Damn, when you're right you're right...
Well it'll never work with that outlook. Wish I had a cheap drill to try, I could make a killing selling underwater torque guns to dock owners.
@LOW1 How many extensions and u-joints to get below water?
That was my initial thought.
I haven't set one of these up, but knew people here have, and it eliminates additional wiring and relays (that not everyone will be able to service) to have issues with later.
Unfortunately your original message used "booster fan" which implies other fans in the system.
What you're really looking to do is install a single inline fan with 3 inlets controlled by 3 separate controls.
I'll think about this and maybe post back later. Using a simple flow switch (triggered...
What are the other 3 fans ducted into? Or are you removing 3 fans and using 1?
Can you draw out a diagram of the full duct? There's some advice in this thread that I think you're overlooking