Two Bilge Pumps - One Through-Hull Discharge:
The plumber in me, would never recommend discharging two (or more) bilge
pumps through a single
through-hull outlet, there are practical issues that might mitigate in favour of manifolding the discharges (ie: reducing the number of hull
If you must:
1. Each Bilge Pump
must have a properly sized & separated discharge hose, from the bilge to the discharge location (transom).
2. Check valves are NOT permitted.
3. The two separate pump discharges are combined only at the transome thru-hull location.
4. The larger combined final discharge hose & thru-hull are large enough to accommodate the combined
pump discharge flows..
For example: (1) Electric and (1) Manual Bilge Pump
(1) 1500 GPH (25GPM) Electric Pump
, plus (1) 25 GPM Manual Cockpit
1-1/8" diameter (free area about 4 sq. in.) hose from Electric Pump
1-1/2" dia. (free area about 7" sq. in.) hose from Bilge Strainer to Manual Pump, then from Pump to transom.
2" diameter (free area about 12.5 sq. in.) hose from (2 - 1) hose manifold fitting to through-hull. (*See A
BYC note below)
Take each Pump hose up to an anti-siphon loop, then turn down.
Connect the two hoses with an appropriate “Y” fitting (1-1/8" & 1-1/2" to 2"), so both discharges point down.
Continue the 2" diameter hose down, to the through-hull discharge.
I recommend locating the discharge above the waterline.
A Seacock is only required when the discharge is below the heeled waterline - but I recommend installation
of a seacock for all through-hulls (secured in “open” position for B.P. discharges).
BYC Section “H-22.8.8"
"If the discharges of several pumps are manifolded to discharge through a single thru-hull fitting, the system shall be designed so that the operation of one pump will not back feed another pump, and the simultaneous operation of each pump will not diminish the pumping capacity of the system. A check valve shall not be used in the manifold discharge system.
This requires that the free area of the final combined discharge hose (at least) equal the free areas of both Pump hoses.
The area of a circle, hence the capacity of a pipe, varies as the square of the diameter. Double a hose's diameter, and you quadruple it's capacity.