Rainmaker, I know our boats are much different, but when I repowered from my old 300 lb V-4 on the transom to a ~430 lb motor set 30” back on a bracket, I had a similar problem. With the old motor I could easily plane at 12-13 mph, but with the new motor and a 3B prop, it was terrible . .min planning speed was about 20 mph, an obvious sign of a very stern heavy boat! It would either just plow along as you describe at 2500 rpm/6-7 mph or with a VERY slight nudge to the throttle (less than 1/8”), it would want to run 3500-4000 rpm and 30+ mph! Part of this is due to the near-vertical torque curve of the E-Tec starting at 2000 rpm, where it makes the transition from very lean stratified-charge mode to a much richer mixture. However I noticed it was a little less sensitive with a 19” prop than a 17”, so I concluded that maybe I needed a little more blade area to absorb all that power. I tried a 4-blade prop, and eureka, problem solved!
I now run a 4B SS 15 x15 PowerTech and I can plane at about 2500 rpm/12-13 mph and boat has an awesome hole shot – literally jumps on plane in 3-4 sec at about 45% throttle. Max rpm is about 5450 with max load. The non-linear throttle response is now easily controlled. (BRP changed a $15 throttle cam on the 2009 150’s that reduced this throttle sensitivity a bit.) I never use full throttle to get on plane, as it tends to throw folks out of the boat!
Your max rpm indicates you’re not overpropped. Although CC’s tend to be more stern-heavy than a Seafari, your last post sounds like you may not be stern heavy either. If you’re too bow heavy, I can see how that could be almost as bad as being too stern heavy. Balance/CG location can make a huge difference in how a boat performs and rides, and that’s why Carl Moesly (a former pilot) used ballast tanks on his race boats to adjust trim/CG for optimum performance in various sea conditions. He also put the gas tank right on the CG on the original production SeaCrafts, so trim wouldn’t change as you burn off fuel! Have you tried moving gear around to see if that makes any difference? Also, what is your min-planning speed?
I have a couple of 4B props you’re welcome to try; a 14x18P Aluminum 4B Michigan Vortex is my spare, which turns about 5550-5600 at max load; also have a 14.125 x 20P SS 4B Michigan Apollo that is too much pitch for my 150, (max rpm about 5250 light load/ 4600 max load) although maybe your 225 could spin it faster. I don’t know if the prop shaft/spline configuration is the same on our motors, but the Michigan 2-piece hub system also used by PowerTech will allow the props to be used on any motor with the right spline adapter piece. Just send me a PM if you want to try any of my props.
You must be digging quite a hole in the water and dragging a hell of a wake @ 9.8 mph if you’re not on plane! I normally run either on plane or at hull speed (fully settled in water) at 900 rpm/5 mph and 10 mpg. Most folks don’t realize how fast gas mileage drops when you run faster than hull speed when in displacement hull mode. The I-Command gage that calculates instantaneous mpg says that at 1100 rpm/5.5 mph I’m getting about 7 mpg, so it doesn’t make much sense to burn 30% more fuel to run 10% faster!
Denny