The Mechanics.
Water is a poor source while in air (heavy, limits passenger capacity, requirements difficult for vessel in flight). Water powered electricity could help increase power vessel begins with.
Solar is promising (recently, one manned plane powered by solar completed test flights within United States). Current human limits are efficiency of absorbance and transfer (easily improved [grew better internally when ship]). They need large surface area to receive enough power (no trans-oceanic flights yet, no crew past one man). Improving it is within their natural grasp (and ours).
I knew they flew (didn't know they taught). Thank you.