[Duncan]

Hi
Ref your boat - at 3 - 5 knots - you are getting into horse drawn territory! - a single horse can pull a 40 ton barge at walking pace

So you really don't need much power

The secret of efficiency with a water screw is large amounts of water pushed back slowly - so a big low rpm prop is best

I don't know about sparks and corrosion but a pair of forklift motors running on 48v would give more than enough power

 

[Duncan]

A large three blade prop is a LOT more efficient than an Archimedes screw

 

[Duncan]

And the safest way to do any conversion is not to do it