Ackerman Steering Compensation For Catamarans

Technical discussion of ARC products
Post Reply
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Ackerman Steering Compensation For Catamarans

Post by Bill Roberts »

Hello Catamaran Sailors,
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 overall 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 overall boat widths and angles are important. Use a turning radius for the hull on the inside of the turn equal to the width of the boat. The outside hull turning radius will be twice the overall width of the boat turning about the same center point as the inside hull. The mid hull length position on each hull should be tangent to the respective curved path that each hull is following. 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. Next draw a line perpendicular to each of these lines passing through the imiginary pintle on each hull. These perpendicular lines are along the center lines of the rudders and are coordinated with the path each hull is turning on. Notice the rudders do not appear 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 22ft boat by 12ft wide turning on an 12ft radius the Ackerman Steering Compensation is 16 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 intersecting at the pintle. At the forward end of the tillers, the tillers appear towed-in toward each other.
The tiller crossbar is connected to each tiller end fitting. The length of the tiller crossbar can be such that when the the boat is sailing in a straight line, the rudders are parallel to hull centerline. But, when the boat is turning, the design Ackerman angle comes into play and the rudders are not parallel. The rudder on the inside of the turn is turning sharper than the rudder on the outside of the turn. Turn the steering system in the opposite direction and the rudder compensation reverses.
Good Sailing,
Bill
Post Reply