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Author Topic:   Merc 4 strokes, props, and gear ratios
FISHNFF posted 01-07-2001 03:55 PM ET (US)   Profile for FISHNFF   Send Email to FISHNFF  
I am having a heck of a time propping my Merc 90 4-stroke on me Alert/Montauk. I just ran with a Rapture SS17" and a DoelFin. With a light load it ran 39.0 knots@5100RPM. I was able to trim out all the way and didn't gain or lose speed or RPM from about 2/3 trim and up. I previously ran a Ballistic 19" w/o a DoelFin with exactly the same results. The 75/90 4-stroke uses the same lower end(#14) as their 100/115/125's at a 2.07:1 ratio, while the Yamaha and Honda 90 4-strokes use a 2.30:1, same as Merc 2-stroke 75/90 and bigfoot 50/60(#13). I am wondering, other than doing the math, if gear ratio has than much of a effect on rpm and speed. The motor has much more torque than my 75 2 stroke but just can't get the RPM's. I was able to turn almost 6000RPM on my 75 2-stroke with the exact prop my 90 4-stroke turns 5100. Hull is the same as is mounting height. A 10% increase in gear ratio should mean 10% increase in prop RPM? I will try a 15" next, but that seems just too low. Anyone else out there share my dilema? I double checked my tach settings with Teleflex and it is right. By the way, the myth of 4-strokes trolling slower is just that. While fuel injected 4 strokes can indeed troll down to 650RPM, my 4 stroke carb gets down to 800, right where my 2 stroke 75 was. At a lower gear ratio, it crawled, while today my 90 "flew" at 3.3 knots.
kingfish posted 01-07-2001 05:55 PM ET (US)     Profile for kingfish  Send Email to kingfish     
Just a curiosity from the peanut gallery here, because I have never owned a 4-sroke, but I thought I understood that 4-strokes ran at lower rippems than 2-strokes. What is the Manufacturers recommended max rpm for your motor?
FISHNFF posted 01-07-2001 10:29 PM ET (US)     Profile for FISHNFF  Send Email to FISHNFF     
WOT is 5000-6000 with max power at (I think) 5800RPM.
triblet posted 01-07-2001 11:00 PM ET (US)     Profile for triblet  Send Email to triblet     
Change one thing at a time or you'll "loose
the rabbit". This is a lesson I learned a
long time ago racing sports cars. You
changed three things: Added Doel Fin, changed
prop type, changed prop pitch.

Either take the Doel Fin off, or put the
first prop back on. See where you stand.

Also, different prop types required different
mounting heights.

And you're boat is three knots faster
than my Montauk with an Evinrude 90
two-stroke. And your Alert is probably
heavier. So don't feel too bad.


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