Still pretty dead in here. How about some practical FYI?
Just went thru this with a guy on his winter to do list items as he was replacing pumps. May help someone else?
Engineering notes:
1. A 750 GPH pump in the bilge that has to lift the water (in a 7/8" ID hose) more than 12" loses 15% of it's rated capacity. 14-18" lift looses 25%. Any bends (that don't crease the hose) will reduce the flow another 10%+.
2. Adding a check valve reduces the GPH by 13%.
3. Plumbing two 750 GPH pumps into one 3/4" discharge hose is not recommended. Even with a Y fitting and not accounting for the lift reduction the maximum a 3/4" hose can attain is 1100 GPH. Enter in the two pumps discharge flow meeting at the Y can cause cavitation of both pumps. *Yep. A few older hulls came like this.
While most OEM'S install 750 GPH bilge/livewell pumps, the average lift from the bilge to the discharge degrades them to 562 GPH.
Adequate?
Maybe, but maybe not?
Suggested alterations:
Upgrade fuses to 6 amps for both the automatic bilge and manual bilge.
Install Johnson Controls 1000 GPH automatic (sensor controlled not float switch) bilge pump and Johnson Controls 1000 GPH manual bilge pump on separate 3/4" hoses. No check valves and large radius turns in the hose routing. With the average degradation of lift these pumps they will give you the 750 GPH you thought you had previously. Spearing a wave or a rogue back deck wash can fill a boat in a hurry. You want maximum pumping happening then. Best to have the max pumps in place and the upgrade cost is minimal over the 750's.
*These 1000 GPH pumps will draw more amps. On/off use such as rain water pump out is not an issue but if you take a huge volume requiring extensive pump time keep the engine running.
Side note: Any other 12VDC pumps rated over 1100 GPH will require 1 1/8" inside diameter hose.