Harvard NROTC midshipmen who had counted on spending the summer cruising around the Mediterranean may instead find themselves drydocked in Boston.

The Defense Department, in an effort to stem the outflow of gold from this country, has cancelled all cruises to foreign ports by members of the Naval ROTC. About 20 Harvard undergraduates - and scores of other students at the Naval Academy and in 51 other college NROTC programs - will be affected by the directive.

All students enrolled in the Navy's college ROTC program must take at least one summer cruise aboard a Naval vessel. At Harvard, about half of the midshipmen taking cruises in a given summer are assigned to ships visiting European or Far Eastern ports.

This summer, however, midshipmen from Harvard and most other colleges east of the Mississippi will be assigned cruise ships to the Second Fleet, which operates off the Atlantic Coast.

Lt. Robert C. Wiley, assistant professor of Naval Science, said yesterday that "a lot" of ships are assigned to the Second Fleet to undergo maintenance. Wiley said that it was "possible but not likely" that some students might spend the summer attached to ships that were tied up in Boston. He noted that two students who were assigned to ships that would be in dry dock for the summer have been reassigned.