Twin-screw vessel handling

NAVEDTRA 14104 Ch. 9 — Trouble Cause ¶1

Vibration Misalignment Bent shaft Clogged, eroded, or otherwise unbalanced impeller Lack of rigidity in the foundation Insufficient suction pressure may also cause vibration, as well as noisy operation and fluctuating discharge pressure. Rotary Pumps Another type of pump you find aboard ship is the rotary pump. A number of types are included in this classification, among which are the gear pump, the screw pump, and the moving vane pump. Unlike the centrifugal pump, which we have discussed, the rotary pump is a positivedisplacement pump. This means that for each revolution of the pump, a fixed volume of fluid is moved regardless of the resistance against which the pump is pushing. As you can see, any blockage in the system could quickly cause damage to the pump or a rupture of the system. Y…

USCG Boat Crew Seamanship Manual §10-A

Screw current and propeller side force A propeller rotating to drive the boat ahead draws water in from every direction forward of the blades and forces it aft in a stream. The flow into the propeller's arc of rotation is the suction screw current; the flow out of it is the discharge screw current. This holds whether the propeller is turning ahead or astern, and the discharge screw current is always the stronger and more concentrated of the two — which is why a vessel handles quite differently going astern than going ahead. Besides thrust along the shaft axis, propeller rotation produces a side force on the stern. Several effects contribute: the propeller works in the layer of water the hull drags with it (frictional wake); the discharge current acts on the rudder; and the blade at the to…

USCG Boat Crew Seamanship Manual §10-C

Backing a single-screw vessel Vessels are designed to go forward and many will not back in a straight line. Higher freeboard and superstructure forward give a large sail area up front, so many boats back into the wind. As sternway develops the apparent pivot point moves aft and the bow may swing through a greater arc, so the coxswain watches the bow as well as the stern and keeps firm control of the helm to stop the rudder swinging hard over. On a single-engine vessel propeller side force is the major obstacle to backing where intended, and the rudder has little effect until sternway exists. A workable sequence is: apply full rudder toward the side that opposes the expected swing before starting astern; use a quick burst of power astern to get the boat moving, accepting the swing side for…

USCG Boat Crew Seamanship Manual §10-D

Twin-screw handling — asymmetric and opposed propulsion A twin-engine vessel can vary the amount or the direction of thrust between its two engines, and any difference in thrust changes the boat's heading. The range runs from a small RPM difference used to hold a course up to splitting the throttles — one engine ahead, one astern — to turn the boat through 360° in its own length. This is often described as twisting the boat, but pivot point, propeller side force and the vessel's turning characteristics all still apply. Because the drives sit off the centerline they apply a turning moment to the hull, and where that moment is applied matters: twin outboards on a bracket apply it well aft of the hull's pivot point, while twin inboards apply most of it at the first thrust-bearing member of t…

USCG Boat Crew Seamanship Manual §10-E

Single-screw compound maneuvering A single-screw vessel never has asymmetric or opposed propulsion available, so its coxswain reaches the same ends by combining the basic forces in sequence at low speed: rudder position, short bursts of thrust ahead and astern, and the propeller's own side force. Ahead with the rudder over, the discharge current is directed to one side and the stern moves strongly; astern, the rudder is weak and side force dominates. Alternating short bursts ahead against the rudder with bursts astern therefore walks the boat around with little headway — the compound maneuver that substitutes for a twin-screw twist. These are learned in calm water with no current before being attempted against wind and current.

Boat Crew Seamanship Manual (M16114.5C) Ch. 10 — Basic Maneuvering ¶1

Introduction To learn the basic handling and maneuvering characteristics of a vessel, a trainee must first observe a skilled coxswain. Also, one must first learn to operate the vessel in relatively open water, away from fixed piers and moored vessels. In this Section This section contains the following information: Title See Page Learning the Controls 10-17 Moving Forward in a Straight Line 10-19 Turning the Boat with the Helm 10-23 Stopping the Boat 10-27 Backing the Vessel 10-28 Using Asymmetric or Opposed Propulsion (Twin-Screw Theory) 10-30 Performing Single-Screw Compound Maneuvering (Single-Screw Theory) 10-33 Learning the Controls B.1. Description When stepping up to the controls of any vessel for the first time, the coxswain should immediately become familiar with any physical cons…