Peter Emanuel fell on his head in July while mowing the lawn on a sharp hill at his home on River Road in Waterford. Not thinking much of it, he did what guys do: dusted himself off and kept on cutting.

Then, several weeks later during a regular checkup, his doctor recommended a CAT scan and X-rays on hearing that Emanuel was having trouble with coordination while walking. When the results came back, the doctor immediately gave Emanuel, a retired history and music teacher at the Williams School in New London, a direct order.

Get to the hospital immediately.

Emanuel did what he was told and soon found out his brain had been slowly developing an internal pool of blood. The acute brain bleed, if left undetected, could have killed him. Now, after three surgeries, Emanuel, a well-known musician in the area who had just completed publication of his first book a few months before his September diagnosis, has returned to his old self, with a scar on his head but seemingly none the worse for wear.

Clearly Emanuel is a determined sort, and his book titled “Course Change” is a testament to his ability to stay focused on a fascinating slice of local history that no one has previously told in full. The book, subtitled “The Whaleship Stonington in the Mexican-American War” (Globe Pequot Press, $32.95 hardcover, available on Amazon and Bank Square Books, among other outlets) recounts the previously little-known tale of a ship based in New London that in 1846 was ordered into the service of the U.S. Navy off the coast of California, and whose men were largely responsible for saving the city of San Diego from a band of locals aligned with Mexico.

“Just one little smelly old whaleship changes the course of history,” Emanuel told me when I visited his home overlooking the Niantic River on Wednesday. “California wouldn’t have been the California we know (without the crew’s intervention), not for a long time.”

The Stonington whaleship had been to sea about three years and was returning home to Connecticut when it sailed to San Diego looking for supplies and someone who could fix a damaged rudder. What they found instead was a town in distress, facing imminent attack from a larger force of so-called Californios intent to confront the rising power of American settlers who would later go on to establish the state of California in 1850, two years after the conclusion of the Mexican-American War.

And only the 40 people on the whaleship could provide the instant manpower and firepower to save the American contingent in San Diego. They didn’t have to do it. But seeing it as their humanitarian duty (and later getting a prod from the U.S. government that essentially conscripted them in the war effort), the crew reinforced Americans in danger of an attack and ditched the ship’s payload of whale oil in favor of installing cannons on a reconfigured deck.

Amazingly, there was no hero’s welcome for these men when they returned to New London, or a story in the local newspaper, nor was there ever a monument erected to them in San Diego to tell their story. But now people everywhere can read Emanuel’s well-researched book, based largely on ship logs, that names the heroes and puts a bit of personality to their four-month military adventure by imagining some of the dialog they might have had with one another in a story that uses a technique called historical narrative.

“There’s very, very little dialog in the actual log book,” Emanuel said. “It’s pretty terse.”

All the events really did happen, and all the characters are named in the ship log, but some of the scenes are recreated to bring out personalities that can be discerned from source material but never definitively verified. Through it all, the whaleship’s second in command, the 23-year-old Alanson Fournier of Long Island, turns out to be Emanuel’s main character, eclipsing the influence of captain George W. Hamley, an irascible New London shipmaster who winds up being captured by the Mexicans as Fournier contemplates a rescue before bringing the ship home.

The story puts the local whaleship right in the middle of the war with Mexico as it transports troops to and from battlefields, helps supply food and water to soldiers and townspeople and rounds up livestock to keep the hungry masses fed. It also includes encounters with some of the most famous historical personages of the time, including John C. Fremont, who went on to become the first Republican Party candidate for president; Edward F. Beale, a naval officer during the Mexican-American War who delivered the eulogy for President Ulysses S. Grant, and Robert F. Stockton, a naval commander who later became a U.S. senator.

Along the way, Emanuel takes us inside the cabins and living quarters of the whaleship and introduces us to an interesting cast of characters who face all the exigencies of being at sea, along with the difficulties of war and their own personal foibles. It’s a true melting pot of American life, including several African-American characters who are able, way ahead of their time, to be judged for their personal skills and character rather than being consigned to inferior status based on their skin tone.

Emanuel’s book odyssey began about a dozen years ago when he got a stipend from the Mystic Seaport to update its website by doing research and writing about several artifacts in its archives. One of the items he heard about in his research, though he didn’t write about it initially, was the ship’s log from the whaleship Stonington, which in the early part of the voyage was kept by the captain and then, later, by Fournier.

Emanuel was able to access a copy of the log from the Library of Congress, but “what was really cool was to have the physical logbook” provided by the Seaport. Having access to the real copy rather than the transcription enabled him to discover lines and words from the book that had been missed by the transcriber, plus there was a slip of paper tucked inside that had not been transcribed which served as a epilogue for the story.

The story told in the log book intrigued Emanuel, and in his spare time, mostly during the summers, he started laying the groundwork for writing a book of the whaleship’s Mexican-American War adventures by filling in the blanks of how the Stonington’s experience fit into the broad expanse of nearby battles.

“The only way I was able to finish the book was by retiring,” Emanuel said.

Still, when he initially sent out a book proposal, Emanuel had only 50 pages written. This was the story as a pure history book, without embellishments. But he was told a typical book had to be at least 250 pages, so at the suggestion of an editor he decided to imagine the characters on board the whaleship, and soon they were “talking” to him through his knowledge of their backgrounds enhanced by imagination.

In the course of his research, Emanuel has taken a couple trips to San Diego to get the lay of the land, and his descriptions of the territory were enhanced by these sojourns along with visits to museums in the city. It’s a rollicking adventure story that Emanuel has crafted in a way that pays homage to the tenacity of these local whalemen who helped America lay claim to California.

And as Emanuel found out when he spied that slip of paper in the log book, they may have returned home from a long whaling ship without any of the lucrative oil that would have allowed some to enjoy a few days with money in their pockets, but the ship owners did provide to the U.S. Navy proof that the men (and the ship) had been actively engaged in military service during their time in California.

“That’s the one question that a lot of people ask,” Emanuel said. “Did they get paid?”

Most likely they did, though Emanuel couldn’t find definitive proof. But, like in many a whaling voyage, pay was more of an afterthought anyway; it was the adventurous spirit that drove the sailors, not thoughts of riches.

In Emanuel’s book, they finally get some measure of recognition nearly two centuries after the Mexican-American War for some of the best characteristics of humanity: bravery, compassion and teamwork. He will talk about the book at 11 a.m. Saturday, Nov. 16, at the East Haddam Grange Hall now that his own “Course Change” has taken its course and he is fully recovered from the kind of calamity that the characters in his book knew all too well.

Lee Howard is The Day’s business editor. To reach him, email l.howard@theday.com.