One boat like his has a variable pitch propeller, it was a hundestad unit. Very well built but performed very poorly, hub took up entirely to much of the usable diameter. Guessing the 3:1 was the selection based off of loaded weight. Boat probably curbs 55-70k and loads 2x that weight.
When they were developed they fished for terminal wild stocks. In the last 30 years the shallow aspect for much of area m, chignik, kodiak and pws has heavily adapted to deeper open water fisheries intercepting a mix of wild and hatchery fish. Sadly the tiny little egg beater props remain, so we shove rather heavy platforms around.
We tow hard on some sets, but it's a small percentage of what we do. Most boats are increasingly light in the torque department, having been built with 12-14 liter mains in the 400 hp range. Fuel prices and main cost means a lot of boats are trading out smaller, it works because 85% of what we do is low in the power need. Biggest change in the fishery tech wise is our nets, so we're dragging less through the water than 20 years ago.
Crawled under the only 6 foot stretched 50 I'm aware of, think a full flow nozzle would take up a lot of space. Between the prop hub and nozzle clearance would leave precious little room for flukes. Looks like a 2 inch shaft. Quick eye balling it, I'm guessing you could get a 30 inch under there if you followed the shaft line back to the new stern and keep the keel iron at the sams angle. Maybe do an intermediate bearing with a steel keel Ala the stretched leclerq boats. Think a 30 with a bit less pitch would absorb the 3:1 better and be cheaper, as it sounds like you have real diy skills.
