Hi Terry,
Here's the Ackerman steering compensation story. When our catamarans make a turn, like tacking for example, the hulls follow curved paths. The hull in the inside of the turn is making a sharper turn , turning on a path with a shorter radius of curvature, than the hull on the outside of the turn. The rudder behind each hull should be coordinated with the turn that hull is making during the turn/tack. Since the hulls are turning on different radii, the correct coordinated turn angle between the hull centerline and rudder foil centerline is different between the two hulls. The correct compensation, or difference in rudder turning angles, varies with boat length and boat width and turning radius.
Here's how you calculate The Ackerman Steering Compensation for a catamaran sailboat.
Draw a picture to scale, top view looking down, of a catamaran in the act of tacking. It can be a stick figure. Only hull lengths and widths and angles are important. Use a turning radius for the 'inside of the turn hull' equal to the width of the boat. The outside hull turning radius will be twice the width of the boat turning about the same center point as the inside hull. The centers of the two hulls should be tangent to their respective paths at the hull mid point. Now go to the transom of each hull and draw a line from the center of turning to an imiginary pintle on the transom of each hull. Now draw a line perpendicular to each rudder turning line from the pintle aft. These lines are along the center lines of the rudders coordinated with the path each hull is turning on. Notice the rudders are not parallel. Extend these rudder centerlines forward until they cross. The included angle between these two rudder centerlines is the Ackerman Steering Compensation Angle for this boat turning/tacking. For a 16ft boat by 8ft wide turning on an 8ft radius the Ackerman Steering Compensation is 18 degrees. This could be built into the tillers with half the angle built into each tiller. The angle is measured between the effective tiller centerline and the rudder centerline. At the forward end of the tillers, they appear towed-in toward each other.
This is a start on the subject;I hope it helps.
Bill
