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Speaking of Bushwacker, he has a 20 Seafari with 150 ETEC and hung on a bracket. He ghets top ends in the middle to upper 40's. I hope he chimes in here.
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Alex, I would have chimed in sooner but I just got back from a 4 day cruise in the Seafari!
Me and an old buddy that I grew up with spent 4 days and 3 nights on the boat exploring the St. John's River from Sanford to Palatka and back. Ran 236 miles (per GPS) and burned 55.3 gal for an average of 4.3 mpg overall, loaded VERY heavy, about 3800 lbs total load. For this trip I switched to a lower pitch prop, a 4B 15x15 Powertech, which would turn about 5400 with this load. Top speed was only about 38 with this setup and heavy load but it was a bear on holeshot! The I-command gages show % of full throttle opening and I never had to use more than about 50% to easily get on plane! Optimum cruise was about 3600-3700 at 35-36% throttle at about 26-27 mph and 3.8-3.9 mpg. Overall mpg is higher because low speed mileage is in the 8-12 mpg range. (The E-Tec's beat ALL other motors, 4-strokes included, below 2000 rpm because they run so lean in that speed range. This is significant - most folks don't realize how much time they spend at low speed!)
Weight has a significant impact on performance, and this same rig with just me, a full tank of gas, and the big bimini top folded has run 49.8 MPH (gps) with my 4B 14 1/8 x 20" Michigan Apollo; optimum cruise in this configuration is around 3200-3300 rpm/25-26 mph and about 5 mpg! However that prop will only turn about 5000 with the sort of load I had on this trip, and I felt it would lug the motor too much.
The "150" is actually about 165 by BRP's own brochures, so it is definitely plenty of power for the 20, especially the CC's and MA's, which should be a couple of hundred pounds lighter than the Seafari. I probably would have bought a 115 E-Tec if they had been available when I repowered, since I ran this boat for over 30 years with a 1975 "115" (about 90 hp at the prop!). After this trip however, I'm glad I went with the V-6 . . . it's also a little smoother than the V-4 at low speed.
I also agree that 50 mph is about the practical limit for the 20' hull unless you put way more than 150 hp on it, and then you get into balance and CG issues with heavy engines. I bought mine from Brown & Hauptner Marine - Frank Brown and Mark Hauptner knew as much as anyone about making a boat go fast. Frank crewed for Moesly on the racing 21's and Mark raced hydros since he was a kid. Hauptner had a Seafari with an old I-6 Merc that would run about 50, but it was definitely "tweaked", and he told me it was really hard to get that hull much over 50, no matter how much power you put on it, since it really wasn't designed for "speed at all costs". Denny