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A question of capacity


One of the most important aspects of boat design is undoubtedly defining the capacity plan.

The capacity plan, the sizing and positioning of tanks for holding fuel, fresh or black water, lubricating oil etc. Since capacity is a function of many project specifications, their sizing is not always immediate, and very often requires several turns around the spiral of the project to get them close to the required values.

Establishing capacities

The first step in defining the capacity plan is determining the size of the main tanks, particularly those for fuel and fresh water which are generally the largest. The capacity of the fuel tank is a function of two main factors at the basis of the project specifications: the range required and the consumption of the engines installed, which in turn is a function of the required cruising speed. Simplifying considerably, we can say that the capacity of the fuel tank is given by the range in nautical miles divided by the design speed in knots multiplied by the overall hourly consumption of the engines installed in litres per hour at a given working regime that makes it possible to reach the design speed; to this value we add a safety margin of five or 10% to take account of the fuel that cannot be drawn up by the fuel pumps. The same thing, but with fewer factors in play, happens for the freshwater tank whose Capacity is defined on the basis of the number of people on board and an estimated average daily consumption figure. On the basis of the volume of freshwater, the capacity of the black and grey water tanks is calculated dividing the volumes between them with an empirical ratio generally linked to the type of water systems present on board but in any case permitting collection of at least 90% of the fresh water capacity.

Design and positioning parameters

Once the volume is defined, the choice of the type of tank and its construction material is closely linked to the geometry of the available space, the interface with the bearing structures of the hull, hydrostatic, hydrodynamic, regulatory and functional problems and economic factors. Tanks, which are often very large, must not invade the spaces reserved for people on board and so must be positioned so as to be hidden from view but, in case of need, be easy to inspect and/or easy to remove and disembark. It is thus intuitive that the positioning of any tank requires a study of its handling which in turn implies making them easy to remove from the areas where they are housed and dismantling procedures must be specified. These requirements often have great impact on the choice of external geometries or internal furnishings that must necessarily offer openings large enough to allow technicians to handle the tanks to remove them from inside the hull without damaging the surrounding structures and then reinstall them. Naturally the geometries of the tanks must not largely dictate the geometries of the structures of the hull, though these are adapted to actors supports or to contain the tanks; it is thus the job of the designer to find the correct placement which does not always coincide with the most obvious choice. To place large volumes of liquid, which represent great concentrations of weight, it is important to assess carefully the effect they could have on the hydrostatic trim of the boat, on its stability and in particular for planing vessels the impact they could have on the longitudinally position of the centre of gravity, a key parameter for trim when racing. So we need to take great care in correctly positioning the fuel tank, which in 90% of cases is the largest volume and its centre of gravity must be placed in the hull as close as possible to the overall centre of gravity of the vessel so as not to cause variations as the quantity of fuel changes. Often there are significant changes in the performance of planing motorboats due to the variation of the hydrodynamic trim as fuel is consumed. The influence of the moving of liquid weights such as fuel or water to change the hydrodynamic trim is often also used in racing or in planing work boats (patrol boats, military craft etc.) which are designed for long voyages also with very variable loads and sea conditions which do not allow them to maintain an ideal trim. To keep the trim constant, in addition to the classic trim correctors ballast can be used, auxiliary tanks into which liquid can be moved, be it fuel or water taken from the sea. Any tank is a container for more or less dense liquid that is free to move inside. The movement of large masses of liquid in a tank is quite natural, especially because it is extremely rare if not impossible for a tank to be completely full. For this reason the liquid that is free to move inside the tank modifies the distribution of the liquid and so modifies the centre of gravity of the tank, creating heeling moments that are dangerous for the stability of the entire vessel. As we can easily imagine it is impossible to avoid the formation of free surfaces and so to reduce their negative effects to a minimum there is a tendency to split up the interior of tanks that are very wide compared to the beam of the boat into watertight compartments. The effect of the variation of the centre of gravity of the tanks on the stability of the vessel also depends on their height and it is always preferable to keep them as low as possible to limit the negative effects on the vertical centre of gravity (VCG) of the vessel.

Metal tanks

The interface between the tanks and structural and other elements surrounding them is one of the elements that also determine construction material and technology. The most common material is metal, stainless steel or aluminium. Metal tanks (photo 1) are suitable for containing any type of liquid: fuel, mud, oil etc., and though they are always very malleable in their shapes they have limits to the geometrical adaptability that often mean they cannot exploit to the full the space available and may have to sacrifice their capacity. In geometries with several discontinuities of shape every corner is a point that accumulates mechanical tension so as far as possible they are built so as to effect changes of direction in the metal sheets by bending. However they cannot be built from a single sheet and some areas must necessarily be welded, but every weld as time passes (generally several years) could represent a point of breakage or leakage. Finally it must be borne in mind that any metal on a boat, exposed to a salty and damp environment, is always at risk from galvanic currents which could produce dangerous corrosion and as a result leaks; for this reason all metallic tanks must be carefully earthed. Though they are always very reliable, metal tanks have technological and usage limits and significant costs which for small or economic boats have a significant impact on price.

