Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

I think this general theme (Using inboard pumps for propulsion) is
worth pursuing. One interesting aspect is being able to use the same
pump(s) for forward propulsion and bow-thruster/steering.

Can we start with a decent idea of the efficiency of the trolling
motors?

How much current do some typical units draw (all 12Volts?) ??

Are they rated only in static thrust? Or also Horsepower? Horsepower
can be converted to Force VS Distance VS Time. (1.0 Horsepower == 550
Foot-Pounds per second, right??) HighSchool Physics was, um, 50 years
ago :-) Yes, I just see "1 horsepower [electric] = 550.221 382 975
foot pound-force/second" at
http://www.onlineconversion.com/power.htm

So, IF you knew the relationship of Drag (In Pounds) VS Speed for YOUR
boat, you could create a graph of Horsepower VS Speed. (This would
be for "Perfect Horsepower" which certainly will not happen with real-
word trolling motors and propellers, OR real-world pumps and
hoses.. )

But you'd have SOME idea...

If you had a 25 pound fish scale and 100 feet of line, and someone to
paddle the boat OUT so you could pull it IN, you might start to get
some numbers...

Other Related Idea: I have thought about running a medium-large (??)
pump from my inboard boat engine to bow ports for "Bow Thruster".
Anyone seen something like this?? A pump could be engaged with an Air
Conditioner Clutch....

So let's keep thinking about this????