Space Ship Boys by Erik Ritler.
The story tells what happens after the sirens sound for real it is not a practice drill this is it, the evacuation is on. Earth is dying, and those who can, those who are accepted, find a place on one of the massive space ships that have been prepared waiting ready for this day.
Devon and his friends, along with most of the high school boys end up on EV5995 starting an 18 year voyage at near light speed to find a new planet. What follows is a vivid description of coming to terms with life onboard a huge biosphere shared with 5000 other boys. And coming to terms with who they are themselves, how will they live in a world with only boys!
There is intrigue and romance, joy and sadness. The characters are very real and each have their own personality, they are brilliantly written and you will fall in love with them. You will laugh and you will cry along the way.
These are all teenagers, Devon is 17, there is plenty of sex, but that is just one part of their lives, and one part of the story. There are some interesting observations about how life onboard works, and detailed descriptions of the biosphere and how it functions. The story is very well researched, and very well written. The author writes notes with each chapter, setting a particular piece of music to parts or the whole of a chapter. He explains how certain bits came to be written.
It is not often I would recommend a book that is unfinished, but I loved this story and became very attached to the boys. Chapters 1 to 15 are themselves a complete book, which finishes appropriately, but nevertheless does not resolve everything going on in the plot. The story continues with another seven chapters which really do take things forward and develop the story. The final published chapter is a bit of a cliff hanger and that unfortunately is where it ends.
I should mention Devon the principal character becomes a cook onboard, and that results in some interesting recipes. Charlie a very close friend gets to work on manufacturing clothes, and this starts with what seems like the easiest place to start underpants!
I don't think that the author intentionally left the story in mid-air, but rather that something happened to prevent him continuing. Don't be put off by it being unfinished you will live every moment and the boys will become your close friends!