Page 179 - NAVAL SCIENCE 3 TEXTBOOK

BAS IC SEr\~ IANS HIP 185

Discharge Current A single-screw vessel is IllOst difllcult to maneuver at very low
speeds where rudder ellect is minimal, while at the same time pro-
peller sidc force is greatest.
Boat _____ ~ Suction One of the Illost notable characteristics of a single-screw boat
Beginning to ----- Current
Back or ship is its tendency to back to port. Four distinct forces affect a
J boat's steering when backing:
The discharge current from the propeller (which tends to
Sidewise Pressure 01 Lower Blades
I (Side Force) I throw the stern to port)
The suction current caused by the propeller drawing in water
Discharge Current from astern (which adds to the steering effect of the rudder,
although not by any great amount)
Suction
Current The sidewise pressure of the blades (which forces the stern to
Boat port)
Backing Rudder
Rapidly Effect The normal steering efiect of the rudder

Typical Situations
Sidewise Pressure of Lower Blades
I (Side Force) I Let us examine the efiect of the forces just described in il few
typical situations. \Ve will assume there is no wind, tide, or current,
The screw (discharge) current, side force, and rudder forces all except in certain instances where it is so stated.
combine to determine how a boat will respond, as in the backing Boat and screw going ahead. \Vhen a boat is dead in the water,
situation depicted here.
with right rudder on, and the screw stttrts turning over, the screw
boat begins moving from a stationary position, or nearly so, and current hits the rudder and forces the stern to port. \'''ith left rud-
decreases rapidly as the boat's speed increases. Side lorce is greater der Oil, the stern moves to starboard. As the boat gathers way, the
when backing than when going ahead. effect of the screw current diminishes and the normal steering
effect of the rudder controls the boat's head.
Screw cmTCIII, c;:lused by the action of a rotating propeller, COIl-
sists of two parts. The portion Oowing into the propeller is the suc- \ " hen the boat is proceeding ahead in the nonnttl manner and
tion current. and the portion Oowing away from the propeller is the rudder is put right, the boat first tiliis off to port. If the rudder
the discharge current. Suction current is a relatively minor force is put left, the boat gocs to stttrboard. The entire boat is thrown
in boat handling. Discharge Cl11'rent is a major force in two main slightly (almost imperceptibly) to the side, but the stern gives way
respects: it is a strong force acting on the rudder with the screw to tt greater extent. The boat advances two or three boat lengths
going ahead, and it is a strong component of side force when the along the line of the original course before it commences to gain
screw is backing because of the discharge current acting against ground in the desired direction. At higher speeds, this advance
the boat's hull. is slightly less than at lower speeds, and turns are executed more
Single-screw ships and boats ha\'e a single rudder mounted quickly. Because of the advance, trying to execute a turn to avoid
directly behind the propeller. Twin-screw ships and boats usually an obstacle only a short distance ahead call result in disaster.
have a rudder mounted directly behind each propeller, but some Boat and screw backing. ' ·"hen backing down, lolll' distinct
have a single rudder mounted between and just behind the pro- forces are involved ill steering. The), are discharge current, side
pellers. ilasically, a rudder is used to attain or maintain a desired to rce, sllction current, and rudder effect. The combination of these
heading. The force necessary to do this is created by dynamic pres- forces is sllch that it is almost impossible to back in a straight line.
sure against the surface of the rudder. The magnitude of this force ilecause discharge current (from the propeller) and side force
and the direction in which it is applied produce the rudder effect tend to throw the stern to port, most single-screw bonts tend to
that controls stern movement and, through it, the boat's headi ng. back easiest in that direction. The relatively weak suction current
Factors having a bearing on rudder effect include rudder size, rud- acts to throw the stern to the side all which the rudder is, but suc-
der angle, rudder location, boat's speed, direction of propeller rota- tion cllrren t is negligible <11 slow speeds, so the rudder tends to be
tion, headway, sternway, suction current, discharge current, and ineftectivc. As the boat gathers sternway, the water through which
side force. The diverse effects of all of these factors can be lumped the boat is moving acts on the rudder and increases the efiect of
together LInder a single term, resllitnllt force, which indicates thc suction current, so the rudder becomes 1110re effective. This usu-
direction and amount of net force exerted on a boal's stern. ally slows, but does not necessarily stop, the stern's swing to port.
\," hen backing long distances, it usually is neccssaq' to occasionally