How long does it take to build a double-ended municipal ferryboat? Any boat with two bows should have two answers, if not more. If by "build a ferryboat" we mean from the moment we start laying the keel to the moment the boat hits the water, we could say a ferryboat takes eleven months to build. Or anyway, that's how long it took Marinette Marine, Inc., a division of Manitowoc Corporation, to build the first in "the new Kennedy class" - at 310-feet and 7.1 million pounds loaded, with a $40-million price tag, the largest vessel constructed by the yard. Altogether, there are three. "The second two were identical," said Marinette Marine's Duane Roehm, Vice-President, Program Management and Planning, "but during the construction of the first, there was a strike.