shaft housing 192 and screw propellor 193 being visible aft of the engine well fig1 shows in front elevation first hull assembly 100 on an enlarged scale visible members of framework 180 include two y members 184 and 186 much as shown in fig5 and 9 here topped by single transverse member 181 of the framework tie member 121 whose ends are barely visible is also slightly visible between the hull envelopes where it passes through vertical hub 85 of each such y member decking 111 is spaced slightly above transverse frame member 181 by intervening central and side longitudinal box strips 113 fig1 is a similarly enlarged front elevation of second hull assembly 200 whose framework 380 comprises spaced pair of vertical members rising from hubs 85 and being interconnected by x bracing and topped by transverse frame member 381 tie member 321 is seen traversing the space 305 between hull envelopes 314 and 316 as well as the vertical hubs the two outermost envelopes 313 and 317 are omitted from this view and the tie member is cut away at each end accordingly because of space limitations fig1 is a transverse sectional elevation showing in detail typical relationship of a hull envelope a transecting tie member and the framework as at the leftmost portion of fig1 or other comparable location in a hull assembly of this invention hull envelope 16 designated by only two digits to indicate its use in any embodiment of hull assembly has diametral horizontal sleeve 45 therethrough open at its ends and transected by tie member 21 so designated without prefixed hundreds digit to indicate general utility as.