Sailors' superstitions have been superstitions particular to sailors or mariners, and which traditionally have been common around the world. Some of these beliefs are popular superstitions, while others are actually better described as traditions, stories, folklore, tropes, myths, or legend.
The origins of many of these superstitions are based in the inherent risks of sailing, and luck, either good or bad, as well as portents and omens that would be given associative meaning in relation to the life of a mariner, sailor, fisherman or a crew in general. Even in the 21st century, "fishers and related fishing workers" in the U.S. have the second most dangerous occupation, trailing only loggers.
By far the best known sailors' superstitions involve bad luck.
Sailors are taught if the sunrise is red to take warning. The day ahead will be dangerous.
"Red Sky at night, Sailors delight; Red Sky in the morning, Sailors take warning."
This saying actually has some scientific validity, although it assumes storms systems will approach from the west, and is therefore generally correct only at mid-latitudes where, due to the rotation of the Earth, prevailing winds travel west to east. If the morning skies are red, it is because clear skies over the horizon to the east permit the sun to light the undersides of moisture-bearing clouds. Conversely, in order to see red clouds in the evening, sunlight must have a clear path from the west, so therefore the prevailing westerly wind must be bringing clear skies.
A "Jonah" is a long-established expression among sailors, meaning a person (either a sailor or a passenger) who is bad luck, which is based on the Biblical prophet Jonah. Clergymen are considered bad luck, as they are all of Jonah's ilk. Redheads and women are also to be avoided as passengers.
Friday is considered to be an unlucky day in some cultures, and perhaps the most enduring sailing superstition is that it is unlucky to begin a voyage or 'set sail' on a Friday. However, this superstition is not universal. In the 19th century Admiral William Henry Smyth, writing in his nautical lexicon The Sailor's Word-Book, described Friday as
The Dies Infaustus, on which old seamen were desirous of not getting under weigh, as ill-omened.
Sailors are often reluctant to set sail on Candlemas Day, believing that any voyage begun then will end in disaster. This may be related to the superstition to remove all Christmas decorations by Candlemas, a practice done well into Victorian times.
In 18th century New England, rolling clouds and roaring waves were thought as bad omens, so sailing on days under such conditions was considered inadvisable.
Sirens were mythological, often dangerous and beautiful, creatures, portrayed as femmes fatales who lured nearby sailors with their enchanting music and voices to shipwreck on the rocky coast of their island. They were portrayed in both Greek and Roman mythology as sea deities who lured mariners, and in Roman lore were daughters of Phorcys. In the Odyssey, the hero Odysseus, wishing to hear the sirens' seductive and destructive song, must protect himself and his crew by having his fellow sailors tie him to the mast and then stop their own ears with wax (see image).
In another myth, Hera, queen of the gods, persuaded the Sirens to enter a singing contest with the Muses, which the Sirens lost; out of their anguish, writes Stephanus of Byzantium, the Sirens turned white and fell into the sea at Aptera ("featherless"), where they formed the islands in the bay that were called Souda (modern Lefkai).
Another mythological creature, the Scylla, is a similar female sea demon that is both dangerous yet beautiful. Sirens supposedly "lured mariners to their deaths with their melodious, enchanting song", while "Scylla sent countless sailors to the depths of the sea."
The albatross as a superstitious relic is referenced in Samuel Taylor Coleridge's well-known poem The Rime of the Ancient Mariner. It is considered very unlucky to kill an albatross; in Coleridge's poem, the narrator killed the bird and his fellow sailors eventually force him to wear the dead bird around his neck.
Having bananas on a ship, especially on a private boat or fishing yacht, is considered bad luck. This myth can be traced back to the 1700s when the trend of ships carrying bananas sinking was initially observed. As they ripen, bananas release ethylene gas responsible for the rapid ripening of fruits. This ripening effect would hasten the deterioration of the wooden ships, resulting in the formation of leaks and holes, and eventual sinking of the ship, explaining the increased number of ships lost in this time period.
Whistling is usually considered to be bad luck with the possible exception of the sources mentioned below. It is said that to whistle is to challenge the wind itself, and that to do so will bring about a storm. Another tale is that it has been considered bad luck ever since the mutiny aboard HMS Bounty; Fletcher Christian is said to have used a whistle as the signal to begin the mutiny against Captain William Bligh.
Sailors also had a culture of items or practices that were believed to bring good luck, or trinkets to ward off bad luck or the evil eye.
While in many cultures, a black cat is considered unlucky, British and Irish sailors considered adopting a black "ship's cat" because it would bring good luck. There is some logic to this belief: cats eat (or at least kill) rodents, which can damage ropes and stores of grain on board, and they are intelligent animals, a high level of care was directed toward them to keep them happy, and research has backed up this superstition. Evidence published in 2017 by a geneticist the University of Leuven shows that Egyptian cats spread their mitochondrial DNA through shipping lanes to medieval northern Europe. Preliminary results of that genetic study concluded that cats were also carried on trading ships to control rodents, and that practice was adopted by traders from other nations, including Vikings in northern Germany around the 8th to 11th centuries.
