There's no doubt that San Francisco earned its reputation as a place where unwary or unlucky sailors risked being pressed into service on board ships departing for long voyages across the Pacific. Forced labour of this sort was relatively commonplace in the years roughly from the 1840s until about 1915; certainly there must have been many thousands of cases in this period, and an entire group of procurers, known as "crimps," sprang up to supply this need.

There were a couple of reasons why shanghaiing was a relatively common practice and why it was specifically associated with San Francisco. Merchant seamen were itinerants in this period. Rather than serving as part of one ship's crew for relatively lengthy periods that covered several different voyages and deployments, as was the case in the navy, they signed up for only single voyages and had the option of leaving a ship at the end of that voyage if they were not happy with the master, the conditions, the pay, or indeed the next planned port of call. They might also choose to leave a ship out of the necessity of requiring paid employment, since they were paid only while they were actually at sea; a ship that stayed in port for a few weeks or months, awaiting her next cargo, was costing these men money. Thus, while a competent ships' captain would do everything in his power to retain a core of trusted, skilled men around him, he would also typically lose some members of his crew, and hence need to recruit ordinary seamen every time he was in port. In the 19th century, these men were supplied via a largely informal system focused on boarding houses along the waterfront. Sailors looking for work could find it by applying to the well-connected men and women who ran the major seamen's flophouses; masters looking to top up their crews passed out their requirements lists to the boarding house keepers known to supply men, who not infrequently had a reputation for providing these bodies by less than above-board means. A bad master, or a poor or unsafe ship, would always struggle to attract sufficient men, and it was in cases such as these that the crimps turned to illegal methods of forcing unwilling recruits on board.

The problem was particularly associated with San Francisco both because it was the main US Pacific port, and so a starting point for many very long trans-oceanic voyages of the sort no man would want to undertake on a "hell-ship," and because at the very start of the period, in 1849, the California gold rush created such an hysteria that it was common for seamen to desert their ships in large numbers in order to join the rush to the goldfields, creating a major manning problem for the ships arriving at the port that could only be solved by kidnapping unwilling crewmen from the bars and flophouses along what became known, for its lawlessness, as the "Barbary Coast."

Getting the hapless men who became victims of "shanghaiing" (a phrase that dates to 1872) onto ships was one of the riskiest parts of the job, and a considerable folklore has sprung up around it. This lore incorporates not only trick chairs positioned over trap doors (an idea that can be traced back at least as far as the legend of Sweeney Todd, the "demon barber of Fleet Street", which in turn has its origins in the Paris of the early 17th century), but also secret tunnels running down to the shore. And certainly rendering these men insensible, through drink or drugs, was a technique occasionally used by crimps such as the infamous 'Scar-Face' Johnson, Michael Connor – a terror of the 1880s – Paddy West and 'The Shanghai Chicken,' John Devine, the last of whom went to the gallows in 1873. Hiram P. Bailey - who quite literally wrote the book (albeit a semi-fictional one) on being shanghaied out of 'Frisco in the '90s, spoke of being slipped mickey finns in a seafront bar and waking up on board a hell-ship that he calls the Washington, notorious for her vicious master, which was "putting to sea with two clergymen, three bar-tenders, four agricultural labourers - all shanghaied - among a more or less nautical crew of thirty men." Davidson describes the "standard concoction" served by crimps to men they planned to shanghai as a mix of "whiskey, brandy, gin, and opium," capable of knocking a man out for days; this was the "Miss Piggott Special," so named after a Barbary Coast landlady rumoured to serve the drink to unwary custmers. A notice in the California Police Gazette, similarly, warned of "strychnine whiskey," served in San Francisco in South of Market bars, which had an almost identical effect.

