A boat comprises an elevator and a boat hull having a deck and defining a dive opening. The quantization error is stored in a buffer, and fed back through a subtracter to at least one input value of a subsequent conversion. 1 m or less, and said upper magnetic layer has a surface roughness of 2. The squeezable projection has a substantially triangular cross section and contacts a wall surface of the supporting groove as insertion of the supporting piece into the supporting groove progresses. The collection of related data for a particular node can be displayed in an encyclopedia-like format.