[1] The primary purpose of a wing engine is to provide redundancy and safety in the event of failure of the main engine; a secondary benefit assists manoeuvering in port or in a marina.
Whereas the main engine will be larger and invariably mounted on the vessel's centreline, the wing engine will be considerably smaller and positioned to one side.
This can give rise to steering difficulties; but this can be used to advantage in port with the main engine as follows: if the main engine has a right-hand propeller, the "prop walk" when in reverse will tend to move the stern to port.
In these circumstances, the wing-motor should be arranged to have a propeller to the left (port-side) of the centreline, so as to balance the vessel in astern, or to produce (with the main engine in neutral) a vector thrust to starboard.
In such canals the wing engine may be used to propel the boat; but when the vessel puts to sea or navigates a fast flowing river, the power of the main engine would be needed.