Payment by hours never works, payment by results or pricework does. If a worker is paid an hourly rate, they'll stretch the 40 hours to 60 or 80, it's why the pubs here are full of builders at 3pm. They only do what they consider you pay them to do. It doesn't matter how much you put on them.

I ran many massive projects for years and always put a price on the job rather than expect them to sit on it. Some of my clients didn't understand and wanted to see 'bodies' there last thing of the day, but we always got the jobs done, on time, and on budget, with increased profitability.

It's human nature not to do more than you have to. I was exactly the same to my bosses. When my contract demanded 60 hours I still only worked what I considered 40 hours worth. There are many ways to eke out a day.