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Ships Typed According to Means of Physical Support

The mode of physical support by which vessels can be categorized assumes that the vessel is

operating under designed conditions. Ships are designed to operate above, on, or below the

surface of the sea, so the air-sea interface will be used as the reference datum. Because the

nature of the physical environment is quite different for the three regions just mentioned,

the physical characteristics of ships designed to operate in those regions can be diverse.

Aerostatic Support

There are two categories of vessels that are supported above the surface of the sea on a

self- induced

cushion

of

air.

These

relatively

lightweight

vehicles

are

capable

of

high

speeds,

since air resistance is considerably less than water resistance, and the absence of contact

with

small

waves

combined

with

flexible

seals

reduces

the

effects

of

wave

impact

at

high

speed.

Such

vessels

depend

on

lift

fans

to

create

a

cushion

of

low-pressure

air

in

an

underbody

chamber.

This cushion of air must be sufficient to support the weight of the vehicle above the water

surface.

The first type of vessel has flexible

and

enable

the

ship

to

rise

completely

above

the

sea

surface.

This

is

called

an

air

cushion

vehicle

(ACV), and in a limited sense it is amphibious.

The other type of air-cushion craft has rigid side walls or thin hulls that extend below the

surface

of

the

water

to

reduce

the

amount

of

air

flow

required

to

maintain

the

cushion

pressure.

This type is called a captured-air- bubble vehicle (CAB). It requires less lift-fan power than

an ACV, is more directionally stable, and can be propelled by water jets or supercavitating

propellers.

It

is

not

amphibious,

however,

and

has

not

yet

achieved

the

popularity

of

the

ACVs,

which include passenger ferries, cross-channel automobile ferries, polar-exploration craft,

landing craft, and riverine warface vessels.

Hydrodynamic Support

There

are

also

two

types

of

vessels

that

depend

on

dynamic

support

generated

by

relatively

rapid

forward motion of specially designed hydrodynamic shapes either on or beneath the surface of

the

water.

A

principle

of

physics

states

that

any

moving

object

that

can

produce

an

unsymmetrical

flow

pattern

generates

a

lift

force

perpendicular

to

the

direction

of

motion.

Just

as

an

airplane

“

skirts

”

that

entirely

surround

the

air

cushion

with (airfoil) produces lift when moving through the air, a hydrofoil, located beneath the

surface and attached by

means of a surface piercing strut, can dynamically support a vessel water.

s hull above th

Planning hulls are hull forms characterized by relatively flat bottoms and shallow V-sections

(especially

forward

of

amidships)

that

produce

partial

to

nearly

full

dynamic

support

for

light

displacement

vessels

and

small

craft

at

higher

speeds.

Planning

craft

are

generally

restricted

in size and displacement because of the required power-to- weight ratio and the structural

stresses associated with traveling at high speed in waves. Most planning craft are also

restricted to operations in reasonably clam water, although some

“

deep V

”

hull forms are

capable of operation

in rough water.

Hydrostatic Support

Finally,

there

is

the

oldest

and

most

reliable

type

of

support,

hydrostatic

support.

All

ships,

boats, and primitive watercraft up to the twentieth century have depended upon the easily

attained buoyant force of water for their operation.

This hydrostatic support, commonly recognized as flotation, can be explained by a fundamental

physical

law

that

the

ancient

philosopher-mathematician

Archimedes

defined

in

the

second

century

B.C. Archimedes

' Principle states that a body immersed

in

a

liquid

is

buoyed

up

(or

acted

upon)

by

a

force

equal

to

the

weight

of

the

liquid

displaced.

This principle applies to all vessels that float (or submerge) in water---salt or fresh. And

from this statement the name of the ships in the category are derived;

they are generally called displacement hulls. Although this ship type is very familiar, its

subcategories warrant special discussion. For example, in some vessels reasonably high speed

must be combined with the ability to carry light cargo or to move more comfortably in rough

water

than

a

planning

hull.

High-speed

planning-hull

characteristics

can

be

modified

to

produce

a semidisplacement hull or semiplaning hull. These compromise craft, of course not as fast as

full-planing hulls but faster than conventional displacement hull, must have more power and

less weight than the latter. Such types are obviously the result of

“

tradeoffs.

”

The example cited above lies between clear-cut physically defined categories it is

not

a

good

example

of

a

variation

of

a

true

displacement-type

ship.

The

latter

must

be

recognized

primarily as a displacement vessel, and its variations depend primarily on the distribution

of buoyant volume -------------- the extent of the depth and breadth of the hull

below the water.

The

most

ubiquitous

type

of

displacement

ship

can

be

generally

classified

as

the

common

carrier,

a seagoing vessel. It may be employed for passenger service, light cargo-carrying, fishing by

trawling or for hundreds of other tasks that do not require exceptional capacity, speed,

submergence, or other special performance. It is the most common and easily recognizable type

of ship, with moderate displacement, moderate speeds, moderate to large lengths, and moderate

capacities. It usually embodies the maximum in cruising range and seaworthiness. It is the

“

ship for all seasons.

standard

to

which

all

other

ship

classifications

in

the

displacement

category

may

be

referred.