Poking my nose in here, looks like a great build and some savvy folks paying attention! Had a great season with my boat, love it more every time I use it.
Here is the best engine size calculator I've found online:
https://btb.fishing/boat-horsepower-calculator/For reference, my boat, built with okoume plywood (lightest available) plus foam core for much of the cabin, with 9.5 ft cabin, 100 gal fuel, anchor winch, 3 people, loaded for shrimping, fishing, with dinghy, safety gear, spare parts/tools, the engines, a fridge, heater, complete electronics and everything else for a multi-day trip in Alaska weighs 6750 or so. Maybe even a bit more, little things add up! So my first recommendation, as soon as you get your trailer, drag it across a scale and get it's exact weight. Then once the boat is on the trailer, drag it across again to get the hull weight, then you can get a spreadsheet started with all the accessories you plan to add to the boat. My boat was built super light, with no overkill on the glass schedule, so the hull and cabin are as light as you can probably get it, and on top of that I camp for days at a time on my boat so it has more accessories than a pure fishing vessel would have.
Pay attention that this calculator uses statute mph, so you have to convert from knots to mph. Brian designed the boat to cruise around 22 knots, so I use 25 mph for the low cruise, and practically speaking, the boat goes up to about 28 knots at reasonable fuel burn for me so I used 32 mph at the high end. The result is 225 hp for low cruise, and 288 hp for high cruise.
My typical rpms are low 4000's to around 4500 rpm when conditions are decent, going around 25-28 knots with my 250AP Suzuki V6 - same block as the 300 hp. There is no way I'd want that to be a 4-cylinder 200 hp engine - it would be wound up pretty high much of the time.
A note on props: I went with a prop that put me a little lower in the recommended rpm range, around 5600-5700 rpm (recommended range 5500-6100) so that I would get more efficiency at cruise...I'm generally very easy on the throttle and could care less about hole shot. That said, when I'm in heavy seas and need to hit the throttle more aggressively to make a move, it's probably a bit hard on the drivetrain, so the sense of security with that larger engine feels reassuring.
So, if your build is going to be light, with minimal equipment, and you don't pack hundreds of pounds of ice....you might be fine with the 200....but don't take anyone's word for it, use the calculator!
