Integral Watertank Construction

by James Baldwin

Portions of this article first appeared in Cruising World magazine

My troubles with water tanks go back a long way. When I bought Atom, my 28-foot Pearson Triton in 1979, it came equipped with one 23-gallon fiberglass water tank located under the forward v-berth. Since this was nowhere near adequate for extended cruising, I carried extra plastic water cans in every conceivable location and spent a lot of time and backbreaking effort dragging them out to refill the main tank. Eventually the tank worked loose from its mounts and started to rub against the hull. When it began leaking in a place inaccessible to repair, I was forced to consider replacing it.

Before making any modifications, I considered my options. Even if I ripped apart the v-berth and pulled out the tank to repair it, I would still have only a 23-gallon tank. I ruled out replacing it with a prefabricated plastic tank or custom-built stainless steel tank because of the cost and because a tank of the shape and capacity I required would not fit through my companionway hatch. Not only could these tanks develop leaks that would be difficult to repair, they prevent access to the hull. If the hull were damaged behind a rigid tank there would be no way to stop the leak and the boat could sink. Flexible tanks were an option, but the size and shape I wanted was not available and at the time I had no experience or confidence in how they stand up to the possibility of chafe damage or leaking fittings. Since I had in my mind to create sealed watertight bulkheads wherever possible, the flexible tank seemed unnecessary.

My solution was to build an integral tank into the boat incorporating the fiberglass hull and an existing bulkhead in the v-shaped bow area as the tank's bottom and sides. By adding one more side and a top, I would have a tank of exactly the shape and capacity I wanted, without some of the problems of standard tanks. It did require careful design and (as I soon found out) a considerable amount of work.

Building the Integral Tank

After emptying all lockers in the forward cabin, I used a jig saw to cut out the entire top plywood bunk surface to within 3-inches of where it was glassed to the hull, and set the pieces aside until later. I then removed and discarded the old tank and cleaned out the wet mass of filth that had collected in the gap between the tank and the hull. Now began the dirty work of grinding and laying up fiberglass that would make the boat nearly uninhabitable for the next week. Did I mention you don't want to be living aboard during a project like this?

To begin with, I ground all surfaces of the hull in the area of the new tank using an angle grinder and #36 grit discs. To protect myself from fiberglass dust during grinding, I wore a full Tyvek suit with hood, goggles, and cartridge respirator.

Instead of making complicated measurements and calculations, I made a rough guess as to the increased capacity of the new tank by simply adding 4-inches to the height of the original tank. Adding a small amount of height enormously increased the volume of the v-shaped tank. With pencil and tape measure, I marked the location of the tanks new top along the sides of the hull and the existing aft plywood locker bulkhead, which would now form the aft end of the tank. At the forward end of the tank I installed a 3/4-inch marine plywood bulkhead divider near the location of the original tanks forward end. This created a useful separate storage locker between the tank and the chain locker.

Using cardboard, I made templates for the tank's internal baffles and cut them out of 1/2-inch marine plywood. The baffles were placed from bottom to top of the tank in an X-pattern. Holes in the baffles allow the water to flow between the four chambers. The baffles must be strong to restrict the potentially damaging force of water sloshing back and forth in a partially full tank. They also must divide the tank into small enough sections to reduce the noise of that sloshing water. There's nothing worse than trying to sleep in a bunk above a half full water tank that lacks sufficient baffles when the boat is rolling in a swell.

After degreasing with acetone, I heavily reinforced the baffles, hull, and aft bulkhead with several layers of medium-weight fiberglass mat and epoxy resin. If fiberglass cloth tape were available I would have used that instead for its stronger strength to weight ratio and ease of use. As I laid in the wood, glass and resin, I worked on the simple principle that if it looks strong it is strong.

I then made another cardboard template for the top of the tank, cut it out of 1/2-inch plywood and trimmed it to fit evenly along the hull and rest squarely on the top edges of the baffles. In this top piece I cut a 12-inch square access hole in the center where the baffles met which allowed clearance to reach a hand inside all four chambers for future cleaning. Then I removed the top and gave its edges and inside surface three coats of epoxy resin followed by two coats of nontoxic, drinking water safe epoxy paint. (Check with your supplier about the suitability of their epoxy coatings for potable water tanks or check the resource links below.)

