The lack of skilled sailors to run Navy ships is "rapidly approaching a crisis," the nation's top admiral has warned the secretary of defense and joint chiefs of staff in a private memo.

Adm. Thomas B. Hayward, chief of naval operations, in sounding that warning went so far as to offer to give up shipbuilding money to obtain some of the $6 billion needed to raise military pay.

Hayward's memo represents one of the gravest warnings since the Vietnam war that the fleet is losing its ability to fight for want of skilled petty officers to run the ships of the line.

Although the admiral himself did not release the memo, copies of it are now circulating outside official channels as concerned officers try to make the case for raising military pay to stem the exodus of experienced people from the armed services.

The Senate is expected to vote on that issue later this month through an amendment drafted by Sen. William L. Armstrong (R-Colo.). It would give military personnel a 3.4 percent cost-of-living pay increases, effective Jan 1, 1980.

Hayward's memo, a copy of which The Washington Post obtained, was sent to Defense Secretary Harold Brown last month as the Pentagon and White House were wrapping up the final details of the fiscal 1981 defense budget.

Hayward warned that:

Shortages of skilled enlisted people who keep ships running threaten to be "the controlling factor" in how many ships can be sent to sea ready to fight.

For what of experienced sailors, 38 percent of the ships patrolling European waters are in the lowest two categories of readiness.

Shortages of experienced officers for submarines, warships and fighter planes also are jeopardizing the Navy's ability to fight.

Hayward, in elaborating on those warnings, wrote Brown that the Navy's critical problem is not recruiting new sailors and officers, but keeping those already trained.

Instead of retaining the desired 60 percent of enlisted people who have completed two hitches, Hayward said, the Navy retained only 45 percent of them in fiscal 1979.

"If this trend continues," he said, "it will be the controlling factor in our force levels within a year or two." He said the Navy is 20,000 people short in ranks E-5 through E-9, 2nd class petty officer through master chief petty officer. Pay in those ranks, including allowances, ranges from about $11,750 to $24,000.

Raw recruits can only fill the numbers gap, Hayward wrote. They cannot run the engine rooms, antisubmarine warfare gear and other weaponry on today's sophisticated submarines and surface ships.

Turning to officers, Hayward said that instead of retaining the desired 50 to 60 percent of experienced people, the Navy is managing to keep only 30 to 40 percent of them.

"Our nuclear-trained submarine officers," the admiral wrote, "who serve under particularly arduous conditions of extended sea duty, have a retential rate which is projected to decline to below 40 percent this year."

Pilots are leaving in such numbers that their retention rate is expected to nosedive from 62 percent in fiscal 1977 to 28 percent projected for fiscal 1980, he continued. Retention of surface ships officers is dropping to around 31 percent, he said.

The "paramount" reason for this exodus is "the large net loss in purchasing power which military people have experienced since 1972. It is absolutely essential to close the gap," Hayward said.

He estimated it would cost $6 billion to restore the purchasing power the 2 million people in uniform enjoyed in 1972. He said the Pentagon must start restoring that money now to stem the tide of departures from the Navy.

"It is no exaggeration to say that we are rapidly apporaching a crisis," Hayward wrote Brown. "I am willing to offer up hardware, if necessary, to assure an adquate level of compensation for our people."