Advanced Maneuvering – Catamarans and Twin Drives

Here again, we are discussing catamarans and twin-drive monohulls for the sake of their similarity and non-duplication.

Real Exercises

Here are some exercises to try out on the water to experience the maneuvering ability of catamarans. This is assuming you don’t have an azimuth Pod drive but rather a standard inboard shaft drive system with rudders aft of the propellers.

 Exercise M10-2

• With engines in neutral, put the wheel hard over in any direction.
• Apply a forward thrust 2-second burst to 2500 rpm on both engines and observe the boat’s behavior.
• With the boat stopped in the water, apply a reverse thrust 2-second burst to 2300 rpm on both engines and observe the boat’s behavior.

What You Observed

• The boat reacted to the forward thrust to turn the boat in the same direction as the wheel. The boat made no reaction with a reverse burst.

Advanced Maneuvering

Now that we know more about rudder forces, let’s look at how to really maneuver a Catamaran using thrust and rudders.

While the Catamaran has two hulls, two engines, two rudders, and sometimes two wheels, both rudders will always work in unison and pivot in the same direction. The rudders do not operate independently – even if there are two helm wheels – whew!

View the animation below. This assumes the boat is moving forward through the water with engines in forward.

When moving forward through the water and using both throttles equally the Catamaran turns much like a monohull.

Unlike a monohull, however, there is another variable of prop wash that can work independently on each rudder. This can give an advantage of being able to maneuver the Catamaran sideways.

For example: With the wheel to port and the port throttle slightly forward and starboard throttle back, the boat will vector sideways to starboard. Port-side water thrust will act on the port rudder while there is no thrust wash over the starboard rudder because that engine is in reverse. See the diagram below and think about it this way, port forward and starboard aft alone will turn the bow to the dock. When you add in thrust over the rudder from the helm to port – this will push the stern to starboard.

The principle here is that the boat wants to spin clockwise but the thrust on the port rudder is pushing the stern of the boat to starboard. Thus, the effect vectors the boat sideways. Makes sense right? In a monohull, if you wanted to kick the stern towards the dock, you’d turn the wheel away from the dock. Combine this with using the bulldozer effect to turn the bow to the dock and you get sideways force.

Here is an animation to show the same effect (just for fun)

There is an easy way to remember this: Use the bulldozer effect to turn the bow to the dock and turn the wheel away from the dock. Use various combinations of throttle to get the boat moving exactly sideways.

A word of note and caution: The sideways vectoring trick IS NOT powerful enough to overcome the effect of wind. You can not vector upwind using this method.