1) Bison: supplied with extra propeller and parts available online.
2) 40lb: I hope this would be large enough and will use less power than a 55 or 68.
3) The 40 has a different size propeller, so suited to motor power (hopefully). The 55,68 and 100lb thrust has the same size prop, so I would think it better suited to the mid range of those sizes.
4) 65 amp deep cycle battery: Lighter to carry and hopefully enough power at slower speed to make at least a couple of trips before 50% depleted.
From what I've read it's not a good idea to use the top speed for any sustained amount of time, as it draws more amperage than circuitry in the motor can take and drains the battery very quickly. I wouldn't be in a rush to get, to and from Blackjack, but would need something that would cope with the wind and tide even if assisted with rowing in bad conditions.
I would be grateful for anybody's experience or opinion.
Cheers Paul Morris
Have no experience of using an electric outboard but a forum search came up with a post by Martin Watson around 8 months ago.
Martin had bought a 68 lbf thrust Bison o/b powered by a lithium golf cart battery.
I believe that Martin has researched such motors extensively so it might be worth contacting him for advice.
Will be interesting to see how you get on if/when you buy one.