Sink-or-Swim Fatherhood

Ding-Dong!

Oh, hi, Honey! I know we've been divorced seven years and haven't seen or spoken to each other since we signed the papers, but this little girl here is your daughter who I never told you about even to demand child support payments. I have an important conference out of state, and until I get back, she's your problem!

Dad: WHAT!!!???

And now the hijinx begin.

This trope deals with the after effects of either a Luke, You Are My Father situation, or a sudden Street Urchin adoption. Unlike other dads, this man hasn't raised this child since birth and doesn't have the level of experience that most dads at this point in the child's life would be expected to have. He doesn't know how to braid a little girl's hair or get a toddler to eat his vegetables. And even if he took the child in of his own volition and is happy to have a youngster in his bachelor pad, he is going to go through a period of acclimatization.

To qualify for this trope, a male (who need not necessarily be related to the child, simply acting in a fatherly role) must suddenly be given sole responsibility over this child AND not have enough prior experience in this capacity to know what the heck he is doing. Maybe there was a divorce, and the kid lived with Mom but she died or ran off. Maybe he just took in an unrelated child off the streets, or his Kid From the Future ended up on his doorstep. Either way, he has to shape up fast.

Often goes hand in hand with Luke, You Are My Father, Children Raise You, Missing Mom and Resentful Guardian (but only at first). If the 'Dad' in question is not actually the father but some other relation or not blood related at all, it will result in Promotion to Parent. Not actually related to Bumbling Dad. Not to be confused with Sink or Swim Mentor.

Examples of Sink-or-Swim Fatherhood include:

Anime and Manga

In Bunny Drop, Daikichi, despite being a bachelor, adopts his grandfather's illegitimate 6-year old daughter (yes, that makes her his aunt), when no one else in his family would take on the responsibility. It could be a subversion in that he has to ask for advice and goes through a lot of crap at work trying to make time to be able to pick her up from preschool, but all in all he does a really good job.
Otaku no Musume-san is about an otaku who gets his formerly unknown daughter dumped on him by his high school sweetheart. Furthermore, the climax of the story is what happens when the mother comes back and intends to take the daughter back.
In Black Cat, Sven adopts Eve after rescuing her from a weapons dealer. Do clones who came out of a test tube and technically never had a biological mother or father count?
- Yes, there doesn't have to be a blood relationship as long as Promotion to Parent takes place.
- Later on, the gang does discover Eve's creator and biological genitor (that is, the scientist who cloned Eve from herself); both feel a sort of filial connection and there's talk about Eve staying with her but they commonly decide not to.
Sakende Yaruze features a type two, where the father, unaware that he had a kid, has his seventeen year old son show up on his doorstep one day after his mother's death. Though they do manage to build something of a relationship in spite of their conflict, they ultimately can't make it work and the son moves back in with his maternal grandmother at the end.
This happens in the backstory of Yotsuba&!. Koiwai adopts Yotsuba without having any prior parenting experience.
In My Girl by Mizu Sahara, the hero gets his formerly unknown five-year-old daughter dropped in his lap when his high-school sweetheart dies. The story is played for the heart-warming drama.
- In subversion, he got a lot of help from his own parents and his dead lover mom.

Fan Works

Dante as ten-year-old Nero's father in the AU Devil May Cry fic Like Father, Like Son, fully demonstrated in chapter 6. Trish also lampshades it in the same chapter, chastising Dante that he has to "grow up, at least a little bit" and "shape up". Patty doesn't quite count as Dante's kid, as she visits the shop instead of permanently living there.