Plastic tanks

The economic alternative to metal tanks are tanks in thermoplastic material made from plastic suitable for food use or resistant to hydrocarbons. This kind of tanks generally have pre-established shapes, sizes and capacities, so the designer must balance the requirements of his project with the commercial availability, though this is now broad enough to cover a large number of cases though designers sometimes propose custom-made shapes. Plastic tanks are mostly used for reserves of water or liquids and less commonly for reserves of petrol or diesel fuel, generally in small quantities. The biggest advantage of plastic tanks are their light weight and naturally their cost, but above all the absence of the risk of the material deteriorating because of galvanic current, humidity or corrosion. Many plastic tanks are also supplied with attachments for metal connections in various configurations and some also have provisions for floats.

Structural tanks

Both metallic and plastic tanks do not fully solve the problem of fully exploiting available volumes for liquids on board because in both cases their shapes are adaptations of the surrounding volume and so leave on used a percentage of often precious space. To avoid this problem some builders opt for structural tanks, creating between the side of the hull and other elements space for housing liquids, making the perimeters and interiors of this tank elements that play a part in the structural strength of the vessel. Structural tanks (photo 2) are not easy to make and demand great technical ability in handling the materials and careful design down to the smallest detail. For example in the case of structural tanks in fibreglass it is important to have watertight seals in the joints between the perimeter, any internal divisions of the tank and the upper part by gluing and/or patching both externally and internally. It is not always easy to carry out these operations and to do so it is important in the design phase to plan for openings that will then be tightly closed with metal caps that can also be used for inspection. The interior of structural tanks for fuel must by law be covered with paint or gelcoat that resists hydrocarbons or with paint suitable for food use in the case of fresh water. Structural tanks certainly make it possible to exploit to the full the volumes of the hull for stowing liquids, they have no corrosion problems (in the case of hulls in composites), but on the other hand since they are part of the hull and often of the bottom of the hull they are exposed to damage in the case of impact with floating objects and in the case of fuel tanks this could also create serious environmental problems.

Tanks in neoprene

The final and least known type of tank (which at the moment is used only to can tell fuel in vessels where it is very important to exploit all the space available but without building a structural tank) comes from aeronautical and later automotive technology, which have long been using tanks built in a special material consisting of neoprene reinforced with internal fabric and thus are flexible and can be modelled into any shape (photo 3). This technology is a new arrival in the yachting sector and is still rather unusual, but makes it possible to eliminate all the problems of wear and deterioration involved in the use of metal, the problems of building structural tanks and can be used also on existing vessels in the case of refitting. Tanks in neoprene are shaped by the builder on the basis of the geometries supplied by the designer and to avoid the formation of free surfaces, since dividing walls cannot be installed they are filled with plastic material similar to a sponge (photo 4) which reduces the internal volume available by only 5% and maintains its shape. Obviously the weight of a tank in neoprene is much lower than that of any other rigid tank although it is decidedly stronger and safer. Since it is derived from aeronautical technology the construction material is completely fireproof, flame proof and resists pressure as required by EC regulations.

Testing and certification

The construction and manufacture of tanks, whatever technology or material is used, and in particular of fuel tanks is governed by UNI regulations and classification registers which lay down certain production standards. In particular for metallic tanks the workers to carry out the welding must have suitable certifications of professional suitability and the tanks must in any case undergo testing under pressure to make sure they are completely watertight. This type of test, described in Annex A of the UNI-EN ISO 10088 standard, consists in blowing air into the tank with all its outlet pipes sealed until the test pressure is reached, which is the greater of 20 kPa or 1 1/2 times the greatest hydrostatic pressure to which the tank can be exposed and is so much greater than the effective working pressure; the watertightness of the tank is then monitored for a maximum of 30 minutes using a manometer on one of the outlet pipes. A loss of pressure obviously means a defect that could be a weld, an outlet pipe or fault in the sheet metal. After this test the regulations require for fuel tanks also a fireproofing test. This test verifies that the tank has no structural damage which would compromise its watertightness even if exposed to flames. To do the test, Annex C of the regulation requires the tank to be filled to 25% of its nominal capacity with petrol and surrounded by 75 mm of commercial heptane in horizontal projection. The heptane is then lit and left to burn for two and half minutes, after which the simulated fire is put out, the tank emptied and the pressure test carried out again blowing airing gradually until a pressure of 2 kPa is reached. The test is considered valid if the tank does not leak.