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The Case For Integral Tanks

Kasten Marine Design, Inc.
Copyright 2009 - 2013 Michael Kasten

This article is not meant to be an exhaustive examination of integral tanks - merely a beginning discussion.

The use of integral tanks on a metal yacht is a natural, but integral tanks are not limited to metal boats..! It is equally possible to use integral tanks on fiberglass and on cold moulded wooden boats, where the question becomes how to best seal the tanks to prevent the migration of fuel or other liquids into the laminate or the core.

The following discussion will only consider integral tanks for metal boats, even though many of the same advantages can be realized when building in other materials. The questions to be considered are the issues of applicability to different kinds of fluids, strength, access, ease of construction, cost, damage control, and long term maintenance.

In 30 years' worth of being involved with metal boats I can safely say, "I've never designed or built a metal boat that doesn't use integral tanks, even down to a series of aluminum sailing prams between 8 and 14 feet in length, where integral tanks are used for floatation and for keeping the sandwiches dry…"

What Is An Integral Tank?

An integral tank is made of the same material as the hull, and makes use of some of the hull surfaces or other structure as tank faces. In other words, integral tanks are part of the actual structure of a boat. An integral tank will not be removable.

A separate non-integral tank can be made of the same material as the hull, or can be made of another suitable material. A non-integral tank is not part of the structure, and can therefore be removed.

Why Use Integral Tanks?

Provided you attend to the requirements outlined in the following article, on a metal boat integral tanks are nearly always to be preferred. Because they take advantage of and actually reinforce the hull structure, integral tanks can provide a number of design efficiencies, more or less as follows:

Throughout the life of the vessel, integral tanks enable much better access to the hull for maintenance and repair. For the sake of on-going maintenance, one can access the hull structure directly via removable clean-out plates in the tank faces and baffles. The exterior hull surfaces would otherwise be buried behind non-integral tanks, making access nearly impossible… or at least very labor intensive and costly.

Some of the requirements most commonly requested for long range voyaging yachts, include:

Using integral tanks prevents the loss of volume you get with separately installed non-integral tanks. Where regions of the hull are able to be double bottomed, damage to the hull where the tanks are located won't compromise the watertight integrity of the vessel. If you puncture the hull in way of the tank, you will probably lose the fluid in that compartment, but you will not flood the whole boat. Further, that damage will not be hidden behind an installed non-integral tank.

This provides not just enhanced survivability, but vastly improved maintainability. Unbolt the top of the tank, and you can quickly access the part of the hull structure.

Can integral tanks provide complete protection against hull puncture?

Actually, no. An integral tank can offer protection to part of the hull surface below the waterline, but there must necessarily also be a region that lies outside of the tank, otherwise there would be no underwater volume left un-flooded to float the boat....!

That said, one can certainly create an integral double bottom, part of which was not dedicated to containing liquids. I am not ordinarily in favor of this, as it is extremely wasteful of space and limits ready access to storage or mechanical chases, so on a practical basis it is quite difficult to justify.

Our 76' Free Spirit and 80' North Coast designs provide good examples of the use of a 'partial' double bottom.


The 76' Motor Yacht - FREE SPIRIT - Kasten Marine Design, Inc.
The Free Spirit shown above makes excellent use of a double bottom forward of the engine room throughout the forward accommodation spaces.
The portion of the enclosed volume that is not dedicated to tanks is used as an access alleyway between the tank faces.


The 80'' Motor Yacht - NORTH COAST - Kasten Marine Design, Inc.
The North Coast shown above has the lower deck accommodations much lower in the vessel, and a fair distance below the waterline.
The tanks lie entirely below the lower sole, so they create a continuous double bottom below the sole only.
If there were damage below the level of the lower sole, there would be excellent protection offered by the tanks.

What Are The Issues With Integral Tanks?

In my design work, if the boat is to be metal the tanks will be integral, therefore they will be built of the same material as the hull itself. The exception I make to this rule is that all waste tanks are non-integral.

Each material has its own issues as does each type of liquid put into the tank. For example, there is a big difference between containing diesel, which has a small molecular structure and likes to escape, versus containing potable water, which is much easier.


During construction and during eventual maintenance there are dangers and safety concerns with tanks of any type, whether they are integral or non-integral.

Tanks can present a dangerous work environment. By design, tanks don't leak, therefore tanks will also hold vapors. The atmosphere in a tank can become noxious for workers, or even explosive.

Without good ventilation, cutting gasses such as propane can accumulate say from a leaking hose, welding fumes will accumulate, and inert gases can displace oxygen. Tanks are usually coated with epoxy paints containing solvents. These solvent vapors are not only flammable, they are toxic.