Some sailors believed that polydactyl cats were better at catching pests, possibly connected with the suggestion that extra digits give a polydactyl cat better balance, important when at sea. Cats were believed to have miraculous powers that could protect ships from dangerous weather. Another popular belief was that cats could start storms through magic stored in their tails. If a ship's cat fell or was thrown overboard, it was thought that it would summon a terrible storm to sink the ship and that if the ship was able to survive, it would be cursed with nine years of bad luck. Other beliefs included: if a cat licked its fur against the grain, it meant a hailstorm was coming; if it sneezed it meant rain; and if it was frisky it meant wind.
On boats and ships whistling was taboo as it was associated with coded communications between mutineers. The cook was usually excused, because as long as he was whistling, he was not stealing the food.
Traditionally, a type of kobold, called a Klabautermann, lives aboard ships and helps sailors and fishermen on the Baltic and North Sea in their duties. He is a merry and diligent creature, with an expert understanding of most watercraft, and an irrepressible musical talent. He also rescues sailors washed overboard. The name comes from the Low German verb klabastern meaning "rumble" or "make a noise". An etymology deriving the name from the verb kalfatern ("to caulk") has also been suggested. A carved klabautermann image, of a small sailor dressed in yellow with a tobacco pipe and woollen sailor's cap, often carrying a caulking hammer, is attached to the mast as a symbol of good luck. However, despite the positive attributes, there is one omen associated with his presence: no member of a ship blessed by his presence shall ever set eyes on him; he only ever becomes visible to the crew of a doomed ship. The belief in Klabautermänner dates to at least the 1770s.
Sailors have had several patron saints. According to his hagiography, Saint Nicholas calmed a storm by prayer. Brendan the Navigator is also considered a patron saint of sailors and navigators, due to his mythical voyage to St. Brendan's Island. Erasmus of Formiae, also known as Saint Elmo, may have become the patron of sailors because he is said to have continued preaching even after a thunderbolt struck the ground beside him. This prompted sailors, who were in danger from sudden storms and lightning, to claim his prayers. The electrical discharges at the mastheads of ships were read as a sign of his protection and came to be called "Saint Elmo's Fire". Thus, Saint Elmo's Fire was usually good luck in traditional sailor's lore, but because it is a sign of electricity in the air and interferes with compass readings, sailors sometimes regarded it as an omen of bad luck and stormy weather. The mariner's cross, also referred to as St. Clement's Cross, is worn by many sailors to bring blessings.
Mermaids usually are considered lucky, but not universally. In Trinidad and Tobago, sea-dwelling mer-men "were known to grant a wish, transform mediocrity into genius and confer wealth and power."
Mermaids appear in British folklore as unlucky omens, both foretelling disaster and provoking it. Several variants of the ballad Sir Patrick Spens depict a mermaid speaking to the doomed ships. In some versions, she tells them they will never see land again; in others, she claims they are near shore, which they are wise enough to know means the same thing. Mermaids can also be a sign of approaching rough weather, and some have been described as monstrous in size, up to 2,000 feet (610 m).
Hans Christian Andersen's fairy tale "The Little Mermaid" was published in 1837. The story was adapted into a Disney film with a bowdlerized plot. In the original version, The Little Mermaid is the youngest daughter of a sea king who lives at the bottom of the sea. To pursue a prince with whom she has fallen in love, the mermaid gets a sea witch to give her legs and agrees to give up her tongue in return. Though she is found on the beach by the prince, he marries another. Told she must stab the prince in the heart to return to her sisters, she can't do it out of love for him. She then rises from the ocean and sees ethereal beings around her who explain that mermaids who do good deeds become daughters of the air, and after 300 years of good service they can earn a human soul. A world-famous statue of the Little Mermaid, based on Andersen's fairy tale, has been in Copenhagen, Denmark since August 1913, with copies in 13 other locations around the world.
Sailors would look for mermaid's purses on beaches for signs of mermaids in the area.
Davy Jones is a popular character in sailor's lore, especially of the gothic fictional variety. Davy Jones' Locker is an idiom for the bottom of the sea: the state of death among drowned sailors. It is used as a euphemism for death or burial at sea (to be sent to Davy Jones' Locker). The origins of the name are unclear, and many theories have been put forth, including an actual David Jones, who was a pirate on the Indian Ocean in the 1630s; a pub owner who kidnapped sailors and then dumped them onto any passing ship; the incompetent Duffer Jones, a notoriously myopic sailor who often found himself over-board; or that Davy Jones is another name for Satan; or "Devil Jonah", the biblical Jonah who became the "evil angel" of all sailors, who would identify more with the beset-upon ship-mates of Jonah than with the unfortunate man himself. Upon death, a wicked sailor's body supposedly went to Davy Jones' locker (a chest, as lockers were back then), but a pious sailor's soul went to Fiddler's Green. This nautical superstition was popularized in the 19th century.
