12 knot cruise with a 225 John Deere is right on. My 30 young brothers with a 6bta was a 12 knot boat at 2000 rpm. The original 8.2 Detroit was right about the same. 12 knots at 3-4 gals per hour makes for an efficient, moneymaking workboat. Want speed ? Push it up, but be prepared for double the fuel burn and half the motor life. All these guys bandy 20 knots around like its easy, it's not in the real world of fishing
YA, all those guys are the same idiots that are bitching that they blew their engine or gear at 4000 hours. Meanwhile all the lobster boats that do cruise at 12 knots have 20,000 hour engines that are still running like sewing machines. Fuck, most of the guys I know cruise at 8-9 knots and would be thrilled with 12 knots at 3-4 gallons an hour.
Old news- if you run 'em easy they last. Just that Young Brothers sail along easier than most and used to be known for their speed- that was the point. 600 off the top ought to run for a long time no doubt. I don't think anyone is expecting a 20 knot cruise w/ 225 hp. But there are some people who won't run a boat 20,000 hours in a lifetime, so they might run her a little harder in a rec or charter application. Those guys could probably could get good enough service out of a 225 JD at a blistering 15 knots.