Strong’s Marine has added a fourth marina, and also formed a partnership that will extend the 71-year-old company’s boat sales to the South Shore of Nassau County for the first time, executives said.

Strong’s recently bought Mattituck Inlet Marina & Shipyard from the Pape family for an undisclosed amount. The purchase includes 16 acres of woods that could potentially become houses.

The expansion of Strong’s, one of the region’s largest recreational boating businesses, comes as the industry tries to adapt to a declining number of boaters. Boat registrations dropped 20 percent on Long Island between 2006 and last year, according to the state Department of Motor Vehicles.

The Mattituck marina and shipyard, renamed Strong’s Yacht Center, sits along the same body of water as nearby Strong’s Water Club & Marina. That property, once called Matt-a-Mar Marina, was purchased in 2013.

Jeff Strong, president of the Mattituck-based company, said the yacht center has 70,000 square feet of indoor boat storage in four buildings, a woodworking shop and a large maintenance facility. It will cater to yachts from Long Island, Connecticut, Rhode Island, New Jersey and elsewhere, he said.

“This used to be the place in the Northeast to bring your yacht — for restoration, for service” under the direction of then-general manager John Hallinan, Strong said last week. “Our mission is to try and build it back up.”

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He said he plans to spend $2 million on improvements. The property’s five employees were retained and three more have been added so far, including general manager Sal Messina.

Separately, Strong’s recently formed a partnership with Al Grover’s High and Dry Marina Inc. in Freeport to sell boats. Strong said his grandfather and company co-founder Stewart Strong used to buy Old Town boats from Al Grover, who started his business in 1950.

“We’ve never entered a partnership before but the cultural underpinnings of our businesses are similar,” Strong said, referring to the Freeport marina’s current owner Dante Grover. “And this allows us to serve all four quadrants of Long Island.”

Grover added, “Our work ethic and philosophy are one and the same.”

Strong predicted the partnership, called Strong’s at Grovers, will in time add 40 to 50 boat sales per year to his company’s annual total of about 200.

In addition to the yacht center and Freeport sales partnership, Strong’s Marine has three marinas on the East End and multiple showrooms. It employs about 100 people.

Strong predicted the expansion projects will increase company revenue to $30 million in 2017, up from more than $20 million last year.