Twin-screw vessel handling

USCG Boat Crew Seamanship Manual §10-A

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 ac…

USCG Boat Crew Seamanship Manual §10-C

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 …

USCG Boat Crew Seamanship Manual §10-D

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 i…

USCG Boat Crew Seamanship Manual §10-E

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.