You can apply the force at an offset (e.g. at the back of the boat) and directed a certain way to steer the boat.
Boats have high friction in left-right direction but low friction in forward-back direction, that's how you deal with drift.
"applying force at an offset" means you take a point-not-in-center-of-mass, and direction of force, and use them to calculate the torque and force on center, it's simple dynamics.
To make the boat go forward, you apply the force forward. To make the boat go left, you direct the force a little to the right
, so that the boat turns as it goes.
Edited by Zondartul, 04 August 2012 - 01:49 AM.