That's a nice houseboat you've got there. We've run through a few over the years, the current one my brother bought about 10 years ago is something like 50 x 20 feet and runs on two twin 35 evinrudes. The barge captain knows what he's talking about. Big engines are for putting boats up on the plane, and you won't be planing, so you only need enough horsepower deal with displacement speeds. That 70 hp we have is more than enough, we can't even go full throttle without bringing water over the bow and into the cabin. We deal with major winds and tides getting the boat down to the marsh and then back into sheltered anchorage for hurricane season, and it handles those fine. I'm guessing the boat you have there will need maybe 1/4 what we have. Look at Glen-l.com. Glen-l has several houseboat plans for boats about the same size as yours. I'd go with what they're recommending for their boats. our latest houseboat we're building right now, and about to launch the hull part of it next weekend, will be 55x24' and will displace 50,000 lbs. We'll power it with a 100 hp Yamaha 4 stroke.
Ed.