[MHml] Cat Rudders Toed In?

[Tom Henry]

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,


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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