The Optimal Speed module gives you an indication about how well the vessel is performing. Interference between the stern wave and the bow wave can either be making the transversal wave bigger (hump) or smaller (hollow).
If the ship has access to on-line speed through water you can set the select option to “Online“ to see how the ship currently is doing, if you are currently close to a hollow speed and thus don’t make any transversal wave or if you’re at a hump speed where the transversal wave is built from the ships engine.
Setting the select option to manual turns the window into a simulator where you can see how efficient different speeds are in terms of hump and hollow. Maybe it’s possible to reduce the speed, get in a hollow interval and still make port in due time.
Online speed or simulated, you will always save bunker by using this module. In terms of money you will have gained the entire cost for the system in return after a short period of time.