Jets (be they axial like real waterjets or centrifugal like a bilge
pump) are more efficient than larger diameter propellers ONLY when the
boat is moving fast enough that the drag from propeller
strut/shaft/and rudder becomes a significant part of the drag values.

In the real world that works out to be around 25-30 knots. A bilge
pump will never beat a small trolling motor propeller at displacement
speeds.

I've seen Cal 20's pushed by a big trolling motor for a sailing
school. Works o.k. in calm water but in any sort of breeze it's not
enough thrust. The Cal 20 with a 3.5 HP outboard was a LOT faster and
would end up towing the ones with a trolling motor.

For a Thunderbird, a typical 5-8 HP long shaft outboard is the only
solution that makes sense.

Evan Gatehouse