McHale's Navy meets the Time Tunnel
Two things to note up front with this book: First, the description on the book cover is flat out wrong as to the mechanism of time travel. I have no idea why, it's not a spoiler and is revealed within the first few pages. Second, I gather from the reviews that there are two distinct versions of the book, a shorter 1965 version and an expanded 1978 version. I read the longer edition with cover art by Steve Hickman.
It's an enjoyable book with a lot of humor and constant action. A crew of misfit sailors have been assigned to an ancient 89 foot wooden sailing vessel in order to help Dr. Krom, an old navy scientist, test new submarine detection gear that is too sensitive to operate on a vessel with a metal hull. The crew includes the usual assortment of naval misfits including Joseph Rate, the competent captain who wants a better crew and a modern ship, a naively religious sailor named McGrath, at least two sailors, Gorson and Cookie, who are suspected of being smugglers, and a few who are good sailors but trouble makers of one sort or another.
The book wastes no time in revealing that Gorson and Cookie have been building an illicit and very complex vacuum distillation still onboard. By page four the two have got their still going just in time for a highly unlikely lighting strike to interact with the still and a depth sounder, triggering a time jump that lands the ship in the path of a Viking vessel. They survive the Vikings and soon figure out what triggered the time jump. They learn to control when a jump happens but they can't control how far they jump and all jumps seem to take them back another 1000 years.
They tangle with unfriendly naval vessels from several eras with time jumps being their only escape in most cases. Before each jump, they usually pick up one or two new crew members; a woman rescued from the Vikings, an Iman, a small island of women castaways. They also encounter other time travelers who've had problems with lighting/still interactions including a prohibition era moonshiner who operated from a houseboat and a medieval monk who distilled wine for a monastery, both of whom join the crew in hopes of getting back to their respective futures.
Unless they can figure out a way to go forward in time, they're going be in big trouble soon because they're running out of civilized times to escape too. An even bigger worry is what happens if the ship appears on dry land instead of in the ocean after a jump. Both worries are realized when a desperate escape puts the ship in the middle of a muddy prehistoric river basin surrounded by large, hungry animals. The entire crew will have to work together to find a way to stay alive, much less return to their own time.
It's a fun book and worth a read if you run across it.