How to aply force to parent gameObject using childs local space coordinates?

Hi I'm using empty gameObjects to simulate water drag force in boat. Force is bigger in boats width direction than length, and its depended on speed of that empty GO. ( not on boats speed because they can be different when boat rotates). I got this script to work, but I use dirty trick to get force vector to point right direction.

 // This script is in few gameobject that are children of boat GO
 // I Use it to simulate drag, so its bigger in sideways 
 
 var boatRigidBody : Rigidbody;
 
 var dragX : float; // this are used to adjust drag difference 
 var dragY : float; 
 
 
 private var dragForceX: Vector3;
 private var dragForceY: Vector3;
 
 private var previousFramePosition : Vector3; 
 private var speed : Vector3;
 private var speedLocal : Vector3;
 
 function FixedUpdate () {
     // Have to calculate sped this way because this has not rigidBody 
     speed = (transform.position - previousFramePosition) / Time.deltaTime; 
     // speed to local coordianates
     speedLocal = transform.InverseTransformDirection(speed);
 
     
     //  ****************************************
     // this dirty hack I did to get Force vector to point right direction
     
     dragForceX = transform.right*-speedLocal.x*dragX ;
     dragForceY = transform.forward*-speedLocal.y*dragY ;
     // *****************************************
     
     // Apply forces
      boatRigidBody.AddForceAtPosition( dragForceX , transform.position);
     boatRigidBody.AddForceAtPosition( dragForceY , transform.position);
 
 
     previousFramePosition = transform.position; 
 
 }

What would be right way to get vector from world space to local, ( and then back to world). That script does what I want, but I don't like those "hacks".