I'm new to my 303, and up until yesterday had only motored around under 2100RPM (separate story, to confirm the accuracy of my console tach I used an optical tach on the flywheel, 2100RPM as read by a cheapo optical tach on the flywheel corresponds to 1900RPM on the console needle, I trust the new cheapo optical over the 35 year old console...) The GPS will generally read around 5 knots at 2100RPM on a calm day - we motor in a river so that's usually 4.5 against the current, 5.5 with it.
Yesterday I finally throttled up to ~2600RPM and yes indeed it does move faster, but the motion of the hull is much less settled. We were running downstream, 5.5 GPS knots at 2100, and by 2600 the GPS was reading ~6.7, which should be about hull speed for a 25' waterline, though maybe still 1/2 knot short accounting for the current.
The main "unsettling" behaviors were a tendency for the heading to wander, at 5 knots through the water it stays on course very solidly, at 6+ it required constant input at the wheel to hold a course - maybe my nice comfy 11' beam is coming into play here? Also, the back of the boat squatted down about 4", where the exhaust is just above water at 5 knots, it's fully below at 6. There was also a notable increase in exhaust odor, not foul, but notable.
I'm happy to cruise at 2100RPM / 5 knots, but have read here and there about Yanmar (and other diesels) "liking to run" at 75-80% of rated RPM (3400 for my 2GMF) to prevent carbon buildup.
Any thoughts / experience from other Pearsons? Particularly wide-body models like the 303?