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Bilge Pump Discharge - New Thru Hull ???
Yes, you can install two pumps to discharge through a single through-hull; but you should not use an existing (properly sized) thru-hull to also accomodate an extra pump. It may not, therefor, be practical.
1. NEVER install a check valve in a Bilge Pump discharge line. While the valve reduces the amount of water that returns to the bilge, after the pump shuts down; they are VERY prone to failure. It''s difficult for a typical bilge pump to open a fouled check valve (and they DO clogg up) A new & clean 1.5" check valve represents the equivalent hydraulic friction loss of about 14 Feet of SMOOTH-BORE Pipe..
2. You CAN install a "Y", providing a single thru-hull discharge for two pumps.
The "Y" will consist of an assembly that has the free area of the thru-hull equal to not less than the sum of the areas of the pump discharge ports (actually, this is only a close approximation).
The pump hoses will each rise to an anti-siphon loop (risers), then drop to the "Y" fitting, then discharge. This prevents the cross-pummping that Ron mentions.
This riser loop should consist of gentle bends, so as to limit the amount of waterflow turbulance introduced into the discharge line(s). Turbulant waterflow increases friction head, reducing pump output.
3. So, for your example (a 0.75" and a 1.5" pump discharge) will have a "Y" assembly consisting of:
a 0.75" internal diameter upper leg, another upper leg at 1.5" (= 2.2 Sw. In total), and a 1.67" internal Diameter(>2.2 Sq. In.) lower discharge leg. Because 1.67" I.D. not available, you''d have to increase discharge thru-hull leg to 1.75 or 2" diamter.
Perhaps a second 3/4" thru-hull might be easier, after all.
Hope this helps.