I think that in general this is right, and that it is am important insight.
The only thing I would say is that SWL's method is inherently more accurate, and is no more trouble than the traditional method, so the accuracy is without any cost. Therefore, I am convinced that it is superior.
Another thing I would say based on experience -- although they are on the one hand too precise, as variations speed and steering
accuracy and wind
direction will inevitably conspire to mess them up, at the same time, none of these methods is actually precise enough, because they don't consider the fact that the currents are not precisely perpendicular (and deviations of current
direction have a big effect on the results), and because they use coarse-grained current
data from tidal atlases.
When I realized all of that, I just stopped doing it by hand altogether and started using a computer, which has been vastly better, not too mention easier.
Using a computer, you deal with unexpected changes in wind
or boat speed by rerunning the numbers every couple of hours, which only takes a few seconds. It works a treat; I'll never go back.
The computer also deals brilliantly with the situation which is impossible to do by hand -- strong rotary currents like in the Channel Islands.