I gave the inside of the tank a final sanding with the angle grinder to remove sharp strands of fiberglass, and applied to all surfaces another two coats of nontoxic epoxy paint. At the bottom of the tanks aft bulkhead, I installed a plastic thru-hull fitting suitable for a 1/2-inch water outlet hose. In the tank top I fitted another 1/2-inch thru-hull for a vent hose and a 1/2-inch thru-hull for the filler inlet. I installed a clear hose sight gauge into the tanks forward bulkhead that can be viewed through the adjoining locker. I then replaced the top, sealed the gaps along its perimeter with thickened epoxy putty, and strengthened the seam from above with wide overlapping layers of fiberglass mat.

To finish the tank, I cut a piece of plywood for the inspection port cover, sealed it with epoxy resin and paint, fit a rubber gasket under it, and screwed it down on the tank top with a thin bead of silicone sealant. I made this inspection port instead of using the round plastic type from the marine chandlers because they are not large enough and not reliably watertight.

With the tank finished, I moved on to replacing the top bunk panel I had earlier cut out. To add support, I glued strips of overhanging hardwood along the lower edges of the bunk cutout, filled the gaps with epoxy adhesive, and fastened it all from above with stainless steel screws. Before replacing the top I enlarged its access hatch to provide easy access for the locker above the water tank. A 3/4-inch lip, or cleat, fixed around the bottom of the cutout supports the hinged locker hatch. Barrel bolts keep it secured in case of a knockdown at sea.

During a later project I added gaskets to make the locker lids partially watertight.

I ran the tanks vent hose into the chain locker and secured it to the bulkhead near the deck. A fine screen placed on the end of the hose prevents insects entering the tank. Because there was no simple, attractive way to run the tank's filler hose through the cabin to the deck, I installed the tank filler fitting flush on the top of the bunk directly above the tank. (There were some unexpected advantages to this that I will mention later.) From the tank outlet fitting I ran a food grade, algae-inhibiting opaque PVC hose, under the cabin sole to the galley foot pump.

Now that the integral tank occupied the lower v-berth area, I had to re-route the chain locker drain hose to pass over the top of the tank and then down to a shut-off valve in the bilge. To ensure the chain locker would drain, I planned for the top of the tank to be just below the level of the bottom of the chain locker. This is something that should be checked before doing your own installation unless your chain locker has a direct overboard drain. Alternatively, you could fiberglass a false bottom into the V of the hull and pass the chain locker drain hose through that, but I preferred no hidden inaccessible areas in my hull. To protect hoses from chafe, you can pass them through sleeves of short lengths of larger diameter hose where they pass through bulkheads.

Adding a Second Tank

Before using the tank, I scrubbed it out with detergents and then baking soda and rinsed it thoroughly. The capacity of the new 43-gallon tank is nearly double that of the original tank. The extra 20 gallons I now carry adds about 200 pounds of weight to the forward end of the boat. To maintain balance required shifting other heavy supplies further aft.

Since the forward tank worked so well I later added a second integral tank in the nearly inaccessible space between the cockpit floor and the hull. Adding a 30-gallon tank in this otherwise unusable space helped counterbalance the weight of the forward tank. On many boats a tank built into the bilge is another possibility.

Although Atom does sit noticeably lower in the water with this combined extra load of 500 pounds, I dont keep the tanks full at all times. But it's good to know I can carry sufficient water when it's needed for long passages or when visiting places where water is not easily available. To keep on an even keel trim, I first use the forward tank down to half full and then switch over to the aft tank until it is empty and then switch back to the forward tank. On a passage we use about one gallon per person each day for combined uses of drinking, cooking, and allowing one quart of water for a rinse after a saltwater deck shower. When water conservation is not a concern we each use an additional gallon per day, mainly for longer showers. For fetching water from shore I have five 3-gallon plastic cans that I keep stored in their own cockpit locker. I also have two 5-gallon cans in the bilge and three 5-gallon cans that fit into what used to be Atom's engine compartment before I converted to outboard drive. This makes an adequate, ocean-crossing total capacity of 113 gallons.

