OFFENBACH, goes that old one-liner, is not often Bach. True enough. But in "Christopher Columbus," the perplexing comic concoction being staged by the Washington Opera at the Kennedy Center, Offenbach is not even Offenbach.

This supreme master of French light opera is credited with having written more than 80 operettas between 1847 and 1880, when he died, in part from exhaustion. None of these, however, was "Christopher Columbus."

Instead of doing one of the real Offenbach works as one of the two offerings in its abbreviated Terrace Theater season, the Washington Opera is putting on this pastiche cobbled together by an American musicologist named Patric Schmid and an English translator named Don White. It was commissioned for the American Bicentennial by the Ulster Orchestra of Belfast, Northern Ireland,and premiered in 1976.

The 28 numbers are drawn from no less than 22 Offenbach operettas, most of them obscure. And for a story, White shaped a sort of gutter version of the Columbus yarn, in which the exploration is more sexual than geographic.

And the raw material is raw indeed. The nymphomaniac Queen Isabella, stripped to her underwear to go at it with Columbus, draws a pendant from her jewel box and declares rapturously that "I won it in a marathon for indoor sports." The doddering old Ferdinand enters the bedroom with wife and guest indulging athletically under the covers and he sings, "Lady of Spain, I adore you."

Most of the music in the first act is without wit, bottom-drawer Offenbach mixed with one or two middle-drawer numbers, such as Isabella's "Jewel Song" (sung on Monday night, the second performance, with indeterminate pitch and tonal production by mezzo Elaine Bonazzi).

The music in the second act does provide some nice moments. The ensemble performance of "Lovely Night" is a fine one, beautifully sung, and the storm that comes afterward is lots of fun, with a wonderfully funny Santa Maria that gives the illusion of being tossed around all over the stage and revolves in a full circle during the storm. Zack Brown's ingenious sets were the only parts of the evening that were consistently witty.

Vocally, the production is hardly flawless, but there was especially fine singing from Robert Orth, Cyndia Sieden, Dana Krueger and Karen Hunt. David Eisler was musical but dry- voiced in the title role. Robin Tabachnik was having a bad evening.

Conductor Randolph Mauldin and director Roman Terleckyj kept the action moving well. Chorus and orchestra both were in good form.

Had it not been done so well, "Christopher Columbus" might have been a real fiasco. As it was, one was still left wondering if it belongs in the repertory of a serious opera company. Some of the performances, one should add, are sol out.

CHRISTOPHER COLUMBUS -- At the Kennedy Center Terrace Theater with performances this Friday, Sunday, Tuesday and on January 18, 20, 22, 24, 26, 28, 31 and February 2.