AmaWaterways, a privately held river cruise company with 20 ships and plans for more, announced on Wednesday that it has added a new investor.

The California-based operator did not reveal any details of the deal with Certares LP, a travel and hospitality investor group, including the amount of the investment, how large a stake the group now holds, or what the influx of new money will be used for. The other owners are AmaWaterways president Rudi Schreiner, executive vice president Kristin Karst, and two other families.

A representative for AmaWaterways said the company would not answer questions about the deal. And Certares, which has investments in American Express Global Business Travel and Travel Leaders Group, did not respond to questions in time for publication. Travel Leaders Group includes a handful of cruise-focused travel agencies.

AmaWaterways, which marks its 15th anniversary this year, has been growing — though not as fast as some of its owners would like, Schreiner told Skift in late 2015.

“The demand is so strong. The thing is that I’m not going overboard with too many ships, I continued during the recession years to grow the fleet and that’s what I’m still doing,” said Schreiner, the president and co-founder. “I have a steady approach. Some of my partners would like to see even more ships, though. I’d rather have a tight inventory than have to go out and discount the ships. Our preferred travel agents partner really helps us fill the ships.”

The operator took delivery of a new 156-passenger vessel in April. And just last week, AmaWaterways announced plans for its biggest ship yet, which will debut in 2019. The extra-wide ship with capacity for 194 passengers will be “designed to elevate the modern luxury market” with multiple dining options and an open-water sports platform. AmaWaterways referred to the next-gen vessel as a “game-changing design concept to revolutionize” the river cruise industry.

An announcement described Certares as the company’s fourth investor and said all were committed to supporting “long-term growth and innovation” in river cruising. AmaWaterways sails mostly in Europe, but has a small presence in Southeast Asia and Africa.

“We look forward to continuing our growth in the river cruising market with the support of our new partners and our family partners,” Schreiner said in the announcement. “We are proud of the company we have built over the last 15 years, the privilege we have of hosting our guests onboard every day and the opportunities we see ahead of us.”

Certares managing partner Colin Farmer hailed the AmaWaterways shareholders’ “incredible track record of customer service and product innovation” in the statement.

“We are honored to join them in support of this next chapter of the company’s development,” Farmer said.

Photo Credit: The AmaLyra ship is shown in Bratislava in this promotional photo from AmaWaterways. The river cruise company announced it had received an investment from Certares, a travel and hospitality investor group. AmaWaterways