My take on the original question Pete is that mirage drive use depends.
In open sea with good winds I usually get through any tack and only pedal to get in or out.
However, on Sunday I decided to explore Werribee River in Victoria, Australia.
Perhaps I should have known better as the forecast and tide information told me that
going from the mouth, the river would be ebbing in the morning and the wind would
be on the nose. Then after stopping for lunch, the reverse true.
It took me over 2 hours to get up the river to where I ran out of water, at low tide.
I averaged 1.4 knots to cover some 3.1 Nm and pedalled at Bob's walking pace,
90% of the time. The trip back was much the same although towards the end I was
able to do lots of tacks in the narrow river.
Pedalling wasn't a problem as the serenity and bird life was amazing.
This is the up river track -

Cheers,
Brian in South Australia