why is oil used in hydraulic systems

I see that we have people with signifcant knowledge and experience of hydraulic systems around here, so let me ask a tyro question:
Modern hydraulic systems all seem to use specialised fluids, typically mineral oil based. Why? Is this choice really necessary for low duty-cycle applications? My very limited knowledge of hydraulics suggests that hydraulic fluids are designed not to degrade or boil at the high temperatures attained during heavy, repetitive use (I know what it feels like to boil brake fluid on a car with inadequately-ventilated brakes descending a mountain pass too quickly).

The reason for going back to square one and asking this very basic question is the failures seen recently in canting keel rams during the Vende Globe and the Volvo Ocean race. Does anyone here have detailed technical knowledge of the systems these boats use? It seems to me that the rams should only be used to move the keel to a position where a physical detent can latch it in place, but I suspect that s not how they work.

If not, why not? Back to the basic hydraulic engineering question: if a hydraulic system had a very low duty cycle (like a minute or two during each tack, on a boat which typically takes half an hour to tack) would a suitably-designed system running on seawater be adequate? Hydraulic oil and hydraulic fluid are terms that are sometimes used interchangeably, but they are not necessarily the same. While hydraulic oil is a fluid, hydraulic fluid can also consist of other fluids, including plain water, water-oil emulsions and salt solutions.

Because hydraulic oil is flammable, it not suitable for use close to an ignition source. Under high pressure, oil spray can ignite. Hydraulic fluid was developed after World War II as a solution to the problem. Hydraulic oil consists of oils and additives designed to transmit power while acting as a lubricant and coolant. The oil is effective in a wide range of temperatures and reduces wear, rust and corrosion in equipment used in industries like agriculture, mining and construction.

Hydraulic fluid is used for many of the same reasons as oil but has wider use in automobile systems like automatic transmissions, as well as power brakes and steering. Aircraft systems also require hydraulic fluids. Viscosity is of paramount importance in choosing a fluid. http://www. hydraulic-equipment-manufacturers. com/articles/types-and-suitability-of-hydraulic-oils. html http://www. hydraulic-equipment-manufacturers. com/hydraulic-fluid. html

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