I am new to micro controllers. My project is to control a pair of trolling motors independantly on an electric boat. It seems that an arduino can supply the logic to set the speed, but I need a motor shield, or a large capacity H bridge (which may be the same thing). Can anyone piojnt me toward a shield that could control a pair of 12 v 20 amp motors?
George
Conway, SC
You probably won't find a shield that can do that.
If you want a dependable controller that can do what you are looking to do, then look into combat robotics dual motor controllers (for heavyweight combat robots); most are extremely high performance controllers designed for driving larger high-current motors like this, most of the time they use a simple R/C PWM system (just like a servo), so you can use a servo library on the Arduino to control them.
The only problem is that they won't be cheap. But you will be able to easily vary the speed and direction of both motors.
If you want cheap (but no easy way to control the speed) - go with a few 40 amp SPDT Bosch-style automotive relays; wire four in an h-bridge to drive a motor (so you'll need 8). Control the relays using MOSFETs or driver transistors to switch the coils. Go with these style relays; even if you manage to find other relays which may be cheaper, the big advantage to them is that you can walk into any Walmart and pick up a few - which is handy if you are out "in the field" and need such a part fairly quickly (that, and in a pinch you can "steal" one from your own car!). Since they are also meant for automobiles, they are design for a tough environment; protect them from the water using a water-tight box, and they'll almost work forever.
Good luck.
I wanted to mention one other thing:
If you wanted to have a "dual speed" control (low and high) with the trolling motors and relays, it would be possible to set this up when running both of them in the forward or reverse direction; it essentially works by setting up a relays (more of the Bosch-style auto relays) to switch the motors between running them from the batteries in parallel (high speed), and running them in series (low speed). This is the same system used by a PowerWheels toy vehicle (except in that case it is done with a switch).
However, if you only want to run a single motor, you would only have "high" speed available (you could set it up to run the motors on both sides in opposite directions, so that differential steering would result, and keep high and low speeds).
running them from the batteries in parallel (high speed), and running them in series (low speed).
Got that a little backwards, don't you?
However I'm not a fisherman so what do I know about trolling. 
Lefty
You would most likely have to build your own driver. If this is what you want to control
Trolling motor - Wikipedia and you don't want bidirectional control and only speed control you will only need a single transistor. A BJT would be easier to use starting out, but not the best solution though, a MOSFET with a driver IC might be even better for you.