Tanks must be well ventilated. Builders will often need a dedicated breathing air supply and there must be a watch person on hand. If a worker is overcome by the atmosphere inside a tank, the watch person is usually NOT advised to go in to pull them out. Without safety breathing gear or other outside help, the watch person will very likely be overcome as well.

Safety procedures should be put in place to handle various potential situations.


Integral tanks are not something you decide on at the last minute and just arbitrarily add to a vessel. Integral tanks should be planned for early on in the design process.

The US Code of Federal Regulations provides minimum thicknesses for tank plating. In many cases, these minimum thicknesses are more conservative with regard to tank scantlings than the ABS rules are with regard to adjacent exterior hull surfaces.

A tank's contents represent a large mass of weight. Rather than being simple dead weights, tank contents are dynamic: they slosh around and move in directions other than the direction of the boat. Fatigue of the hull and tank structures must be considered.

The tank faces and the surrounding structure must be able to handle the various loads in order to resist flexing which would lead to fatigue of the structural materials. In order to prevent over-stressing the tank and hull surfaces, baffles are used at sufficient intervals to prevent sloshing but to still allow fluids to flow.

As an example, in order to limit the stress on the tank structure on non-integral tanks aboard passenger vessels, the US CFR requires that tank compartments be no larger than 30" horizontally between baffles. (46 CFR, Subchapter T, Part 182.440 (a) (8), re: non -integral metal tanks on vessels under 100 gross tons.)

Typically conservative when it comes to structure, I have taken this to be a reasonably sane recommendation for integral tanks as well. Strictly speaking, the 30 inch maximum baffle spacing is not required of integral tanks, and is not required of pleasure vessels.

There are fatigue issues with metal, however they are easily addressed. Ordinarily, an integral tank acts to reinforce the overall structure, enhancing rigidity. A relatively conservative baffle placement is intended to address both the free surface effects of the liquid in the tank as well as the stress on the tank from the motion of those liquids.

For basic engineering of tank panels, whether using Roark or Mark's or Machinery's Handbook (engineering references) or when using the ABS rule, tank plating and framing scantlings are determined by the "head pressure" of the liquid, figured by measuring from the lowest point of the tank to the fill point or to the tank vent high loop, whichever is higher.


While baffles help prevent excessive loads on the structure, they also serve to limit the effects of the "free surface" of the liquids within the tanks.

The free surface of liquid tank contents has a negative effect on a vessel's stability. The free surface effect is best illustrated by remembering the last time you tried to step into a dinghy that was partially filled with water. All the water will rush to the low side, greatly degrading the stability. Maybe you have gone for an unexpected swim..!

While ordinary baffles with limbers will not prevent the liquid from eventually moving to the low side, the liquid "slosh" will have been immensely reduced, and the liquid will ideally not be permitted to move to the low side within the natural roll period of the vessel.


It is always best to provide a large bolted-on tank top or inboard tank face, plus large bolted-on manholes in the tank baffles when creating an integral tank design for ease of access during construction, and for on-going maintenance throughout the life of a boat.

On the subject of access, the rather conservative baffle spacing mentioned above does present a problem, so I make use of bolt-in baffle plates for integral tanks. This allows the baffle plates to be fastened in as the builder is backing out while finally closing up the tanks, and to allow easy re-entry for future maintenance.

What Are The Cost Factors?

Integral tanks can sometimes be slightly more costly in terms of labor and materials (fabrication, prep, sandblasting, painting, etc.), especially when compared to simply make use of non-integral HDPE tanks. In spite of the many advantages offered by integral tanks, the overall cost of any boat building project is always very much in the foreground, and the cost differences can't be ignored.

In order make a fair comparison however, one should consider ALL cost factors associated with integral tanks, rather than just those that relate to the labor and materials to create them. For example, there could be substantial cost savings in the long term by not having to tear out large parts of the interior to remove a non-integral tank in way of a hull repair.

The primary objections to integral tanks are likely to be voiced by builders who will have to dedicate a portion of their own shop resources to creating them, rather than simply having non-integral tanks supplied from an outside source. This is somewhat a cost issue, but is more of a convenience, scheduling, shop space, and human resource issue for the builder.

What About Tank Coatings?

Whether built of steel or aluminum, tanks should be coated with something to protect the tank structure from the contents. In many cases it is also necessary to protect the contents from the tank structure, for example to prevent contamination by the taste of aluminum in fresh water tanks.

Epoxy is ordinarily preferred as a tank coating for the sake of its flexibility; for its extremely good adhesion; and for its relative impermeability to liquids. Epoxy paints designed as tank coatings will often have 100% solids (vs. around 60% solids in typical epoxy paints).

