單項(xiàng)選擇題A (n) （） is a chemical or electro-chemical attack on the metal surface which may be further increased if the sea-water is polluted.

A.erosion
B.corrosion
C.cavity
D.scouring


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2.單項(xiàng)選擇題In moving forward, a ship imparts forward motion to the water at the stems, this is known as （）.

A.the speed of the vessel
B.the speed of the propeller
C.the wake
D.the wake fraction

4.單項(xiàng)選擇題When viewed from astern, if the propeller revolves in a clockwise direction when going ahead, it is known as （）.

A.ahead direction
B.astern direction
C.right-handed
D.left-handed

5.單項(xiàng)選擇題The pitch ratio of a propeller is the mean pitch divided by （）.

A.the propeller diameter
B.the propeller radius
C.the pitch angle
D.the length of helix