A 9.9 hp with a barge prop is all you need and better still if it could be a four stroke to keep noise down and no oil.
The movie I made on lake Waikaramona https://www.youtube.com/watch?v=DDfSuAtiVkQ
shows a 4 ton load powered with one 15 HP four stroke Johnson, when no load I used both 15hp out boards which speed along at 25mph.
You can see another one of my landing barge designs next to the Myark folding trailer barge with a RV camper on top that this landing bare has plug in torsions suspensions which the barge hull is actually the trailer cutting towing weight down and trailer cost, also can launch on beaches or remote areas with no boat ramps available and floats in a couple inches of water.
That desigh has two 25 hp out boards and does 30mph and would make a good base for trailer camper boat as its torsions allow close to the ground, also easy to walk of and onto a beach front.
I have attached a picture of the lake that the movie was made with my dog and I.