external force
 

hi.
i have a boat that it has 6 degree of freedom and i want to give it an external force in the direction of the longitudinal axis of the boat? (instead of modeling its propeller)
so i need to write an expression for the external force and as i said my problem is 6dof.
at first i wanted to define a coordinate that moves with the rigid body. but i could not define coordinate like that.
so how should i give the boat an external force that always be in the direction of the longitudinal axis of the boat?

Sorry, wrong section