Hi Hugh -
82 lbs thrust on a 24 foot boat...
Because you described this as potentially a "life or death" backup, I'd play it conservative and say: not worth the risk! Find a smaller internal combustion outboard instead and bolt it to the transom. Even the smallest you'll typically find is probably going to have more power & useful range than that e-motor. A used one will cost a lot less than a lithium pack, too.
The playing around I did with the electric boat was just that: playing around. On water flat as glass. Any chop or breeze whatsoever, and my (obviously much lower powerful) motor would have been uselesss. But then I could have sailed.
Hope that's not too discouraging a response.
Darin