Thanks for the welcome aboard... Yes I have figured out that the 3.0 already uses a 1:75 rocker. Just pulled the 2:00 gonna try the 1:65 this week. I have a very lite 18' boat and the goal is 35-40mph cruise at 3200-3600rpm.
Wish I had a dyno sheet on these things. I don't so I'm guessing that they run most efficiently at 3200-3600 rpm. I'll report back and let you fellas know how it works. Going from Richmond out to the beach so a good 8hr ride.
When we set up props on boats, it's not to a midrange speed, it's at full throttle (called WOT). Setting to a target speed and mid range revs setting will get you into the wrong prop size.
The way to get the right prop is to put the drive with the ratio the manufacturer specifies for that engine (in the case of a 3Litre, that's a 2:1 or 1.94:1 drive), put a prop on (for a light 18' boat I'd start with a 19") and take it out, with a normal load, and run it up to full speed. Your target RPM is 4400-4800 (aim for the high end). Based on the revs you do get (and use a shop tacho, not the dash tacho, they are not accurate enough), you can change propeller pitch to get you into the right rev range. Props are supplied in pitch increments of 2", and each 2" of change will change your WOT RPM by about 400rpm. So if your 19" runs the engine to 5100, change to a 21". If it only makes 4200, then decrease to a 17".
Trying to prop for a target speed at midrange rpm could get the wrong prop on, and that could lead to an overloaded engine (the load imparted on the engine by the prop is across the whole rpm range, not just at the top end, so if it's the wrong prop at WOT, then it's the wrong prop for the entire rev range)... Overloaded engines have a tendency to 'rapidly disassemble', in an expensive way!
Chris.........
