If my math is wrong, void this post.

Your math and example may be fine.
I'd have to put on my Kreskin hat to sit at a keyboard and pontificate about
fuel economy on boats I haven't actually observed.

NOYB's example seemed particulalry out of round.

NOYB double checked his example and discovered it contained some errors.

That would not mean that any example somewhat similar to NOYB's, (such as
yours) *must* be equally flawed, just as your experience doesn't vindicate the
original, faulty example.