When the slickness of the Smithsonian museum complex begins to pall it is a relief to get away to the Memorial Museum at the Washington Navy Yard.

The vast old building is just as cluttered as the downtown museums were in the good old days. Some of the stuff is there because some admiral - or his lady - sent it in. Other exhibits are just too damn heavy to haul away without an urgent reason.

The result is cases of junk next to cases containing mavels of the model builder's art, and poignant reminders of the dangers of the sea in jarring juxtaposition with a selection of schlock apparently assembled by an influential chaplain.

There are World War II-era deck guns that children are encouraged to play on; the museum is so lightly attended that parents usually can sneak a turn unobserved.

Some of the exhibits are unexplained, and you could while away a whole rainy afternoon trying to figure out what in tunket they are. Or trying to find someone who can tell you. It can become a game: for instance, is the original genuine regulation hardtack biscuit from the U.S.S. Constitution still on display? Where is it?

It amounts to being turned loose to rummage through the Navy's attic. It even smells like an attic, which is as it should be.

The range is from atomic bombs to Adm. Arleigh (31-Knot) Burke's deskplate, and includes an outdoor display of monstrous pieces of ordnance. And the Navy Yard itself is one of Washington's best wandering-around places. One afternoon this week, for instance, a squad of spit-and-polish Marines marched out onto the dock beside the Anacostia and began, by the numbers, to shoot at each other at Point-blank range. Either the shells were blanks or the Marines still are looking for those "few good men," because nobody fell down.

This being Armed Forces Week, the Navy has laid on an extra. The patrol gunboat PG-93 will be open to the public Friday (noon to 4), Saturday (10 to 4) and Sunday (noon to 4). To get there, take the exit marked Navy Yard from the Southwest Freeway.