You power up in your friend's tournament boat and immediately feel something different. Your boat holds a straight course with hardly any effort, but the steering wheel in your buddy's boat pulls to the right, and if you let go of the wheel, the boat veers off course.
What you're experiencing is a difference in rudder torque, and your friend doesn't necessarily have a problem in his steering system. Rudder torque is a measure of the amount of directional steering system pressure generated by the rudder as it travels through the water. When operating a boat with significant rudder torque, the driver must apply greater steering pressure to counteract the directional “pull” of the steering system.
Generally, the amount of rudder torque varies among powerboats and can usually be adjusted to suit specific applications or driver preferences. While excessive rudder torque is unsafe and tiring to the driver, the positive pressure and “heavier” steering feel created from a “loaded” rudder are preferred in some skiing applications.
Tournament ski boats used for relatively short, straight runs through a slalom course generally benefit from more rudder torque. This pressure on the steering system provides the slalom driver with a frame of reference, helping him to lock into a straight course through the buoys and counteract the strong opposite lateral pull of the skier. This solid frame of reference becomes even more critical in world-class slalom skiing with shorter line lengths and tighter course tolerances.
For recreational skiing, boarding or cruising, though, this constant pressure on the steering wheel can become annoying and just plain tiring on the driver. In this case, a more neutral rudder, one that creates less torque and steering system pressure, tends to be more comfortable, more forgiving, and safer.
The hydrodynamic force of the water passing around the vertical hydrofoil that is the rudder creates torque. In most cases, rudder torque can be reduced or exaggerated by manipulating the surface and shape of the hydrofoil to change its hydrodynamics.
Flattening the right side of a rudder, for example, will increase a boat's tendency to pull to the right, while flattening the left side of the same rudder will reduce the right-hand pull.
In addition, all inboards have a natural tendency to pull left or right based on the direction of prop rotation. This is known as prop torque. Grinding a rudder to pull in the opposite direction of prop torque
is not recommended, as it generally has an adverse effect on both steering performance and wake characteristics.
Since rudder tuning is not an exact science, it should be performed by a trained professional and through a gradual process of trial and error. The rudder shape is carefully altered by grinding; then the steering is tested on-water to see if additional adjustment is needed.
“Most high-quality rudders are manufactured with a precise, symmetrical hydrofoil shape,” says Charlie Pigeon, president of Tigi Boats. “But just like propellers, we lake-test every one to make sure, and sometimes we find one that needs some tuning.” Because Tigi boats are designed for versatility, Pigeon says, they have relatively neutral steering. “A loaded rudder can create some tired arms at the end of a day on the lake, so we go with the two-finger steering approach.”
Pigeon adds that customer preference, however, comes first, so if rudder torque is requested, Tigi will make the necessary modifications.
“It's really a matter of taste and how you use your boat. If you're in the slalom course all day pulling a serious skier competing at a world-class level, a boat with a heavier rudder is definitely beneficial. More neutral steering would be a handicap in this case. But if you're cruising, towing boarders or most skiers, you probably don't want to be fighting the steering wheel all day. The important thing is that rudder torque can be easily modified to fit your needs, but it should only be done by someone who knows what they are doing.”