UNC Resources of Falls Church, taking another step to diversify its uranium-based business interests at a time of reduced activity in that industry, yesterday announced plans to acquire a manufacturer of large aluminum marine vessels.

Under a definitive agreement, UNC would buy the privately owned Swiftships Inc., of Morgan City, La., for--"among other things"--UNC securities ultimately convertible into about 1.4 million shares of common stock. With UNC common trading on the New York Stock Exchange at $6.75 a share, the common shares would be worth about $9.5 million.

In addition, UNC President Keith Cunningham said two principal executives of Swiftships, who have agreed to stay with the business, may be entitled to other payments based on future earnings.

Swiftships was described as the leading producer of aluminum vessels over 100 feet in length, which are used primarily in offshore oil service and military applications. Swiftships also manufactures steel vessels and provides vessel repair services.

Revenues of Swiftships this year will be about $60 million and are projected to grow to more than $100 million in 1982, Cunningham said.

"This acquisition is a major further step in UNC's transition into a diversified company less dependent on a single market or customer," he added. Along with another subsidiary acquired earlier this year, UNC now has a substantial niche in the growing offshore oil and gas support business.

Swiftships employs 800 workers at four locations in Louisiana and Texas. Since the firm's first aluminum vessel was completed in 1970, some 300 ships have been delivered. Customers include more than 40 oil companies, 19 foreign governments, and the Navy, Coast Guard, Air Force and Army Corps of Engineers.

In addition to uranium mining and marine services, UNC is engaged in mineral exploration, oil and gas exploration and production and precision manufacturing, with annual revenues of more than $300 million.