“Your idea that the stored chemical energy in a battery is “outside the
system” in a way that is different from stored chemical energy in ones
body, is mistaken.”
No, no. That’s not what I said at all. In fact I said the exact opposite – that both are energy stores that could potentially be converted to other forms.
What you need to do to understand this is draw a force-vector diagram. What the turbine fan is doing is slapping the air to move the air relative to the fan. It’s not at all a matter of energy conversion. It’s a matter of mechanics.
The reason the fan works for this is because it’s moving a stream of air relative to the boat. That’s a function of the angle of the blades as they spin. They slap the air forward, creating a low pressure zone that is immediately filled with more air. That’s how the air stream is generated.
When you huff and puff, the mechanics are different. You can only push the air our of your lungs by exerting a force against the frame of your body. If your body’s going to stay inside the boat, it has to push against the boat with as much force as it uses to blow the air out of your lungs – and do so in the opposite direction from which the expelled air leaves your lungs.
If you want to move the boat using metabolic energy, the way to do that is to paddle. Your body being attached to the boat then becomes an asset instead of a liability because you are pushing the water relative to the boat with each stroke.