It is said that necessity is the mother of invention, and it is hard to imagine any necessity more pressing than survival.

So it makes sense that a Hancock County man who survived 21/2 months at sea after being shipwrecked 19 years ago would use his experience to invent a boat designed to spare others the same kind of ordeal.

Lamoine resident Steve Callahan, 49, wrote a book called “Adrift” about his misadventure in 1982 when he spent 76 days floating in a life raft in the Atlantic Ocean. More recently, his creative efforts, when he is not writing for sailing or boat building magazines, have led to the development of The Clam, a folding fiberglass dinghy with an inflatable tube lining its rim.

Callahan has had the rough idea for The Clam for many years, but he and his partner, Phil Andrews, owner of the HydraNova boat shop in Ellsworth, have refined the boat’s design to the point where they are just beginning to manufacture and market it.

“Some details are still evolving in it, but it’s essentially finished,” Callahan said recently.

The boat is designed with many of the needs of shipwreck survival in mind, but is not being marketed as a lifeboat, according to Callahan. He said he and Andrews have decided not to get the boat officially sanctioned as a safety vessel. Life rafts or boats are not required equipment for recreational sailors anyway, Callahan said, so getting the boat officially approved isn’t necessary for their target market.

The Clam’s unique design also makes it difficult to classify as either a life raft or a lifeboat.

“We’re in a niche that nothing really applies to,” Callahan said.

Still, The Clam has features that can help people who find themselves shipwrecked at sea, Callahan said.

Charles Doane, a senior editor with Sail magazine in Rockland, does not own a Clam but has taken one out for a trial run. He said the boat combines the maneuverability of a rigid-bottom boat with the stability of an inflatable boat.

“It sails and rows like a real boat,” Doane said. “Most inflatables don’t.” But, like most inflatables, it is easier to climb into from the water than regular rigid-hull boats, he said.

“If I was equipping a boat for a long-distance ocean cruise, I would certainly think about [equipping it with The Clam],” Doane said.

Callahan, a lifelong sailor, was sailing across the Atlantic alone in his 21-foot sloop Napoleon Solo – a boat he made at his home in Lamoine – when it sank in 1982. He was several hundred miles west of the Canary Islands, off the coast of Morocco, when the boat was struck by an object in the middle of the night, he said. Callahan said he still doesn’t know what rammed his vessel, causing water to spray in through the wall of the cabin like a geyser.

“It came from the side,” Callahan said. “It’s just one of those things I’ll never know.”

It took about 60 seconds for the Napoleon Solo to become fully submerged in the water, Callahan said.

For the next 76 days, Callahan lived in a circular raft with two inner tubes stacked on top of each other for walls and a single-ply rubber sheet for a floor. It offered few options for piloting and was not easy to maintain or repair with the little resources he had while adrift, he said.

Callahan said his experience would have been different if he had The Clam on board when the Napoleon Solo sank. By either rowing or sailing the folding boat, and by living off supplies stowed in water-resistant compartments in the deck, Callahan says he could have reached the Cape Verde Islands off Senegal well before he drifted to the Caribbean.

“It probably would have taken me no more than two weeks,” he said. “It certainly would have made my life a whole lot easier.”

Besides worrying about the obvious, such as exposure and starvation, as he drifted across the Atlantic, Callahan said he also had to be concerned about fixing leaks and spearing fish without puncturing the raft.

Before he reached land, Callahan suffered from extreme fatigue and disorientation when exposed to the afternoon sun. Sometimes forced to sit or lie down in water that collected in the raft, he also developed “well over a hundred” saltwater sores before he was found.

Callahan said he had lost nearly one-third of his weight, 50 to 60 pounds, by the time he was found by fishermen five miles off Guadeloupe.

“If you’re in a lifeboat or life raft for more than two weeks, you’re in serious trouble,” Callahan said.

But often, sailors do not even have a lifeboat or raft onboard because of storage concerns, he said. That’s why he developed The Clam. Its 11-foot unfolded length makes it relatively seaworthy and its 5-foot-4-inch folded length makes its easy to store, he said.

Andrews, 60, said he founded HydraNova, his Route 1A boat shop, a few years ago with the idea of manufacturing the folding boat designed by Callahan. He said a 10-inch inflatable tube on the rim serves many purposes in the design of the craft. It helps deflect water away from the boat’s interior as it moves through the water and acts as a secondary flotation system.

“You could put a hole in the bottom and still not sink,” Andrews said. “You could puncture a tube and still not sink.”

Though there are storage compartments in the floor of The Clam for survival gear, the boat does not come with any survival equipment. Callahan said there is a good reason for this.

“I think it’s a good thing for people to put together their own kits,” Callahan said. Such practice allows people to customize their own survival kits for their own needs and to become familiar with the equipment in the kit before they need to use it, he said.

Despite the survival considerations that went into the design of the boat, it is well suited for everyday use, according to Callahan. At 108 pounds, it is lighter than most dinghies comparable in size, and the tube comes in handy for novice sailors. It acts as a bumper if The Clam gets too close to another boat, he said.

“It’s a great trainer for kids,” Callahan said.

The Clam sells for about $3,800, but comes in a cheaper, nonfolding model. The Clam’s sailing assembly, including a carbon-fiber mast and a battened sail, is sold separately.

Also sold separately is a canopy that fits over the boat’s interior to help keep the elements out of the boat while it is being sailed or rowed.

Andrews said some marine-supply businesses have expressed an interest in selling The Clam at their stores, and some direct sales have been made through HydraNova’s Web site, www.hydranova.com. If demand goes up, HydraNova can hire more than its current 11/2 employees, he said.

“Now it’s just a matter of manpower,” Andrews said. “We know how to build this boat well.”

Patents are pending for The Clam’s boat-folding system, according to Callahan. He said there may be other types of folding boats, such as a kayak, made in the future.

“I’m really happy with the boat,” he said.