"A hősök hajója kiúszik az óvónő tavára."

Translation: The heroes' ship sails out onto the kindergarten teacher's lake.

[richardkiss]

Why is it "hajója" when "hősök" is plural?

[JanTatouse]

Have a look at this page http://www.personal.psu.edu/faculty/a/d/adr10/hu8.html and look up the sentence "A gyerekeknek nincs jó [tanáruk/tanára]". So if I understand it correctly, hősök is enough to indicate the plural possessor. Another indication is not necessary. Without the word hősök, the sentence would probably look like this: "A hajójuk kiúszik az óvónő tavára".

[vvsey]

Yes, that is correct. Btw, this is only happening with the third person plural.
Here is another discussion with some explanation.
https://www.duolingo.com/comment/18828701