Page 178 - NAVAL SCIENCE 3 TEXTBOOK

184 NAVAL SKILLS

Propeller Forces
Generally speaking, a boat is moved by forces resulting from
pressure diiTerences. For all practical purposes, water is incompress-
ible; therefore, when a propeller or screw rotates, high- and low-
pressure areas ,ll'e created on opposite sides of the propeller blades.
This force, called propeller (111'1/5( , is transmitted along the propel-
ler shaft ti·om the high-pressure area toward the low-pressure area.
Utility boats, like this personnel boat, are used for transporting \Vhen a right-handed propeller, the type normally used on a sin-
people and light equipment. gle-screw boat, is rotating clockwise as viewed from astern, the low-
pressure area is on the forward face of the propeller blades, resulting
Utility boats are essentially open powerboats of sizes ranging in forward movement of the boat. \Vhen the propeller rotntes COUI1-
terclock'wise, an opposite effect occurs, and the boat backs.
from about 25 feet to about 75 feet, designed lor hauling personnel
Next in importance to propeller thrust is side force, which tends
and light cargo between ships and fleet landings ashore.1\'Iost have
to move the boat's stern sideways in the direction of propeller rota-
a single screw and rudder.
tion. The upper blades exert a force opposite to that of the lower
OJJicers' boa($ arc smaller, traditional-looking powerboats with
blades, but the lower blades are moving in water of greater pres-
cabins intended to carry senior officers and other ship's officers. They
sure. Consequently, the force of the lower blcldes is greater. It is as
,He usually of less capacity than comparably sized utility boats. Most
though the lower blades were touching the bottom and pushing
ships have only one or two of these, reserved for lise primarily by the
the stern to the side. \Vhen going ahead the stern tends to star-
ship's captnin or higher-ranking officers who mily be embarked on
board, and when backing, to port. Side force is greatest when the
the ship. The captain's boat is c<llled a gig. A boat used by an officer
of higher rank than the ship's captaill is called a uarge.
IHodel'll iI/fi(/tables come in many shapes nnd sizes. Traditional
double-ended whaleboats of years past have largely been replaced
by rigid hull intlatable boats (RHIBsJ to fill the small boating needs
of most Navy ships. The latest RHIRs are water jet propelled, elim-
inating the need for a propeller and rudder.

Coxswain
The person in charge of handling a civili,m bont, regardless of its
size or type, is cllstomarily called the boat's mptnill. This tradition
is followed almost universally in both commercial and pleasure
boating. In the N,wy, the person in charge of a sailboat is also called
the captain, but in the case of powerboats of all sizes and types, the
person in charge of the boat, its crew, and any pnssengers is called
the coxslI'ain (pronounced coc'-sun). The coxswnin is responsible
for the maneuvering and sa let)' of the boat under all circulllstances
except when a commissioned line officer is embarked as either a
boat officer or passenger; these officers may then give directions
under certain circumstances to the coxswain.

Boat Handling
In order to become a good boat handler, a person must understnnd
the forces that act on the boat and cause it to move in one wny or
another. These forces can be broad I)' classified into two types: those
that are controllable by the boat handler, such as the propeller force
and rudder pressure, and those that are uncontrollable, such as wind
and current. It is the interplay of <.111 forces acting simultaneollsly that Rigid hull inflatable boats (RH IBs) have become the workhorses
among the small craft of the fleet, doing duty as lifeboats, personnel
determines how a boat will react in any given situatiol1.
boats, fast patrol craft, and utility boats.