Ballast tank fluid dynamics

[Polarity]

Originally Posted by marshmat

Not only the pipe and fittings are at fault, but there can still be resistance to the flow after it enters the tank, due to whatever medium is already in there.
When you fill from the top, your water is exiting the pipe into... air. At ~600 times less dense than water, this presents little resistance, thus the flow loses very little momentum as it exits the nozzle. (Ever notice how jetboats almost always have the pump nozzle above the operating waterline?)
When you fill from the bottom, the static head you have to contend with is much less than when you fill from the top. At least, it is at first. Once you are discharging into water, though- and with a free surface, no less- things get unpredictable. If the velocity of the outflow is high, thus a fair bit of momentum and a fair bit of kinetic energy tied up in the moving water, the back pressure on the outflow stream can (note I say can, not will) increase significantly. I doubt your tanks are designed to maintain clean, laminar flow within the tank? Because there's no way in hell that they can. So you get turbulence, lots of it, and you get an outlet stream that is decelerated very rapidly upon exiting the nozzle. My understanding is that this translates to a higher pressure in the nozzle and its supply line, thus a higher pressure at the pump outlet- possibly as high, or higher than, the pressure on the pump outlet in the top-filling case where there is no additional resistance after the flow exits the nozzle.

Great! Thanks Mat
Thats what I was looking for, the output velocity is high, and the pump and scoops are back pressure sensitive, so it looks like (especially combined with Will's comment) that the turbulence and deceleration will cause a greater back pressure loss than the loss of the always higher head of the top fill.
Now to see if I can make it work in reality. I have an idea of bringing the top fill from the pump through the tank face at a shallow angle to avoid the 90 deg bend. bringing the scoops in is more complicated as they are also used to drain the tanks and the weight and complexity of additional piping might outweigh the advantage.
Is there a formulae for a rough calculation of the difference between the two?

Cheers!

Paul