Holding Tank Relocation

By Rob Farrow

One of the first things that needed to be addressed on our Alwest 370 was replacement of the holding tank which was originally located under the vanity sink in the head.

Working on (someone else’s) old sewage system is never an appealing task, so I decided to remove everything and start fresh.

From head, to tank, to pumpout fitting and everything in between, would all be new.  Along the way, the tank would be relocated to an accessible location in the main bilge, completely freeing up the under counter storage space for future use.

A Todd 20 US gallon (10″ x 16″ x 30″) fits nicely in this bilge space as shown.  It rests directly on the hull and is this supported for it entire length along each side.  The framing around the tank keeps it in place.

The first step was to extend the existing bilge access (in front of the fridge) all the way forward to open up the main bilge area.

Crosswise floor beams were cut out and longitudinal stringers installed to end up with an open space to just fit the tank and provide access to the connections at the forward end.

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Originally, the new crosspieces were screwed into place above the tank.  A subsequent modification was to make “pockets” along the longitudinal beams to set these into so that they are easily removable.

This allows the tank to be pumped completely dry by lifting the aft end during pumpout.

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The hoses were routed up through the floor in the head, and then per their original routings.  When I constructed the new vanity for the head, I left a “toe space” pre normal cabinetry construction and all hoses run under the cabinet floor.


       click on picture for larger image


       click on picture for larger image


My theory on holding tank systems is that every clamp and fitting should be easily accessible for inspection (and maintenance if necessary).

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I installed this tank to be easy to service.

That was 10 years ago, and it has been trouble free ever since.


Rob Farrow

Catherine Ann

February 2007