My approach was;
Find a holding tank shape that fitted the space available under one of the forward berths … as well as a suitable diaphram-type pump for the location I had in mind.
Fit the necessary pipe connections into the holding tank then fit the holding tank onto some prepared supports and secure in place with two straps.
Fit the diaphram-type discharge-pump in the chosen location … ensuring space for the pipe connections and for operation of the pumping lever.
Disconnect the existing sewage discharge pipe from the bowl-pump, shorten to suit and connect to the diaphram-pump outlet.
Drill holes for the remaining sewage piping route and run the two new pipe lengths connecting them as needed. One from bowl-pump to holding tank and the other from holding tank to discharge-pump.
Drill holes for the breather pipe and run the pipe from top of holding tank to an external vent.
The items and costs (Dec 2001) were;
Holding tank (20 litres … the largest I could fit without some major work) with holes for top inlet, bottom outlet, and top breather connections. >> Cost $120
Diaphram-type hand operated pump (chosen for simplicity and high pumping capacity). >> Cost $250
Length (2m) of reinforced oil-resistant black-water 38mm hose. >> Cost $60
Length of clear flexible 16mm hose for use as breather pipe (not ideal but chosen so that any overflowing of holding tank can be immediately seen) >> Cost $20
Pipe elbows, SS clamps and breather vent to suit. >> Cost $70
To help meet the budget (cost was $520) I did not add a filter to the breather pipeline (could add later if odours become a problem) and I only fitted single hose-clamps at each connection (vs the recommended two) … so hope I don't pay for this with a failure in the future.
After two seasons use, the verdict of the skipper and mate is that it is one of the best improvements that we have made to our H28. Tank capacity is fine, operation simple, and we have had no system failures … so far. While some odours do occaisionally occur our approach to minimise these is to use a marine toilet deoderiser / seal lubricator plus fresh water to flush the tank when back at the marina. So it works well … however we still do have a bucket on board … just in case.