Film

In Jungle 2 Jungle, Tim Alan's character is suddenly dumped with his thirteen year old son who has been raised by his mother with indigenous people in the amazon, because his mother told him when he was a man he could go visit his Father, and based on tribal traditions he's a man at this age.
In The Game Plan, Dwayne Jonhson's character is a professional football player who suddenly is stuck with his 8-year old daughter who he didn't know existed until she showed up on his door step. He doesn't take it well.
In Matchstick Men, Nicholas Cage's character suddenly has to take care of his 14-year old daughter, who isn't actually his daughter and isn't 14, but part of a plan to distract him while his partner steals him blind, whom he hadn't seen since she was in the womb. Considering he's a neurotic, obsessive compulsive, bachelor con artist you can guess how well prepared he is for it.
In Despicable Me, Gru adopts three little girls as part of his own insidious plans. At first he ignores them and does a terrible job parenting, having no intention of actually being their father, only to use them, but eventually he comes around and really gets into being a Dad, and takes on the challenge for real.
Buddy's biological dad in Elf is this, although the focus of the film is elsewhere.
In Breakfast With Scot, the titular Scot's mother dies of a drug overdose, and custody is supposed to go to her ex-boyfriend (who isn't Scot's biological father). Said boyfriend being a jerkass off in Brazil, Scot ends up with the jerkass's brother and his partner.
In Big Daddy this happens to the main character's roommate but the roommate is out of town when the kid arrives so the main character tells child services he's the kid's father. Eventually child services finds out the main character was lying and a trial ensues where the child's real dad saves the day.
Three Men and a Baby (and it's French original version, Trois Hommes et un Couffin).
The movie Parenthood has Larry, whose response to having his newly discovered son dumped on him is to in turn dump the kid on his parents.
Gender-flipped in Raising Helen, where high-living Helen ends up as a single mom to her newly orphaned nieces and nephews. Her new love interest, the local pastor, adjusts far more easily to the kids.
Old Dogs: Robin Williams' character discovers that his short marriage resulted in twins when his ex-wife dumps them on his doorstep and asks to take care of them while she faces jail time for an environmental protest.

Literature

In Mostly Harmless, Trillian leaves her daughter in Arthur's care, saying that she is his daughter, too. However, it's only true in biological sense: her pregnancy was via artificial insemination and Arthur just happened to be the only human sperm donor left in the universe after the destruction of Earth.
In The Dresden Files novel Changes, Harry learns that he has a daughter with his ex-girlfriend Susan, who has been kidnapped by vampires and needs to be rescued. While not exactly an exact example of this, he does remain Maggie's only parent after Susan's death at the end of the novel.
The Suvi Kinos series begins with the titular character being orphaned in a car accident. She ends up in the care of her uncles. Her five hulking, academical doctor uncles. They are varying mixes of caveman and nerd, so hilarity ensues. For example, when little Suvi cries to not be pacified by default maintenance, they look up a childcare book, and end up desperate because there's no index entry for "vocal alarm". The entire first book, dealing with Suvi's years 0 to 6, is a big zigzagging of this trope.

Live Action TV

Ally McBeal somehow managed to reverse this trope an have a sink or swim mother.
My Two Dads is made from this. Woman dies, wills that the father of her daughter should raise her - but it could be one of two guys (it started before genetic testing was mainstream, though this was addressed in a later episode). So they both raise her. Neither one knew of the child until the death of the mother.
Happens to Worf twice on Star Trek: The Next Generation: the first time, he learns about his son Alexander shortly before Alexander's mother is killed, and about a season later, Worf's parents (who have been looking after him) send Alexander to the Enterprise to live with his father.
Happens in Badger. Much of the domestic plot is driven by Wilf, the teenaged daughter Tom McCabe never knew he had, who rocks up on his doorstep.

Video Games

In the backstory of Triangle Heart 3 ～sweet songs forever～ (and thus, presumably, Magical Girl Lyrical Nanoha), Shiro Takamachi in his youth had an affair with a woman named Kaori, who left him, then returned a year later with their baby son Kyoya and ran away again, leaving him to raise the boy alone.

Western Animation

The Family Guy segment with Peter's imaginary sitcom "My Black Son" where a preteen boy shows up on his front step and the lyrics comment "Don't even remember sleeping with that lady."
- Al-so-he's-a-nin-ja!