I'm not up for an argument either, just using this example to point out the lose of knowledge due to simplified text books.
As some homework, have a look into why gliders carry water as ballast on cross country flights. Its very interesting and will help to increase your understanding on the implications for your powered aircraft.
This is real world knowledge that you guys need to know and backs up my opinion that some text books are not worth the paper they are written on.
P.S. Don't trust everything you find via google, it could be written by someone with the same or less knowledge then yourself!