It’s one of the grandest American musicals, so somebody was bound to give Show Boat the chamber-size treatment. The less-is-more approach has been all the rage lately, with scaled-down Les Mizes and Lights in the Piazzas littering the theatrical landscape, and it’d be downright criminal of the Rodgers and Hammerstein Organization not to page through its catalog and offer up this particular crown jewel for display. Thus the “No Boat Show Boat” that opened this week at Signature Theatre, with a cast of merely 25 and a slenderized set of spanking-new orchestrations by Jonathan Tunick. It’s hardly a pocket-size production, but neither is it the sort of Ziegfeldian excess that was on display in 1998, when Hal Prince steered the Cotton Blossom into the Kennedy Center with all flags flying—and it’s no stretch to assume that the R&H crew want the nation’s mid-size theater companies to see that Old Man River can be made to roll on something less than a Broadway budget. They’ll be satisfied if perhaps not thrilled by Eric Schaeffer’s dutiful staging, which does well enough by the saga’s four-decade sprawl but rarely gathers much musical steam; thematic threads about social tides and generational shifts make more of an impression here than classic tunes like “Bill” and “Can’t Help Lovin’ Dat Man.” That’s partly because the musical numbers seem haphazardly staged (with much of “Bill,” for instance, offered lackadaisically from behind a piano) and partly because the vocal casting itself seems surprisingly careless: Too many crucial numbers suffer at the hands of singers with pitch problems, or with voices of insufficient heft, or an indifferent way with a melodic line. Stephanie Waters’ ingénue is prettily sung, true, but her Magnolia—who’ll turn out to have more spine than her name suggests, let’s remember—generates precious little heat, whether it’s in a clinch with her gambler paramour (a suave, full-voiced Will Gartshore) or in a confrontation with her disapproving mother (a deliciously starchy Kimberly Schraf). This Show Boat’s most indelible presence, in fact, is its unflappable rascal of a captain: Harry A. Winter’s Cap’n Andy is front and center in Schaeffer’s staging, and it’s never been clearer that the river-running impresario and the gentleman gambler have the same wildness at heart.