[MHml] Cat Rudders Toed In?

[Dan Frenette]

I could see where the flex in the cross arm would let the toe in flex to 
parallel. This would however be a loss of energy. The reason they do 
this on cars is to remove the play from the system.

If you have toe in and you go fast the cross arm or the rudders or ... 
will flex or it will just explode.

I don't know if the magic toe in number would be less than 2 degrees or 
1/2 a degree. You would start with the angles on the gungons etc. You 
can figure out how much play you have and work back. Or just go back and 
measure it and compensate. In an ideal world you don't need that but 
with dissimilar metals, hull flex, etc you maybe better off working that 
into the equation.

But I don't do this for a living so it's JMHO.

Dan Frenette

Tom Henry wrote:
> As has been mentioned, there are two issues at work here
> 
> (1) the angle ( or not) for straight ahead sailing
> (2) the Ackermann angle for making turns
> 
> 
> When sailing straight ahead the rudder blades should be essentially 
> parallel.
> 
> Typically, catamaran tillers are toed inward substantially compared to
> the rudder blades, which are typically parallel to each other and
> aligned with the hull centerlines when the boat is sailing straight
> ahead (not allowing for leeway ;-)
> 
> What is referred to as 'Ackermann' enables the rudders to be more
> correctly aligned during turns, the more the tiller bar is moved, the
> more the angle between rudders increases, so the tighter the turn you
> are trying to negotiate, the more it helps you.
> 
> See http://hem.bredband.net/b262106/Boat/acker.html for a rather helpful
> diagram.
> 
> To accomplish this, the tiller is set at an angle (inward towards the
> boat centerline) to the rudder blade (approx 30 degrees, and you'll get
> LOTS of feedback from the list on how to figure the best angle for your
> boat).
> 
> As a consequence of designed-in Ackermann, one would expect your tiller
> bar to be shorter than the distance between the rudder posts.
> 
> IMO - IF your tiller is parallel to the rudder centerline then the
> designer has NOT built in any allowance for Ackermann -- and -- you
> should _very_ seriously consider correcting this oversight.
> 
> HTH,
> 
> 
> ======================================================================
> List Friends,
>      Our new to us 35' Cat (designer unknown) appears to have both
> rudders toed in about half a degree from parallel.  Is this a normal
> thing with cats or a builder's mistake?  My experience is with trimarans
> and I would think they should be parallel with the centerline of the
> hulls.  The rudders are a tapered NACA 0012 foil with the shaft at 25%
> of the chord.  I'm reattaching the tiller crossbar at the ends of the
> tillers, and it is 1 1/2" shorter (center of tiller hole to hole) than
> the rudder shafts are (14' center to center)  The tillers are 58" long.
> Thanks for any advise!,  John Matthiesen, Sugaree Too
> 
> 
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