Not a Design but a Use Case change: Prop changes as needed?

[Swamplizard]

Thanks CDog - getting there. Replacing Trim Pump today which will make big difference I think in terms of finding best economy cruise speed and perhaps a slower on-plane speed when wanted. Hoping these props are the right set but wont know until I get flat water and get to WOT for a few minutes.

 

[onlineentity]

For small mud motored vessels I've found that trial and error of many size and shape props is very common thanks to the affordability of the props. They'll even send free ones to try sometimes. That is a very different case with a large brass propeller I'm sure you would know well .. There are calculators also to help out, here are a couple tools I love this one Free to use Propeller Sizing Calculators https://www.vicprop.com/free-propeller-sizing-calculators and a different one here Prop Pitch Calculator https://www.omnicalculator.com/physics/prop-pitch

Aside from that usually people here use a small kicker outboard for low speed cruising and fishing instead of the inboard. Good to have that backup for redundancy anyways especially up here, it gets super remote and the lake tossing makes sediment movement causing engine failures common along with a million other things.
I'm working on closing my design in on 20mpg so I can't say I am envious of your consumption rate
Another consideration is engines are meant to run at a certain RPM range for peak efficiency and power output, the vicprop tool outputs that after you do a calculation. It's like 70-80% range for a gas inboard meaning the low speed operation will inevitably be less efficient.
Maybe it's comparable to car on freeway has less fuel used per mile but that is also related to gearing very differently

 

[Swamplizard]

20MPG in a boat - Wow!
Pretty happy with props I am testing as my daily runners. The 16,000 lb 42 footer acts like a 30 footer with them - gets on plane quickly, cruises well, and what I lose in top speed will allow me to load her up with humans, fuel, water and beer without overloading the engines.
For those longer "trawler speed" trips in the Keys and Bahamas I think the spare set of 29-pitch props will work....if I can get my 1,000rpm speed from 6mph to 10-12 I would be doubling my MPG.....over a 300 mile trip that adds up. Havnt had a chance to test them yet but will report back.

 

[Bryan carroll]

picked up some high rake 16.5x17 3 blade rolla props. Boat now gets on plane easily, can cruise at 3800 at 24mph (one adult, 200g fuel, light water) and pull 5000 rpm with some room to spare. Hopefully now she can plane when full of fuel, water, and people and not overwork the engines.

I have pair of 4 blade rolla cleavers, slightly larger if you wanted to put the (5000rpm with room to spare) to use