I've got a 1993 17' searay , 3.0 4cyl alpha 1 I/O. I'm runnin an aluminum 3 blade 19 pitch prop. (well it says 48-78120-19 on the prop). I'm wondering if switching to a 4 blade with a higher pitch would save on gas at a lower rpm and would go faster at WOT with out loading up my engine too much, any ideas?
Not knowing what your wide open throttle rpm with an average load currently is, we have no idea what would happen. Theoretically the answer is yes, increasing pitch would increase speed at the same rpm as before, or conversely, engine rpm would be lower at the same speed as before. BUT -- there that word is again, ONLY if the engine has enough power to twist the extra pitch. Lower engine rpm does not translate automatically to fuel savings. If the throttle needs to be open more because you are lugging the engine then fuel consumption would be worse. Give us some numbers to work with -- for all you know you may already have the best prop.
The merc 3.0 is running 42 mph trimmed out at about 4,500 rpm and thats WOT for it. just wondering the outcome if changing to a 4 blade with the same pitch or with a higher pitch. just wanting to experiment but get some guidence on where to start w/o blowin a buch of money.
Keep in mind that each degree of pitch that you add will take approximately 200 RPM off your WOT. If your engine is rated for 4500 WOT, then you may find that the current prop is the best fit. At 4100 RPM you may be lugging the engine. I recently jumped from a 19 degree SS on my 2007 Sea Ray 210 with a 350 Mag MPI to a 21 degree. WOT dropped from around 5100 to just over 4600 and top end really saw no improvement. Hole shot took a big hit, and it struggles to get on plane if 2 adults are in the back as I pull a tube. I'm going back to the 19 degree and selling the 21.