Beam
- The breadth of the ship at the broadest point is called
the beam. Molded beam is measured amidships or at the
widest section from the inside surface of the shell plating.
Maximum beam or extreme breadth is the breadth at the
widest part of the ship, and is equal to the molded breadth
plus twice the plating thickness plus the width of fenders,
overhanging decks, or other solid projections.
Displacement
- The displacement is the weight of the water of the displaced
volume of the ship; for static equilibrium it is the same
as the weight of the ship and all cargo on board. Therefore,
displacement is directly related to displacement volume
and it can be found by multiplying the volume with the
specific gravity of the water in any set of consistent
units. For example if the volume is in cubic feet, we
may divide it by 35 to get the displacement in long tons
in seawater, or by 36 in fresh water.
Draft
- The vertical distance between the waterline and the
deepest part of the ship at any point along the length
is the draft. Drafts are usually measured to the keel
and are given as draft forward (Tf ), draft aft (Ta )
and mean draft (T or Tm ). A ship's forward and after
draft marks are seldom at the perpendiculars and mean
draft is not necessarily amidships; the slight errors
introduced by using drafts at these points can be discounted
if trim is not extreme. Molded drafts are measured from
the molded baseline, while keel drafts are measured from
a horizontal line though the lowest point on the bottom
of the keel extended to intersect the forward and after
perpendiculars. Navigational or extreme drafts indicate
the extreme depth of sonar domes, propellers, pit swords,
or other appendages which extend below the keel, and are
therefore not used to calculate hydrostatic properties.
Draft scales for keel drafts are usually placed on both
sides of the ship at each end as near as practical to
the respective perpendiculars. The external draft marks
are generally Arabic numerals, with height and spacing
arranged so that the vertical projection on the vessel
of the numeral heights and vertical spacing between numerals
are both six inches. The draft figures are placed so that
the bottom of the figure indicates the keel draft. Drafts
can thus be read to the nearest quarter-foot (3 inches)
in relatively calm waters. This measurement can vary greatly
depending on the amount of fuel, weapons, stores, etc.
on board. The salinity of the water can also affect the
ships buoyancy and therefore her draft. The official figures
for draft are derived from the Designer's Water Line,
which is determined during the design process. This number
does not take into account the distance of any protrusions
below the keel, such as the sonar dome. Navigational Draft
is the term to define the shallowest water in which the
ship can operate. The Navigational Draft has been published
in several figures anywhere between 25' and 30'.
LBP
(Length Between Perpendiculars)
- The length of a vessel along the waterline from the
forward surface of the stem, or main bow perpendicular
member, to the after surface of the sternpost, or main
stern perpendicular member. This was believed to give
a reasonable idea of the ship's carrying capacity, as
it excluded the small, often unusable volume contained
in her overhanging ends. On some types of vessels this
is, for all practical purposes, a waterline measurement.
In a ship with raked stems, naturally this length changes
as the draught of the ship changes, therefore it is measured
from a defined loaded condition.
Maximum
Speed
- The maximum published speeds
for ships have always been a point of contention. Many
sailors are happy to boast that their ship was the fastest
in the fleet or in their class, but there are many factors
that influence the maximum achievable speed of any ship.
A lightly loaded ship (without weapons, stores, and full
fuel loads) in shallow water will achieve greater speeds
than that same ship fully loaded in deep water. This is
why the maximum stated speeds of many ships were achieved
during builder's trials (often in a bay or on a river).
The only meaningful measure of a ship’s maximum
speed (determined by the designers) is under normal, fully
loaded, deep water conditions. For the Farragut class,
that number is 33.5 knots.
Here are some of
the major factors that influence speed:
Load
– This actually refers to 2 types of loads. The
first is weapons, fuel, and stores. Without these loads,
the ship will have a lighter displacement and therefore
ride higher in the water. The higher the ship rides,
the less surface area of the hull has contact with the
water, providing lower resistance and drag. The second
type of load is that from the ship’s systems.
Basic physics tells us that you can’t get something
from nothing, so any ship’s system (pumps, generators,
etc.) takes power away from the main engines.
Hull
- The single largest factor that influences speed is
the hull itself. The calculations are quite complex,
but there comes a point where the SHP that is required
to overcome wave making resistance increases exponentially,
requiring huge increases in power with only very small
increases in speed. The propagation speed of deepwater
waves is proportional to the wavelength of the generated
waves, and the wavelength of a boat's wake is based
on its waterline length — so there is a direct
relationship between the waterline length (and thus
wave propagation speed) and the rate at which drag increases.
A simple way of considering wave-making resistance is
to look at the hull in relation to its wake. At speeds
lower than the wave propagation speed, the wave rapidly
dissipates to the sides. As the hull approaches the
wave propagation speed, however, the wake at the bow
begins to build up faster than it can dissipate, and
so it grows in amplitude. Since the water is not able
to "get out of the way of the hull fast enough",
the hull, in essence, has to climb over or push through
the bow wave. This results in an exponential increase
in resistance with increasing speed.
Propellers
– The propellers are the device that transfers
the energy from the engines to the water. At low speeds,
the propeller can efficiently transfer the energy with
minimum turbulence, but as speeds increase, so does
that turbulence. This turbulence, called cavitation,
is characterized by the formation of air pockets at
the trailing edge of the propeller. Since the next blade
of the propeller has to go through that air pocket,
much of the energy is lost there instead of being imparted
to the water. To sum it up, the efficiency of the propeller
decreases with the speed of rotation.
SHP
(Shaft Horse Power) - Shaft horsepower
is the power delivered to the propeller shaft of a ship.