A long time ago when I was in college there was a problem in the back of a chapter on thermodynamics in my freshman chemistry textbook: A newly designed ship travels the tropical seas by using the warm tropical seawater,converting the heat from the seawater to work, and then expelling ice cubes back into the sea after energy was extracted from the water. That is it. The question for this problem may have been to explain why this is impossible, why it is impractical or something like that. Anyway, the problem was not assigned and there was no discussion of it in class. Years later having forgotten most of my thermodynamics I still remember the problem.Is it impossible? Why? For ease of calculation let's say the temperature of the tropical seawater was 80 degrees F. After doing the work the ship expels water at 33 degrees Farenheit. I know the Second Law says that you can't convert heat into an eqivalent amount of work,not with 100% efficiency.But there are billions of gallons of tropical seawater in ocean. Even if the conversion is only 10% efficient, it should be enough to power the ship across the ocean, wouldn't it?