There is added safety in keeping water in two separate tanks in case one becomes contaminated or develops a leak. Each tank has its outlet hose going to a shut-off valve before joining the common inlet of the galley foot pump. On small cruising boats, I consider pressure pumps unacceptable because of their power usage, noise, unreliability and the way they encourage water wastage. Unless you enjoy washing your hands one at a time (something I never learned how to do) I would not choose a galley hand pump except as a back-up to a foot pump or as a saltwater pump for washing dishes.

The problem of having the forward tank's water filler fitting below deck is that if you overfill the tank, the water ends up spilling on the forward bunk. This disadvantage is offset by the ability to refill the tank from rainwater collected on deck. On Atom I fit the two deck scuppers with valves that allow me to divert the deck drains to a hose below deck that I insert into the water tank. If the water filler were mounted on deck I would have to fill containers down below and then haul them on deck to pour them into the tank. There are times on passage when trying to refill water on the deck of a small boat is impossible due to the extreme motion and the possibility of having seawater enter the filler. Now I can safely refill the tank by siphoning water into it from a can resting on the bunk. Since the water tank filler fitting is located directly below the forward deck hatch, I normally fill the tank from on deck using a funnel connected to a 4-foot length of hose that I drop through the hatch into the open tank filler. To avoid overfilling, I use the sight gauge to monitor the incoming water level.


Forward 43-gallon water tank built into hull underthe V-berth which was raised to increase storage. Top of tank is visible below open locker.


The 30-gallon aft water tank was built into the otherwise useless space between the cockpit footwell and hull.


Each tank cost me about $300 (in 1994) for materials including - plywood (bought as off-cuts at a discount), fiberglass, epoxy, paint, and fittings. Labor amounted to roughly 40 hours per tank. This is not an easy job by any means, but it is within the ability of anyone experienced in general boat repair. My tanks have worked so well that I've since constructed additional tanks on other boats. Although all my tanks have not had the slightest problem, I suspect some lightly built modern boats may not be suited to integral tanks because of risk of tank bulkhead damage due to hull flexing. For them, flexible tanks may be the way to go. And since flexible tanks are much easier and cheaper to install, many of my customers choose flexible tanks, though they do occasionally have issues with leaks.

You might think a watermaker would be a reasonable alternative to increased tank storage capacity. On a small, self-sufficient cruiser such as mine, watermakers are out of the question because of their expense, energy use, unreliability, and maintenance hassles. When your watermaker does crap out in a place scarce of fresh water, like the Bahamas, you'll be thankful for the extra storage capacity.

There is also the concern about the possibility of causing osmosis blisters by having water on both sides of the hull, as is the case near the integral tank. My 46-year-old boat has had integral tanks for 15 years now without any sign of blistering. After working for many years building and repairing fiberglass boats, it seems that a fiberglass boat will blister or not, chiefly depending on the quality of its lay-up and materials during construction and the coatings used to seal it thereafter. If the boat you are adding tanks to is showing signs of blistering, then of course that problem should be addressed first, or the adding of integral tanks may accelerate the problem. Modern barrier epoxies are, for practical purposes, impermeable to water. In short, if your boat is not blistering, an epoxy-lined tank should not start the problem.

There are several advantages of integral tanks over other types of tanks. Integral tanks are more space efficient. They occupy less room and hold more water than any other type tank because they utilize all the available space of the tank compartment. They add massive strength to the hull. They act as a watertight locker in case of collision. They fit in areas of the hull where other tanks cannot. With an integral tank you can choose exactly the size tank you want and not be constrained to use whatever size tank is commercially available. Properly built integral tanks are much less likely than other tanks to leak or need replacement. If you get rolled they can't break free and cause further damage or loss of precious drinking water. Whichever type water tanks you choose, it's worth remembering that fresh water is your most important and most used resource and its storage deserves careful thought.

Video of building an integral tank under the v-berth of an Alberg 30


Instructions for West System water tank construction:

FDA-certified epoxy water tank barrier coating: