Question on propping a 4-stroke

Discussion in 'Prop Shop' started by TomFL, Feb 11, 2009.

  1. TomFL

    TomFL Well-Known Member

    Never fiddled with 4-stroke propping before and I have a question.

    On the merc 25 EFI 4-banger, the manual says RPM range is 5000-6000 RPM.

    I had an 11" pitch prop, ran about 31mph, but with just me on the boat and normal load, it revved up to about 6180 rpm which getting pretty danged close to the 6250 rpm fuel cutoff. And is above the reccomended RPM range.

    I switched to the same brand, same diameter 12.5" pitch prop, and the speed went up slightly, maybe 1 mph. But my RPM's dropped drastically to mid 5500's max, most of the time I'm in the high 5400 range.

    Not sure how I lost all that RPM with just 1.5" of pitch change; I thought the school on props is 200 RPM for every inch of pitch??

    Anyways, here's my question: is it better for 4-strokes to run close to the max RPM's as a 2-stroke would, or is it better to have them lugging along?

  2. Brett

    Brett > PRO STAFF <

    2 strokes are for screaming with your throttle jammed to the breakin' point.

    4 strokes are for running at 4800 rpm, in the zone, leaving the throttle a little room left to zoom... :cool:

    a little reading for ya...

  3. The_Skiff_Shop

    The_Skiff_Shop Well-Known Member

    You say you switched pitch with the "same brand and diameter" but did you change prop style? Something else is going on IMHO if you lost 850 rpm with 1-1/2" of pitch.

    The 200 rpm per 1" of pitch rule of thumb applies to larger motors. Motors below 40 hp are tricky.

    While 4 strokes offer more even torque throughout the rpm range I would not load it down with too big a prop. Think of starting your car out in 4th gear. ;)

    Unless your a speed demon, I would just cup the 11 pitch you have to dump a few hundred RPM. You'll have an awesome hole shot and should be able to plane at slower speeds. But that's just me.

    As always, just my .00000000000000000000002 sense.

  4. Flyline

    Flyline Won &quot;Do More With Less&quot; Award!

    I will suggest a powertech or quiicksiler 10 x 12 will bring around 5800 RPM due the stainless steel polished propeller has thinner blade surface and doesn't flex. Aluminum propeller has more resistence (sp?) than S.S propeller.

    Also I will suggust what Capt nRon about 11" pitch with CUP that will also help.
  5. Frank_Sebastian

    Frank_Sebastian Well-Known Member

    Brett a couple of things struck me as odd in the article in regard to 2 strokes vs 4 stroke. One was the "over oiling" that caused piston failure. The second was 650 hours on the engine.

    If he hung onto the throttle for 12 hours per day then early in the morning on the 55 day the engine would have to be replacesd Also I have seen many engines with hour meters reading in the high 3,000 and doing well. A typical commercial fisherman works about 150 to 200 days a year. Most of that time is spent handling fish and running from one location to another. At least 3 hours per day. A crabber will run his engine almost 10 hours a day although mostly at fast idle. (some race between the traps) as they set farther apart than normal. If they can't use an engine fro 3 years they complain about it. Now I don't want to call shenanigans here, but from my own personal experience I think the two strokes are what I would buy given the choice. Lighter, simplier (much) easier to make and cheaper to buy. They have seemed to endure well if run conservativly and with the newer materials they would be my first choice.

    In regard to pistons seizing up because of oil, The opposite seems to be the case. What happens to a piston is a ring will break (or stick) when that happens the piston will overheat as the contact between the cylinder wall and piston is via the rings and that conducts the heat away.

    Best regards,
  6. Brett

    Brett > PRO STAFF <

    Good points Frank, makes the article look a bit biased.
    The article is out of the north shore of Australia. No idea
    the quality of fuel or oil being used in the engines that "failed".
    Running too oil rich will result in heavy carbon build up inside
    the piston firing chamber. That carbon build up will lead to
    premature ring failure. Doesn't even say the brand of the engine
    that was failing. Then again, I was more interested in the results
    of his testing of the two Yamaha 40 outboards.
  7. TomFL

    TomFL Well-Known Member

    After switching back and forth between the two props, honestly the 12.5" pitch prop makes the motor sound happier. Cruising and WOT.

    Cruising with the 12.5 pitch, if you want to gas it to bridge a wave or boat wake, the boat jumps. Cruising with the 11p you're almost at max RPM's at what the motor seems comfortable at, and it feels like there's not much left in it.

    There is a little to be desired for the holeshot compared to the 11 pitch naturally, but it's a small price to pay honestly. I'm switching from a 24v troller setup back to a 12v which will drop about 40 lbs from the boat, so that should help a bit.

    If anything I would think dropping just 1/2" in pitch might be perfect.

    Ron, when funds allow I will get in touch with you for a SS prop and call it done.