We replaced our 19 gallon holding tank with a 35 gallon one mounted under the v-berth, forward of the vee.  The tank was from Oceanlink (www.oceanlinkinc.com), Catalog # OLT 890.  It's actually a bow tank meant to mount horizontally, but it fits perfectly under the v-berth of the T37 sitting vertically.  Our design has all the plumbing exiting the tank as close as possible to center to avoid waste entering the hoses when heeling over.  The tank, with the fittings, cost us about $450 back in 2004.

Design:  We've had bad luck with y-valves in the past, so all our toilet waste gets pumped into the holding tank.  Once again, to avoid a y-valve there are two separate exit hoses on the tank.  Both are 1.5 inches in diameter and extend to the bottom of the tank.  One goes to the pumpout fitting on deck, and the other goes to an outgoing seacock via a hand pump (mounted in the port hanging locker).  The latter allows us to pump waste overboard when allowed, by law, to do so.  The tank is also fitted with a 5/8 inch diameter hose which is vented to the outside.

Tank Installation:  Linda did a brilliant job with the installation of the tank.  See the pictures below for details.

The tank prior to installation.  It can't be seen in the picture, but the "bottom" of the tank is slanted upward and narrows as it goes forward.

Marine plywood (3/4") was used to construct the support for the tank.  The support does not touch or put any pressure on the hull.  Thick pieces of teak support the bottom, bolted into the forward and aft bulkheads.  The bottom is angled upward to match the angle of the bottom of the tank.  The sides are beveled to match the angle of the sides, and are securely screwed into the bottom support.  The forward edges are supported by 1x1 pieces of teak screwed to the forward bulkhead.  It is important that this construction fits the outside of the tank exactly, so no pressure points are created, and the tank cannot move at all.

At this point, put the tank into place.  Bolt a teak board to the forward edge of the vee on top of the tank as shown.  The 2x6s in the picture were measured and cut at this point.  The boards are secured on the forward edge with 4" lag screws (2 each).  The side supports are screwed into the 2x6 boards using three 4" lag screws each.  Finally the aft teak board is screwed into the 2x6 boards with single 4" lag screws.  The hoses were attached after the tank was secured.