Hi Bo!
Thank you and there is no such thing as "ignorant" as we all see situations differently which makes for the stimulation of great ideas! Counting the revs is a great idea and this sub has two motors. I guess you could add the two motor RPM's together and then divide them by 2 for average RPM. In a turn you speed up one motor and slow down the other. While I do not have an answer as to why this is not used in aircraft (for example) it seems there must be a reason. Aircraft seem to use a pitot tube while boats seem to use either pressure or a "paddle wheel" system. Maybe it has more to do with "flow" such as a "head wind" in aircraft? Best, Ed