Manifolding multiple discharges
You can either provide a separate thru-hull for each discharge, ~or~ manifold several discharges together, as follows: From ABYC Section H-22.8.8 (Electric Bilge Pumps) “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 discharge manifold system.” In practical terms, this means that the two discharge hoses must have a backflow prevention trap (like a vented loop), consisting of loops (in each pipe) above the level , (and hydraulically ahead) of the manifold and thru-hull. The loops should be about 18" higher than the thru-hull, to assure no backflow. Because of the difficulty in achieving this, most boats have separate thru-hulls for each discharge. The single thru-hull should be about 1.5 times as large (diameter) as the (2) discharges (ie: 2 x 1" pump discharge hoses require minimum 1.5" diameter thru-hull). HTH, Gord May
