Hydraulic Steering in Multihulls

[Talbot]

The tie bar is by far the easiest way to set up your rudders so that there is an akerman (sp)system. This means that when for and aft, the rudders are parallel, but when you turn the wheel, the inner hull on the turn has a rudder that is turning tighter than the one on the outer part of the turn.

[44'cruisingcat]

Originally Posted by Talbot

The tie bar is by far the easiest way to set up your rudders so that there is an akerman (sp)system. This means that when for and aft, the rudders are parallel, but when you turn the wheel, the inner hull on the turn has a rudder that is turning tighter than the one on the outer part of the turn.

It's not difficult to set up ackerman geometery on hydraulic steering. I seriously think it would be a complete waste of time. It's neccessary on a car because the tyres don't (normally) slip.

Rudders are entirely different, they HAVE to slip to work. They work because they have an angle of attack, and so create lift. So the only effect having ackerman geometery, perfectly set up, would have is that each rudder would develop the same amount of lift, whereas without it there could be a slight difference.

For ackerman geometery to work you would need to know what your boat's turning circle would be at a given rudder angle. You also need to know where the centre of rotation will be. And I actually think both of these parameters could change with varying speed. ie. rudders will slip less at higher speeds.

[Talbot]

This was discussed at length some years ago by builders and users at Multihull Sailing - Information and Members Forum The agreed recommend was that akkerman steering does make a difference.

The additional "lift" on the outside hull provides a turning momentum which helps to counteract the tendency of a cat with LARS to skid outwards on a turn.