A better picture of the Helena Wharf Boat is found here:

http://en.wikipedia.org/wiki/SS_Sultana

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WHARF BOAT

a boat moored and used for a wharf at a bank of a river or in a like situation where the height of the water is so variable that a fixed wharf is impracticable

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a boat supporting a platform sometimes used as a wharf in rivers or in other situations where actual wharves do not exist, or where they are impracticable from the great variation in the height of the water.