This said, there is absolutely no credible evidence for the use of trapdoors to secure victims, as illustrated here, either in San Francisco or anywhere else; ditto for secret tunnels, in San Francisco at least (there is some evidence they were used in Portland). Bill Pickelhaupt, author of one of the key studies of crimping and shanghaiing, studied oral and written accounts for years without coming upon a verifiable case, and there are a couple of good reasons why this should be so: first, the practice would have risked injury to a man who needed to be able-bodied to be of much use to his new employer, and, second, the law required sailors to sign ship's articles, something there was no guarantee a man who had been kidnapped or forced aboard would do. It was far easier to target men who, while they might not want to undertake the voyage, grudgingly accepted they had little alternative but to do so.

Mark Strecker comments that while crimps

used drink, drugs, coercion, and trickery to shanghai men, most preferred to employ financial blackmail. It worked like this: seamen chronically lacked money, so a crimp extended them far more credit than they could possibly repay. The crimp then offered those indebted to him the choices of debtors' prison - something many American states and territories had - or taking a berth on a ship of the crimp's choosing. Most did the latter. The crimp took what they owed him from their advance... The loss of this portion of earnings caused the affected to have short wages at the voyage's end. To make up the difference, they had to get credit from another boardinghouse master, starting the process over.

It would be remiss of me not to mention that there is some evidence that crimps did make use of trapdoors at one key point in the shanghaiing process – Davidson mentions "dead-falls" in wharf-front warehouses, "through which shanghaied sailors were shoved into rowboats waiting below." But Strecker's book also gives us an interesting example of the consequences of employing the sort of coercive methods you are interested in:

One Sunday night a seaman named Patrick Grant went carousing along [the Shanghai] waterfront. The next morning he awoke on a strange ship as the newest member of her crew. Feeling ill beyond the effects of a hangover, he informed the second mate he could not work because he had a nasty fever. During the four days [the voyage] lasted he received not a morsel of food. Somehow, his plight caught the attention of Mr Brown, a British vice-consul, who charged Grant's master, Captain Murphy, with illegally bringing a man on board without first having him sign the ship's articles, a violation of the Merchant Shipping Act of 1854. For his defense, Murphy produced articles with Grant's signature, saying he had put it there himself as Grant could not write. Grant proved his literacy and went free. The judge fined the captain a mere £10 for his violation as he felt Murphy had meant no harm.

Shanghaiing continued to flourish until the very early 20th century, in part because, as Davidson points out, "as members of a migratory class unable to vote for protective legislation, sailors formed a classic lost community in a democracy." It took organisation and unionisation to force the legislative changes that outlawed crimping and made shanghaiing a thing of the past. The Dingley Act (1884), a Federal law that prohibited payment of advance wages by masters to crimps, was one significant blow to the shanghaiing system, as was the formation of the Seaman's Union in 1885 and the agitation that led to the Maguire Act (1885) and the outlawing of the crimps' other major stand-by, the attachment of sailors' clothing. This was followed by the White Act (1898), which limited the amount a sailor could be required to pay to any "original creditor".

Between them, these three acts – and the end of the sailing ship era, which had required a large number of semi-skilled crewmen used largely as muscle – pretty much finished off the crimps of San Francisco – but that does not mean that sailors did not continue to risk shanghaiing elsewhere in the world for many years to come. That risk, though, was more mundane than legend suggests; as Milne concludes, "Crimp stories were ... an inside joke rather than a serious attempt to deceive... They signal the core nature of the crimp's trade, which was to extract every possible monetary value from the seafarer. Old seamen gave the impression that it had been an honour to be shanghaied by famous crimps, albeit after many years in which the memories had been softened through yarning."

Sources

Hiram P. Bailey, Shanghaied Out of 'Frisco in the Nineties (1925)

Lance S. Davidson, "Shanghaied: the systematic kidnapping of sailors in early San Francisco." California History 64 (1985)

Graham J. Milne, People, Place and Power on the Nineteenth-Century Waterfront (2016)

Bill Pickelhaupt, Shanghaied in San Francisco: Politics and Personalities (1996)

Georgia Smith. "About that blood in the scuppers." In James Brook et al, Reclaiming San Francisco: History, Politics, Culture (1998)

Mark Strecker, Shanghaiing Sailors: A Maritime History of Forced Labor, 1849 to 1915 (2014)