# prop getting up to 5,100 rpms



## steve77777 (Nov 12, 2013)

Im getting up to 5,100 rpms and the boat runs good ,Im happy with it but dont most engines rev to 5,500 when full throttle--I have a 40 mercury with a 13 p prop 10 and half diameter if i tilt the engine up anymore when doing 5,100 and full throttle it blows out?


----------



## Dillusion (May 21, 2012)

Just because a motor is rated for 5500 doesnt mean it needs to run at 5500 or that your best performance is at 5500. You could be getting the best optimal performance at 5100 as opposed to 5500 where your prop would be slipping and you wouldnt gain any MPH or performance.

If your prop is blowing out with any amount of trim from the very bottom then it's either mounted too high or you need lots more cup because you have a tunnel?

On my merc 40 with a 13p prop I can trim up with a count in my head of about 4-5 seconds from the bottom before it blows out and I dont have a tunnel.

My engine pushes a 290lb boat and I get around 5400RPM WOT, if you're boat is heavier than 300lbs I supposed 5100 is just fine...


----------



## joebucko (Dec 29, 2012)

If you motor is the Mercury 40hp 4 stroke the recommended WOT is 5500-6000 rpm. If all you can get is 5100 then your engine is lugging which isn't good. To develop its full horsepower the engine really needs to be in the middle to upper part of the 5500-6000 range. If your boat is heavy this could explain it (600-800 lbs) and changing to a lower pitch would probably bring the rpms up but if your boat is lighter in weight then something else is wrong. 
What kind of boat are you running?


----------



## steve77777 (Nov 12, 2013)

i have a palmer critchfield ,same as like a boggy skiff about 15 ft but i think without motor its 400 lbs,the engine doesnt have a jackplate but looks like they mounted it alittle high because it doesnt sit directly down they definately brought it up and mounted a few inches above .,so when you say i may need a bigger cup what is that pitch or diameter ? i have a 13p 10 and half diameter now,what would help change things?


----------



## Snookdaddy (Jan 5, 2008)

> Just because a motor is rated for 5500 doesnt mean it needs to run at 5500 or that your best performance is at 5500. You could be getting the best optimal performance at 5100 as opposed to 5500 where your prop would be slipping and you wouldnt gain any MPH or performance.
> 
> If your prop is blowing out with any amount of trim from the very bottom then it's either mounted too high or you need lots more cup because you have a tunnel?
> 
> ...


It is hard to believe that you're only getting 5400rpm on your Devil Ray when I was getting 5750rpm on my 40hp 4 stroke on my 18' Waterman...

I was running a 10-3/8" x 13 pitch Merc Vengeance  with a 2:00 to 1 gearcase...  We put my prop on my friends 40hp 2 stroke 16' Whipray and it got up to 5700rpm and 36+mph with 2 peeps.  His 2 stroke is 1:83 to 1 gear case and seemed to be the perfect prop for his skiff.  I'd do a compression test to see if you pulling at least a 125psi on each cylinder on the 2 stroke.  Something seems out of place...

Personally, I would not run a prop that only gets to 5100rpm as you are certainly lugging the engine and would benefit in rpm and probably speed by dropping down an inch or two..  Probably two imho


----------



## Dillusion (May 21, 2012)

> > Just because a motor is rated for 5500 doesnt mean it needs to run at 5500 or that your best performance is at 5500. You could be getting the best optimal performance at 5100 as opposed to 5500 where your prop would be slipping and you wouldnt gain any MPH or performance.
> >
> > If your prop is blowing out with any amount of trim from the very bottom then it's either mounted too high or you need lots more cup because you have a tunnel?
> >
> ...


It could be 5500-5600 RPM...I'll double check if I can fish this weekend in the snow. but I swear I saw 5400RPM...the tach was on the second red line between 5000 and 6000RPM so that means 5400. That's what I remember at least lol


----------



## steve77777 (Nov 12, 2013)

so what would i drop the diameter or pitch,i guess you mean diameter? how about these rpm gauges are they always pretty correct ? i mean the boat sounds good and hauls butt,i think im doing around 35mph... but i just wanna make sure if its really 5,100 max what could i do and you guys are definately helping out thankyou for the feedback...


----------



## steve77777 (Nov 12, 2013)

and i have a 2 stroke 2003


----------



## steve77777 (Nov 12, 2013)

also what if the engine is alittle high ,will a smaller pitch blow out the same way?


----------



## DuckNut (Apr 3, 2009)

> so what would i drop the diameter or pitch,i guess you mean diameter? how about these rpm gauges are they always pretty correct ? i mean the boat sounds good and hauls butt,i think im doing around 35mph... but i just wanna make sure if its really 5,100 max what could i do and you guys are definately helping out thankyou for the feedback...


Typically you would drop your pitch, not diameter.
The only way to know if your tach is accurate is to hook up another or have have a mechanic test it for you. I have never seen a truly accurate tach on small engines. You can tell if you are bogging by the sound.

If you are getting blow out with that prop then you should drop your motor down and retest. Having the ability to tilt it when the conditions call for it is good to have. Also you should be able to fine tune the ride with the trim and that will also affect the rpms's.


----------



## HaMm3r (Dec 11, 2006)

> and i have a 2 stroke 2003


If I remember correctly, that year 2-stroke 40hp Merc had a 5000-5500 RPM range, so you're not bogging and don't need to do anything unless you want to change some specific performance characteristic.


----------



## CurtisWright (May 9, 2012)

Less pitch = More RPM.


----------



## steve77777 (Nov 12, 2013)

Thankyou everyone,yes I have been reading as much about this as I could,and yes less pitch =more rpms,,Its weird when you think bigger is better-ha not always.


----------