In relation to language and cultural practices, such sailors' superstitions are the consequence of folkloric practices or traditions whose meanings were once important signifiers, but now are lost to most modern sailors and laypersons alike.
Sailor tattoos are a visual way to preserve the culture of the maritime superstitions. Sailors believed that certain symbols and talismans would help them in when facing certain events in life; they thought that those symbols would attract good luck or bad luck in the worst of the cases:
Sailors, at the constant mercy of the elements, often feel the need for religious images on their bodies to appease the angry powers that caused storms and drowning far from home.-- Tattoo Archives 
For example, the images of a pig and a hen were good luck; both animals are not capable of swimming, but they believed that God would look down upon a shipwreck and see an animal not capable of swimming and would take them into his hand and place them on land. In reality, most of the smaller livestock aboard ships would float in their wooden crates and would be among the only survivors of wrecks, adding further reason to getting the tattoos on sailors' feet, as it was believed to help them survive a wreck. Another example of superstitions is the North Star (Nautical Star or compass rose); sailors had the belief that by wearing this symbol it would help them to find his or her way home. Sailors designed mariner motifs of their own, according to their travel experiences in the ocean. By the 19th century, about 90% of all United States Navy sailors had tattoos.
The anchor is commonly used in sailor tattoos, which were supposed to prevent a sailor from floating away from the ship, should he fall overboard. The words 'HOLD FAST' tattooed on the knuckles would prevent a sailor from falling from aloft.
The Line-crossing ceremony commemorates a sailor's first crossing of the Equator. Its practices invoke good luck on the new sailor. The ceremony of Crossing the Line is an initiation rite in the Royal Navy, Royal Canadian Navy, U.S. Navy, U.S. Coast Guard, U.S. Marine Corps, and other navies that commemorates a sailor's first crossing of the Equator. The tradition may have originated with ceremonies when passing headlands, and become a "folly" sanctioned as a boost to morale, or have been created as a test for seasoned sailors to ensure their new shipmates were capable of handling long rough times at sea. Sailors who have already crossed the Equator are nicknamed (Trusty) Shellbacks, often referred to as Sons of Neptune; those who have not are nicknamed (Slimy) Pollywogs (in 1832 the nickname griffins was noted ).
After crossing the line, Pollywogs receive subpoenas  to appear before King Neptune and his court (usually including his first assistant Davy Jones and her Highness Amphitrite and often various dignitaries, who are all represented by the highest ranking seamen), who officiate at the ceremony, which is often preceded by a beauty contest of men dressing up as women. Afterwards, some wogs may be "interrogated" by King Neptune and his entourage. During the ceremony, the Pollywogs undergo a number of increasingly embarrassing ordeals (such as wearing clothing inside out and backwards; crawling on hands and knees; being swatted with short lengths of firehose; kissing the Royal Baby's belly coated with axle grease, etc.), largely for the entertainment of the Shellbacks. Once the ceremony is complete, a Pollywog receives a certificate  declaring his new status.
U.S. President Franklin D. Roosevelt described his Crossing the Line ceremony aboard the "Happy Ship" USS Indianapolis with his "Jolly Companions" in a letter to his wife Eleanor Roosevelt on 26 November 1936:
Marvelous costumes in which King Neptune and Queen Aphrodite [sic.] and their court appeared. The Pollywogs were given an intensive initiation lasting two days, but we have all survived and are now full-fledged Shellbacks"-- FDR 
As of 1/1/16some cruise ships also have a line-crossing ceremony for their passengers.
The phrase over a barrel, meaning to be in a dilemma or "a weak or difficult position", may refer to the first aid practice amongst sailors of placing a drowning victim's head over a barrel, and rolling his body over it, in an attempt to remove aspirated water from the person's lungs. However, this etymology is challenged, and may come instead from the custom of punishing a prisoner by flogging or paddling him while he is strapped to a barrel; there is no documentary evidence it was actually used specifically as a nautical phrase. Either way, the image created in the mind is that of total helplessness and loss of control, which is a common anxiety of sailors in fear of corporal punishment.
Many theories have formed as to explain these phenomena and why they exist. Jakob Jakobsen theorised that such practices existed with the intent to confuse and ward off fairies and protect the sailors. Lockwood concurred as well with the general line of thought and concluded it was also done so as to not summon dangerous animals such as whales or ravens. Solheim follows this same line of thought and considers it also be done to protect loved ones back on land from such animals and spirits. Bairbre Ní Fhloinn critiques this line of thought concluding it to be reductionist and insulting to the intelligence of fishermen however she does admit it may be a factor.