The circular steel structure installed around the propeller of a towboat is the（）.

单项选择题The bow thruster generally is ineffective at（）.

A.over 3 knots headway
B.at any speed astern
C.at any speed ahead
D.over 1 knot sternway

单项选择题Structural bulkheads on a ship are usually（）.

A.continuous
B.watertight
C.transverse
D.non-watertight

单项选择题Signs of racking stresses generally appear at the（）.

A.bow and stern shell frames and plating
B.junction of the frames with the beams and floors
C.garboard strake，at each side of the keel
D.thrust bearing of the main shaft

单项选择题Sidewise force of the propeller tends to throw a vessel’s stern to the right or left，depending on rotation. This force is caused by（）.

A.back current from the rudder
B.greater pressure on the right or left side of the propeller，depending on rotation
C.lower pressure on the right or left side of the propeller，depending on rotation
D.torque from the velocity and angle at which the surrounding water impinges upon the propeller blades

单项选择题Shell plating that has curvature in two directions and must be heated and hammered to shape over specially prepared forms is called（）.

A.compound plate
B.furnaced plate
C.flat plate
D.rolled plate