[Someone]

A front rudder would not need to move before its axis of rotation. It could easily be just like the rudder of a rowing shell (http://www.rumerys.com/Avanti/a3.jpg), but mounted close to the bow.

Also, many ships want/need to have the ability to steer both the front and the back of the boat. As an other reply already says, bow thrusters (http://en.wikipedia.org/wiki/Bow_thruster) are common in larger ships nowadays.

And yes, people have tried to build serious sailing boats with two true rudders:

http://articles.philly.com/1986-10-17/sports/26058791_1_radi...

I even could find a mention of a boat with only a front rudder: http://www.craigmarine.info/yacht_boat/builders/power_boat/B... (scroll down to 'Canard')

So, be a front rudder, but be good at it!

[frenchman_in_ny]

A number of high performance sailboats have front rudders -- it's called CBTF or Canting Ballast Twin Foils. One example is the Schock 40 [1].

[1] http://www.wdschock.com/boats/schock40/s40_specifications_b....

[astrodust]

The down-side to a front rudder appears to be that it creates turbulence underneath the hull of the boat, where the bow rudder creates turbulence in the water behind the boat.

A rudder also creates drag. Having drag behind the boat is a lot easier to deal with, it has a damping effect on movement, than drag on the front which would make it more twitchy and erratic.

For a racing boat maybe you're willing to deal with twitchy.

[Someone]

Aha! I knew that these existed, but didn't know they had two rudders/foils, too. Sailing boats that can't really be used if their engine isn't running :-) (they need power, typically hydraulic, to move the keel during sailing)