Navy leaders sail forth today on what threatens to be their roughest legislative voyage in years as they defend before Congress a military budget they opposed in several key areas.

To make their mission even rougher, Navy leaders start their testimony before the House Armed Services Committee without having the long-range shipbuilding plan Congress had requested. Denfense Secretary Harold Brown has held up the plan because he believes it needs more study.

Rather than wait any longer concressional committees will be tempted to design the Navy of the future on their own, perhaps earmarketing money for a new aircraft carrier even though President Carter did not request it.

Navy Secretary W. Graham Claytor and Adm. James L. Holloway, chief of naval operations, will defend Carter's $126 billion defense budget in hearings today and subsequently. But they will be shot at with their own ammunition because some members of Congress have obtained secret Navy memos challenging the budget decisions.

On such decision that Navy leaders have challenged is Carter's insistence on buying Northrop F18 fighter and attack planes rather than a combination of Grummand F14 fighters and Vought attack aircraft as Navy leaders had recommended. House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) is a staunch defender of the F18, which will get its engines from the General Electric plant in his home area of Lynn, Mass.

Navy leaders assert that the F18 is not enough of a fighter to combat the Soviet threat to the fleet.

"The major maritime air threat of the 1980s is the [Soviet] Backfire" bomber, Under Secretary of the Navy james R. Woolsey noted in his memo that his assessment was shared by the entire Navy leadership.

The Soviet Backfire patrolling broad stretches of the Atlantic and Pacific oceans could attack U.S. ships 300 miles away by firing missiles, Woolsey wrote.

"The Navy must be capable of defeating or deterring this threat," he continued. "It is clear that only the F14 offers such a capability. The F18 does not.

"The F18 is not, as is often stated, a lower cost complement to the F14. It is rather a low capability substitute . . .

"The only marginal advantage claimed for the F18 is its greater dog-fight ability," Woolsey argued further. "In addition to being irrelevant to the most serious threat, this claimed advantage is degraded by the decreasing likelihood of future dogfight encouters because of the introduction of much improved all-aspect air-to-air missiles."

If the F14 got new engines. Woolsey said that even the dogfighting edge of the F18 "most likely" would be reversed. Brown, in going over the proposed Navy budget, cut out the request for new engines for the F14 add four NAVY.

The Navy could save almost $2 billion for shipbuilding and other programs short of money if the whole F18 program were canceled and its roles filled instead by the F14 and A7 war planes, Woolsey wrote. He noted that "our ship construction program, which was at 31 ships only a few months ago, now stands at 16" ships - presumably because of the budget squeeze.

Despite the arguments of Claytor, Woolsey and Holloway, President Carter, in his final defense budget, reduced the production of F14 fighters to 24 planes a month and refused to buy more A7 attack aircraft.

The competing plane manufacturers have taken their arguments about what should be purchased with the new defense budget to Congress. Vought has even gone so far as to tell O'Neill it would like to put two engines instead of one in the A7 attack plane, adding pointedly that the new engines could be built in the General Electric plant in Lynn.