channel
layout and design, and vessel projected areas to wind and currents.
