New Ballast Question

[Esco]

I couldn't find anything with a search, so I apologize if this info is readily available.

My question is this: Does a through-hull scoop for ballast water intake end up filling the bags while underway if the ball valve is left open?

I don't want to have to close the valve while we are boarding as I want to retain the ability to adjust the system on the fly.

[srud]

Unfortunately it does on my center ballast tank, but never has on the rear tanks. Someday I'll get ambitious and put a valve in the fill line for that center tank. It must be some sort of siphoning taking place because when I first drop the boat in the water, I can drive it all day and it will not fill. However as soon as I fill it once and then empty it, it starts filling itself anytime the boat is moving, despite the fact that the fill line is looped up above the waterline.