HOW THE SYSTEM WORKS
A hydraulic steering system consists of a steering pump, a cylinder tied to the rudder or to the outboard or sterndrive engine and the connecting nylon or copper tubing.
Under normal operating conditions, a turn of the steering wheel will send the oil from the helm unit into one of the two connecting hydraulic lines.
The oil will be pumped into one of the chambers of the cylinder and either extend or retract the cylinder rod, depending on the steering wheel rotation.
The fluid going out from the other chamber of the cylinder is returned to the helm via the other hydraulic line.
There are two basic components in all the hydraulic steering systems: the helm unit and the cylinder, connected by nylon or copper tubing. The helm unit consists of both a hydraulic pump and a valve assembly.
The valve assembly prevents outgoing fluid from returning along the same line, isolates each steering station, locks the rudder and eliminates rudder “feedback” to the helm.
The cylinders are double acting and may be balanced or unbalanced, (in which case the rod extends through only one end of the cylinder).

SUGGESTIONS AND WARNINGS WHEN SELECTING A HYDRAULIC STEERING SYSTEM
The strength required to drive a boat equipped with a hydraulic steering system is inversely proportional to the number of turns of the wheel lock-to-lock. The wheel turns are determinated by the ratio between the cylinder volume, pump displacement and the free movement of the rudder.
With the same type of cylinder installed on the engine, the less are the turns, the faster is the response but higher is the effort; more wheel turns, slower is the response and lower is the effort.
Other factors that can influence steering effort are:
Vessel speed 
Rudder dimension, or engine power 
Propeller selection 
Hull type (displacement, planing, etc.) 
Sea conditions 
Oil viscosity