Hydraulic steering moves the rudder with oil. Turning the wheel drives a pump in the helm, which pushes fluid down a pair of hoses or tubes to a ram on the rudder's tiller arm. It's smooth and powerful until fluid gets out. Andrew McDonald, a marine service owner writing in Canadian Yachting, says oil leaks are the main cause of problems on hydraulic systems, and that air in the system makes steering spongy or even makes it fail altogether.
How a surveyor checks it
The reservoir. Check the fluid level at the helm, and the pressure on systems that are pressurised.
The leaks. McDonald's list: the whole system, with particular attention to the ram and helm unit seals. Look at the ram shaft for oil, and the bilge under the ram. Check flexible hoses for dry rot or cracking, and follow tubing runs through bulkheads, where chafe and corrosion often occur.
Air. Spin the wheel hard over and count the turns lock to lock, listening for noise. Then hold it hard over and let go. If it bounces back, there's air trapped in the system.
Under way. On the sea trial, the wheel should answer without lag or creep, and the boat should hold a course without the wheel drifting.
What decides the grade
Whether the boat can be steered. A spongy wheel, a slipping wheel, an active leak at the ram or a cracked hose is an A. Weeping seals, low fluid or air is a B. A sound, dry system with no service record is a C.