The high ratio of solids offers several advantages:


Polysulfide vs. Epoxy

Epoxy paint is the most commonly used tank coating. It is impermeable to fuel and water, and if designed for potable water tanks, imparts no impurities to the water. With 5% to 7% elongation before failure, epoxy paint is ordinarily sufficiently flexible to last a very long time as a tank coating.

On aircraft, which are subject to thermal cycling and which have much more flexible structures, micro-cracking of epoxy coatings is an issue. For large integral tanks, aircraft make use of polysulfide coatings, which can have nearly 200% elongation before failure.

Polysifude coatings are relatively thick and rubbery, have been in use for thirty years, resist cracking from fatigue and thermal cycling, and are impermeable to fuel and water.

By design, metal boats ordinarily do not flex enough to cause failure of epoxy paint. However in way of a dent due to impact, or possibly in a region of the hull subject to regular slamming, a more flexible tank coating can provide an advantage.

In these locations, the use of polysulfide tank coatings can provide an extra factor of safety and durability for integral tanks, or for that matter even as a corrosion protection coating for the rest of the hull.

One manufacturer of polysulfide tank coatings is PRC-DeSoto International, who developed their PRC-1422 tank coating for the Navy for use on riveted steel fuel tanks. Polysulfide tank coatings were found to be sufficiently flexible to seal the seams and rivets.

Another polysulfide coating is 3M-101 Marine Sealant, which offers proven chemical resistance, adhesion, toughness and flexibility.

Applying The Coatings...

It isn't usually considered necessary to coat the inside of steel tanks holding diesel. However, I often specify doing so because fuel tanks are not always kept full. This allows condensation, which then inevitably causes corrosion.

For steel or aluminum tanks containing fuel or water, I specify painting with epoxy in order to prevent corrosion in the case of steel tanks, and in order to prevent contamination of the tank contents in the case of aluminum tanks.

In order to create a long lasting tank coating, whether using polysulfide or epoxy paint, the coatings must be correctly applied. Tanks must be designed for good access, with no hidden corners or sharp edges.

Correct preparation of the surfaces before coating is essential, as is quality assurance during the coating process itself. For good adhesion of the coatings, the best surface preparation is to first thoroughly clean the surfaces to remove any possible contamination by oils, and then to sand blast all surfaces.

While there are many other proprietary surface preparation methods pandered by various manufacturers, none are equal to a properly sandblasted surface.

Needless to say, excellent access to all regions of the tank must be provided in order to achieve this. Large tank covers are ideal. If inconveniently large for subsequent maintenance, tank covers can be provided with separate access plates.

As when painting any type of structure, it is not uncommon to miss a few spots during each coat. Usually a minimum of three coats will be needed to make sure all areas are completely covered. For epoxy coatings, twelve mils dry film thickness (0.305 mm) is a typical minimum coating thickness recommended.

This brings up the issue of workmanship when constructing integral tanks of any material. In order for integral tanks to succeed long-term, there must be attention to detail during the design, as well as a high standard of workmanship during fabrication of the tank surfaces.


As an example of the types of access provided, please see the structural profile and arrangement drawings of two of our designs, the 80 foot North Coast, and the 75 foot Free Spirit. Both vessels are aluminum motor yachts, and both have a similar hull shape, but each has a very different approach to the tank arrangement.


76' Motor Yacht - FREE SPIRIT - Kasten Marine Design, Inc.
On the Free Spirit shown above, the tanks were arranged to port and starb'd below the accommodation spaces,
and a generous central access alley was provided between the primary tank faces.
In this case, tank man-hole covers were placed inboard facing the centerline for the best access.
The tank covers were placed on raised flanges, in order to provide a leak-free bolting ring.
Internal longitudinal baffle plates are bolted in place, but transverse baffles are able to be integrally welded,
so that they become an integral part of the transverse framing of the vessel.


80' Motor Yacht - NORTH COAST - Kasten Marine Design, Inc.
On the North Coast shown above, even though the vessel is larger the tanks did not have as much room due to the extensive below-decks accommodations.
Here, the tanks create a nearly continuous double bottom to the boat throughout its length.
There being no extra space for a central alleyway, the tank lids were designed to be placed in the tops of the tanks.
In this case, both transverse and longitudinal baffles are designed to be welded in place.

On both vessels, the removable tank lids allow free access during construction, and for future maintenance. In both cases, the overall range of tank length has been intentionally limited in order to allow flexibility in managing the loads and the consequent trim of the vessel. Tank width has intentionally been limited as well, even in the North Coast design (by subdivision) primarily in order to prevent excessive free surface effects and other stability issues.

What About Cases Where Tanks Cannot Be Integral?

