your sog faster with the current, sog slower against the current is indeed a fact. My dog instinctively knows this, too, as he has to put it into practice every time he retrieves something from the water and comes back in a cross-current.
My comment was on the notion (theory) of accurately calculating the current effects for a voyage up or down the inside passage. The data necessary for accurately doing this simply doesn't exist along much of that route. So all one has to work with are general current averages. That inaccuracy combined with the extremely variable local currents along the way in the maze of channels and passes one goes through make it pretty impossible to come up with a truly accurate prediction of time and fuel consumption.
In other locations--- sfo bay perhaps--- where the current data is complete and the geography does not have the same extremely varied effect on local currents over such a huge distance, it is much easier to accurately predict time and fuel use for a cruise.
That's my only point.
