I am not sure what technologies you are using or what you have already tried, but all you should need to do is apply a force* along the ship object's forward axis, i.e. the vector that corresponds to the ship's facing. So for example, if the ship's positive Z axis points in the direction of travel, you would direct its movement along the positive Z axis (relative to the ship, not the world space). Meanwhile, steering would just rotate the ship along its vertical axis without applying a directional force. This would mean that regardless of how the ship is rotated, it will always continue moving along its own forward axis.
I am not an expert in nautical physics, so you may be trying to do something more complicated, but this should handle the basic case.
* Substitute translation, acceleration, etc. for whatever method you're using to calculate movement.