Originally Posted by backinblue
So full disclosure, I was not familiar with the term "Ackerman". Spending a little time time on ChatGPT I now understand the theory and why it's important in cars. I asked a few follow-up questions relating to dual rudder boats. This was the latest reply:
The dual rudder system used in boats is typically not designed using the Ackerman geometry like in land vehicles. While the principles of the Ackerman geometry are based on the fact that the inside and outside wheels of a vehicle need to turn at different angles during a turn, this is not necessarily the case in boats with twin rudders.
The dual rudder system in boats is designed to improve handling and control, particularly at high speeds or in rough seas, by providing better stability and control. The two rudders can work independently or together to provide more precise control over the boat's heading and direction, but the design of the system is typically not based on the Ackerman geometry used in land vehicles.
That being said, some boats with twin rudders may have steering systems that incorporate some principles of the Ackerman geometry, but this is not a common design approach for dual rudder boats. Instead, the design of the steering system in twin rudder boats is typically customized to the specific requirements of the boat and its intended use, based on factors such as the size and weight of the boat, the type of water it will operate in, and the desired level of maneuverability and control.
Yes, plenty of twin rudder boats keep the rudders parallel through a turn. Unless it's an extremely wide boat (like a catamaran), that's not a bad compromise. Turning the inside rudder (relative to the turn) slightly further (Ackerman) can reduce drag in low speed turns depending on how far apart the rudders are (I don't think it helps much at higher speeds where the boat isn't pivoting so tightly).
However, my boat has the exact opposite of beneficial Ackerman. The outside rudder in a turn is turning 5+ degrees more than the inside rudder when at full deflection. That's the bit I find odd, as I can't imagine any situation where that's beneficial. If it just had no Ackerman at all I'd probably be happy to call it good enough.
Thinking about it, this just gave me a thought... I need to take a close look at the tiller arms on the rudders. If they're toed out slightly relative to the actual rudder position, it's possible that they were just installed on the wrong sides originally and switching them would give a more normal Ackerman effect. That won't be a project for this year though, as the rudder installation requires dropping the rudders to remove the tiller arms. And any work on the steering gear is slow and unpleasant, as it has to be done hanging upside-down over the aft end of the fuel tanks...