I have considered this - my guess - like Don's - is that a single big motor
would be more efficient.

And if comparing two 15's to one 30, I would definitely opt for the 30.
But my boat (a 21' sharpie) probably needs 60-100 lbs. I got a 40 at
Walmart, cause it was cheap and available, and it is OK, but not quite
enough against a breeze. But above that size, the purchase price of a motor
increases dramatically - far more than in proportion to the thrust.

So I'm thinking two 40's. In addition to the purchase price economies, the
advantages of redundancy seem to be compelling. And 90% of the time, one
motor will be plenty.

Now - how to charge the d***ed batteries - maybe put them in the bed of my
pickup, wired in parallel with the main battery? Or keep the boat
near-shore on the lake, and run a long extension cord through the water....

Sal's Dad

-- My initial and un-scientific guess it that you would get something less
than
the paper calculations might show because you have the added physical drag
from the second motor that MIGHT be greater than that from a single motor.
I would suspect that the actual displacement of two motors would be
greater
than that of a single motor I.E.; more stuff in the water equals more
drag.
Secondly, if the single motor is rated at a higher efficiency than the two
motors are, you've lost ground in battery life. Conversely if the smaller
motors are more efficient, you're ahead of the hounds.