One of the great biannual migrations is about to begin as cruise ships move from their winter homes in the Caribbean to Europe for the summer.

These transatlantic repositioning voyages are a rite of spring. The reverse occurs in the fall, when vessels flock back to Caribbean waters.

It can be tough for lines to sell these longer, shoulder-season voyages.

“A lot of people don’t want to do it because they don’t have the time — 10 or 12 days, of which nine may be at sea,” said Peter Carideo of CRC Travel, a luxury specialist in Chicago.

But for cruise fans with time on their hands, they’re an opportunity to sail with big savings.

Most repositionings are in March, April and November. Seasoned travelers such as Jennifer Anderson of Chicago find allure beyond the savings, saying the ship becomes the destination — like an all-inclusive resort.

In November, Anderson and her husband crossed from Lisbon, Portugal, to the Caribbean. Their Seabourn ship called at the Portuguese island of Madeira, then spent nine days at sea before arriving at Barbados. Other transatlantics they took were similar, sometimes visiting the Canary Islands or ending in Florida.

Anderson said she swims and tans, and “I read 20 books.”

Never too late

It’s not too late to plan a spring voyage. Carideo said they rarely sell out. A check of spring departures found availability across a range of lines.

An exception: Seven Seas Splendor, a new ship for luxury line Regent Seven Seas Cruises. Regent loyalists Nancy and Jim Kopnisky of Sarasota, Fla., booked six months ahead to ensure a berth on April’s inaugural cruise from New York to Barcelona.

Later, when Jim’s daughter and her husband decided to join them, few suites were available. The inaugural-season pricing was no bargain. But they still favor Regent for its all-inclusive rates that cover shore excursions.

This will be the Kopniskys’ first repositioning cruise and, like Anderson, they weren’t seeking ports.

“Been there, done that,” Kopnisky said. “This will give us an opportunity to see how it feels to spend six days at sea without the hustle-bustle of having to get up early in the morning. We’re looking forward to sitting back and relaxing.”

Jim’s daughter, Kathy Kopnisky, an early retiree in Silver Spring, Md., and her husband plan to hit the gym every day, and they’ve booked a class to learn how to make paella.

The cruise lines pack in enrichment activities such as lectures, classes in everything from digital photo editing to music, plus sports competitions and games.

Ted Eastwick grew up sailing transatlantic with his family. He’s taken two repositioning voyages on Crystal Cruises. One was from Hamburg, Germany, to Miami, where he lives; the other was from Honolulu to Vancouver, British Columbia.

Eastwick estimates repositioning voyages save up to 70%, even on the five-star lines. And they seem to foster more of a bonding vibe among passengers than your typical cruise.

“There’s an esprit de corps that you don’t usually find on cruises today. ... It’s like, ‘We’re all in this together as we head out to sea,’ ” Eastwick said.

Options abound

With cruise ships sailing globally, there is a greater range of repositioning cruises on all types of lines.

“It’s not just your traditional repositioning across the Atlantic,” said Vicki Freed of Royal Caribbean International. She pointed to Ovation of the Seas’ trans-Pacific cruise from Australia to Hawaii, with the option of continuing to Vancouver.

Ovation will sail from Sydney on April 20 and visit five destinations in New Zealand and French Polynesia, with 12 sea days sprinkled in.

What’s there to do during a dozen days at sea? Plenty. Riders can soar high above the ship in a glass viewing capsule on an extending arm, catch a movie on an outdoor screen, rock climb or test the surfing and skydiving simulators. Ovation’s SeaPlex playground offers bumper cars, a ball court, roller skating and a circus trapeze school. The adults-only Solarium houses a glass-enclosed pool.

Fares for this 18-night journey start at $2,253 in an interior cabin and $3,178 for a balcony room. Taxes, fees and port expenses of $183.52 are extra.

Other repositioning cruises include Carnival Legend’s 14-night crossing from Tampa, Fla., to Barcelona, leaving March 25.

Legend stops in the Bahamas, Bermuda, Portugal’s Azores Islands and Málaga and Valencia, Spain. This is an “extra value” sailing, with fares starting at $759 per person in an interior room and $899 in a balcony cabin. Taxes, fees and port expenses are an additional $223.89.

Norwegian Cruise Line’s 15-day crossing from Miami to Barcelona leaves April 18. The journey begins with six sea days before arriving at the Azores, two more sea days before Lisbon, then Cádiz, Málaga, Cartagena and Palma de Majorca in Spain. With the 30% discount at press time, fares start at $993 per person in an inside room and $1,490 in a balcony room, plus taxes and port fees.

For a boutique-type experience, Ponant, a French line with small, sleek ships, caters to international clientele. Expect gourmet cuisine, a spa and a theater for lectures and concerts on these yacht-like vessels.

In early April, its 184-passenger Le Dumont-d’Urville will sail from Cayenne, French Guiana, to Dakar, Senegal. There are no stops. For the eight-night voyage, Ponant offers bonus savings of up to 30%, with fares starting at $2,200. And there’s a special offer for solo travelers: no single supplement.

Not for first-timers

Carideo said people who haven’t cruised before probably shouldn’t dive into a repositioning ship as their maiden voyage.

“If they don’t like it, they’re stuck on it,” he said.

Boredom, seasickness and bad weather are possibilities.

“It’s about your expectations,” said Anderson. “If you want to be entertained all the time, maybe it’s not for you. I go to relax, decompress, meditate, try to tune out the world a little. I like the [ship’s] movement, the serenity.”