Just wanted to submit a brief thing about the use of human forms in my fanfics.
First: why? The concept of human forms was influenced by the Transformers fandom and the idea of "holoforms" or solid holograms. I got the idea after watching TUGS and thinking "if they don't have human crews, then how are they capable of doing some of these things?" Well, and some lines like "I'm coming aboard!" which obvious a boat can't do. But, if they can project themselves as humans, then it would make sense.
How does it work? Obviously, the engines and boats have some kind of soul and personality. For them, using their human forms is kind of like astral project: letting their spirit leave their bodies for short periods of time.
What are the human forms like? They are like solid ghosts. They feel real (skin like real skin, fabric like real fabric) and can pick up/interact with objects. However, they don't breathe, eat, drink, use the bathroom, bleed, or do anything else a living human might do. The one exception is that engines may consume food to fuel their human projections for longer.
Restrictions: Human forms always appear in the cab/wheelhouse of a vehicle, no matter where they dissipate from. Engines have a range of about a mile from their real bodies, ships can go further. A human form also usually only lasts anywhere from about an hour (for engines) to a few hours for ships. Using a human form is exhausting. Time can be extended however, by consuming food to convert for energy.
If a living machine falls asleep in their human body, they automatically revert to their real body. A good shock (like a fall, or a punch in a fight) can also send them back to their real body.
Clothing: Most have an automatic outfit that appears whenever they go human. A living machine can change this to anything they like with some concentration, however.
"Adult things" There is generally no desire at all to have sex for living machines, even in their human forms. The act would be fairly unsatisfying anyway, since their genitalia are not real and do not respond the way that a real human's would.
However, since human forms are a projection of a machine's soul, touching in human form is like touching souls. It can be a very powerful experience allowing for the exchange of energy, or even thoughts and feelings, if done right.
How to tell a false human form from a true human: Beyond the whole "not breathing, cool skin" thing. Their eyes have no iris, only a black pupil. (This is generally the most obvious sign.) Their bodies may also glow slightly, or seem "unreal". Other living machines can easily spot their own kind simply because they 'feel different' from real humans.
Anyway, that's all I have so far! May edit this later if I come up with other stuff. Feel free to ask questions if you want or whatever.
Listening to: Worthless
Reading: Rocketeer Adventures
Watching: Iron Man:Armored Adventures