I found if you're willing to take apart the supporting part of the V berth, but not the V berth itself. you can easily fit in a 20 gallon holding tank and 38 gallons of water. The attached photo kinda shows how they fit. I put a 20 gallon holding tank on the port side of the V berth back, almost up against the bulkhead. I put an 18 gallon tank for water on the starboard side (opposite the holding tank). It drains into a 20 gallon tank that rests in the center. The boat's water supply comes off the center tank and the deck fill is connected to the "upper" tank. The attached photo kinda shows this.

I suggest you make "mockups" of the tanks BEFORE you order and make sure you can fit them in. I've already sent back a water heater that didn't fit because I didn't bother to do a mockup beforehand. These mockups can be really simple. I'll attach a pic of two I did for my forward area.

About the weight distribution question. My boat had the original 27 (?) gallon tank in the V berth. The former owner moved it 24" forward so an 8 gallon holding tank could be fit in. What I've done is added a larger holding tank, but moved it approx 20" farther back than the original and off to the port side. I added an 18 gallon water tank on the port side to offset the holding tank weight and moved it back almost 3 feet from the original water tank, then added another 20 gallon water tank, centerline to the boat, but still almost 2 feet back from the original tank. So even though I've added 11 to 13 gallons of water, I've moved it more toward the center of the boat. I've also moved the 25 gallon gas tank forward about 20" to get it out of the starboard cockpit locker, and added a 4 gallon water heater on the port side to balance it out. Someone smarter than I can analyze what I've done to the trim, but I've tried to balance the tankage and move the weight more to the center of the boat. At this point, I'm just hoping it doesn't roll over and sink when I put it in the water.

There are actually a number of tanks that will fit in the V-berth, depending on what else you want to put up there. I will look to see if I can find the tank numbers that I used. When choosing your tank(s), I suggest you build simple mockups to make sure that the tanks will fit in the V-berth area the way you want them to (and also fit through the companionway & V-berth door). I looked at Ronco, Peal and Oceanlink tanks. When looking through their catalogs, I discovered that they all had the same tank molds, all were 3/8 and seamless rotomolded. I chose Ronco because they offered me a 50% discount.

Fittings: The holding tank is fitted with two 1 fittings (inlet & outlet) at the rear of the tank in the deepest part. I chose to put the tank on the port side about 2 forward of the V-berth bulkhead (to move the weight as far back as possible. The tank also has two fittings for vents (on the top of the tank about 12 from the ends): one forward to scoop air into the tank, one aft to let the air out of the tank. The thru-hull for the inlet vent is just forward of the anchor locker bulkhead. The vent lines run uphill from the thru-hulls for a bit so rain/splash wont run into the tank. The outlet vent thru-hull is in the head area just aft of the V-berth bulkhead. Youll have to decide where your fittings will be located on the tanks based on your installation.

Mounting: The tanks didnt match the curve of the hull so I fashioned wooden blocks to match the curve of the hull on one side and the angle of the tanks on the other. They are fiberglassed to the hull They cradle the tanks so they cant move forward, aft, or sideways. (see the pictures of the fuel tank mounts) The tanks just lie in these blocks, then more blocks were fitted between the top of the tanks and the underside of the V-berth interior deck. I used foam to help support the tanks underneath, and to help make them more immovable. However, even with all that, the tanks can be removed/replaced if need be. I cut access panels in the interior V-berth deck so I have access to all fittings and inspection plates on the tanks. This still left a substantial storage space between the tanks and the anchor locker bulkhead. I did the plumbing to allow the addition of a 10 gallon flexible tank to be added in this area for cruising if need be.

Plumbing: The head flushes directly into the upper rear fitting on the holding tank (about 18 run). The tank empties thru the lower rear fitting with a short run to a Y valve located very low in the storage area behind the head. From there it can be pumped out dockside or thru a macerator and thru-hull. Ive included a picture to show the tank fittings coming thru the bulkhead into the head area, I think youll get the idea. I havent decided whether to use flexible hoses or solid pvc plumbing for inlet and outlet.

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