See also model basin for tow tanks.
Tanks are compartments for the storage of fluid consumables, liquid cargo, or similar products which may shift. They are water or oil-tight. Consumable tanks are usually kept low in the vessel (though loose tanks, header tanks, or anti-roll tanks may be higher).
The layout of tanks is described by a drawing known as a tank plan. Tanks contribute the bulk of non-cargo deadweight to a vessel, and as such are generally kept low to increase a vessels stability. Tank placement and division are important to minimize trim and heel issues. When the vessels tanks are filled to capacity (and with full cargo), the vessel should sit level. The tank boundaries should be chosen such that all parts of the tanks are accessible, they do not impart undesirable free surface effects, and can be efficiently plumbed for ventilation pipes (goose necks), sounding tubes, pumps, pipes and pickups. Consideration should be given to the order in which consumable tanks will be consumed. Tanks with volatile products should have multiple manholes to aid in de-gassing for cleaning. As much as possible, sloped tank tops should proceed inboard to prevent entrained air from limited overall usable volume. Some tanks will require coffer dams or additional weld/plate thickness between them - for example, lube oil and fuel oil must be separated; or fresh water and fuel oil. Potable water tanks will require specialized coatings.
Tanks should be fitted with means of measuring the quantity inside. This can be by sounding tube or sight glass. Fuel oil tanks are provided with a overflow tank to prevent accidental spillage if overfilled, which is sized based on the filling pumps rate. Tanks with corrosive products may be fitted with a header tank, which allows the tank to remain pressed and prevents air from aiding oxidization in the main tank. The filling and emptying of tanks is done either by dedicated pumps or through a valve chest.