Although this post is specific to the classic Monk 36 trawler, the system description may be helpful to owners of boats of other manufacturers and other hull designs.
Sanctuary is fit with two Capilano (now Teleflex) model 250V helm pumps. Hydraulic systems based on these pumps were very common in the mid-80s to early-90s. They are a generation older than the model 1250V, which succeeded the 250V design in the early-90s. These systems are filled with hydraulic fluid. When properly purged, there is no air in the closed hydraulic system. The 250V system is not pressurized. In systems containing multiple helm or autopilot pumps at different vertical elevations, fluid is added from the pump at the highest elevation in the system. The highest pump in the system has an air space that functions as an expansion reservoir. The expansion reservoir is vented to the atmosphere. The overall system is maintained at its full level by gravity.
Hydraulic systems containing Capilano 250V pumps also contain a Model 50 Uniflow valve. The Uniflow valve converts the cylinder from an “unbalanced” design to a “balanced” design, and provides a return path for the flow of hydraulic fluid.
The Capilano 250V helm pumps have built-in pressure relief valves that vent hydraulic pressure when the mechanical limit-of-travel of the cylinder is reached. The mechanical limit-of-travel is referred to as its “stop.” When the steering wheel is turned fully “hard-to-port” or fully “hard-to-stbd,” at the point that the “stop” is reached, the check valve opens. At that point, the helmsman will feel only a very slight increase in back pressure. The wheel (pump’s shaft) will continue to turn indefinitely. However, the pump’s internal pressure relief valve opens to protect the pump and hydraulic system from any hydraulic over-pressure damage or ill effect.
On the model 250V pump, there is a knob beneath the pump’s shaft that adjusts the number of turns from stop-to-stop. To the helmsman, this can be thought of as a helm sensitivity adjustment. The knob adjusts a wobble plate internal to the pump. The wobble plate changes the fluid displacement volume of the pump. The hydraulic fluid displaced by the pump ranges from 1.7 in3 to 3.4 in3. The least number of stop-to-stop turns is around 4 full wheel revolutions, and the greatest number of stop-to-stop turns is around 7-3/4 wheel revolutions. A low number of stop-to-stop turns requires more steering effort on the part of the helmsman but also results in greater responsiveness to steering adjustments. Generally, this adjustment is made to suit the personal preference of the individual helmsman. To manually “center” the rudder, the helmsman will have to learn to “feel” the point at which the internal 250V pressure relief valves release. When that point is reached in either direction, stop turning the wheel. While counting the number of turns, turn the wheel the other way until the check valve releases. Finally, turn the wheel back 1/2 the total number of turns, and the rudder will be centered. Over the course of the day, steering wheel indexing may change, but that is never a problem in operation.
Hydraulic fluid does not evaporate, nor is it consumed in use. Loss of hydraulic fluid is always related to a leak in the system. The most common hydraulic fluid leaks occur at the shaft seal of the 250V pump shaft or at the seals of the ram piston shaft. Less frequently, leaks can occur at hydraulic fittings anywhere in the system. If the level of the fluid in the pump reservoir gets low enough, air will be introduced into the sealed hydraulic systems and become entrapped in the system. Air in the system will generally cause the helm pump to “chatter” as air bubbles pass through it. The overall capacity of Sanctuary’s hydraulic system is 3-1/2 quarts of hydraulic fluid.
A temporary “fix” for a low fluid condition can be achieved by simply replacing the lost volume of fluid; i.e., “topping off” the reservoir. The definitive fix, however, is to find and correct the source of the leak. Hydraulic fluid is added to a level of 1″ below the fill ring of the highest elevation pump in the system. Manufacturer specs allow the use of Dexron III automotive transmission fluid on these steering systems, but I personally prefer to use Teleflex hydraulic fluid as sold in most chandleries sell. Follow manufacturer procedures for bleeding air from the hydraulic system. For Monk owners, I have posted those procedures here: http://groups.yahoo.com/group/Monk36OwnersAssociation/files/08.%20Mechanical/Early%20MONK%20Steering%20System/.