|MadSci Network: Physics|
from the basics of rocket propulsion, we know that the thrust is equal to
product of the mass flow rate and nozzle exit velocity plus the product of
nozzle exit area and the difference between the nozzle exit pressure and
The product of the nozzle exit area and the ambient pressure is referred to as the back-pressure. Since the density of water is greater than that of air, it follows that the back-pressure in water is greater than in air; and thus, the thrust is less.