Although aluminum and stainless are popular materials for non-integral tanks, possibly the best alternate tank material is high density polyethylene.

Unless space is extremely tight, I prefer to specify heavy wall HDPE tanks for black water. Black water tanks are not ordinarily very large (by comparison to fuel and fresh water tanks), so can more readily be planned for easy removal. For a retro-fit on any boat, HDPE tanks also make good sense.

HDPE is easily fabricated by "welding" so these tanks can be made to fit a compartment of nearly any shape. Properly done, the joints are very strong. Scantlings should be robust.

For example, even for small HDPE tanks it is preferred that wall thickness be a minimum of 1/2" up to 70 gallons, or 5/8" up to 120 gallons, with greater thicknesses as tank size increases. We typically specify that baffles in poly tanks be located no farther apart than 18 inches.

What Regulations Apply To Integral Tanks?

The CFR (Code of Federal Regulations) has the following to say about integral tanks aboard "T" boats (the ABYC recommendations more or less reiterate the same, with a few minor exceptions).

Potable Water tanks are not mentioned. There is no wording to prevent or encourage integral tanks of any material. ABYC recommends that the materials of construction not impart a taste or odor to the water. Presumably, epoxy coated structures would be acceptable.

Gasoline tanks may NOT be integral regardless of material. Diesel tanks may be integral provided that the hull is constructed of steel, aluminum (or solid composite).

Waste Tanks are not mentioned by either the CFR or the ABYC. As with potable water tanks, there is no wording to prevent or encourage integral waste tanks of any material.

Integral tanks must pass a 5 psig pressure test, provided by being filled and having a stand pipe of 11.5 feet (3.5 meters). If the head pressure will be greater in service, the stand pipe used for testing must be to the greater of the two.

And to Non-Integral Tanks...?

Per ABYC, all non -integral fuel tanks (gasoline or diesel) must pass a fire test, a pressure test, a drop test while filled, and a repetitive beating type of test, then must still hold fuel without leaks for a specified number of days afterward.

Presumably all builders will be equipped with a gang of guys in helmets and white coats with matches, hammers and other torture devices...!

The approved materials for gasoline and diesel tanks are: Aluminum 5086, 5083, 5052; Stainless type 316-L or 317-L; Aluminized Steel; Steel; Silicon Bronze; Copper Nickel; and Monel.

The ABYC materials tables for gasoline and diesel tanks do not include plastic tanks, but the wording of the ABYC rule permits non-metallic tanks which can pass the specified fire and torture tests.

Per the ABYC recommendations, non-integral tanks of any material must bear a label stating that they have been tested in accordance with 33 CFR 183.510 (a), meaning they survived various torture tests, which differ according to the tank size.

Are Bladder Tanks An Alternative?

I believe bladders to be an extremely poor solution, unless viewed as being entirely temporary or only for occasional use.

Although they are quite strong, often being used to transport huge quantities of fuel or other liquids on flatbed trailers, on a boat a bladder tank will move around and will be subject to chafe and puncture. Bladder tanks would be very unlikely to pass the requisite tests as fuel containment tanks.

Obviously, bladder tanks provide an easy way out for the builder... However, they are really only sufficient as a temporary solution, especially for boats of high-quality construction.

There are always exceptions to any rule… I used a small bladder as a black water tank in my own steel schooner in order to comply with regulations. I knew I would rarely use it and I didn't want to use a large amount of space for the tank. It mostly stayed collapsed and empty, so it worked fine, but I wouldn't ordinarily specify a bladder for any type of permanent tank.

In Conclusion...

While integral tanks have been well accepted in commercial metal vessels, yachts are a slightly different story. Even with metal vessels there may be resistance voiced by builders, and for the reasons expressed above.

One must always qualify anything said about tanks by first considering the type of liquid that the tank is to contain. Materials and methods appropriate for one kind of liquid may not be for another. For example, HDPE or GRP should not be used for gasoline. Due to corrosion issues, aluminum is not recommended for black water tanks. Galvanized steel is not used for diesel, etc.

For metal vessels, the advantage of vastly improved access to the hull for maintenance offered by integral tanks is extremely valuable. On some vessels, the option of designing the tanks to offer a double bottom for part of the underwater hull can be an advantage.

In all cases, integral tanks offer the best access to the interior of the hull, and save considerable interior space, often allowing a lower center of gravity and enhanced safety.

Obviously I'm a strong advocate of integral tanks, but there will always be occasions where they are not appropriate, perhaps due to cost considerations, convenience, fluid type or regulations.

For more information on the question of hull materials, please see our web articles on the following:
Aluminum for Boats | Aluminum vs. Steel | Steel Boats | Composites for Boats | The Evolution of a Wooden Sailing Type