The ram is really the key part or “foundation” of the steering system (in so many words, the “RAM” carries the load). The helm pump (at the other end) is just the fluid “pumper” and does not carry the load except while actually turning the rudder. Once the rudder is static, regardless of its actual position, the ram and steering lines (up to the check valves in the helm pump) are carrying the load, or pressures, developed in the system (the rudder is always being pushed, one way or another, from the vessel movement and / or water movement around or past it. Even a very small & inexpensive helm pump could turn a very big rudder on a large vessel quite easily, but this is not always a practical solution. As to selecting a RAM as to its base quality, to us this is a no-brainer. Brass, Bronze, and Stainless Steels alloys are the only acceptable materials that should be used for the construction of any RAM that even gets close to saltwater, or is used on any type of vessel larger than a run-about. Aluminum has no place in Hydraulic Steering RAM construction except (maybe) for fresh water use and super light duty applications.
The internal & external size of a ram is measured in a few ways—Common terms:
- BORE: The piston bore diameter—Inches or in millimeters—common sizes might be 1.25”, 1.5”, 1.75” and 2”
- STROKE: The total travel of the ram – 7” and 9” seem to be most common, but ram strokes come from about 5” to about 12”
- DISPLACEMENT: This is the mathematical computation of the bore and stroke measurements less the volume of the rod itself. This is the most important number as it tells you the relationship you will have with the helm pump as to how many turns the pump will have to make in order to move the ram a full stroke. An example—A 1.5” bore ram with a 7” stroke might have an internal displacement of 10 cubic inches; A typical helm pump may have an internal displacement of 2 cubic inches ( or 35 cc’s) per revolution.. This would mean that it takes approx. 5 complete turns of the helm pump to move the ram its 7” of travel. But in reality, always add about 10% to compute turns vs the real number you will end up with. Why (?) , because of internal hydraulic slippage that is always part of any hydraulic system that uses fluid to move something