Using too high of pitch prop, ok?

[mxz]

Is it ok to use a prop that has too much pitch and therefore will not reach max rpm? Tried putting my 28" RE4 on our Vegas V with 2.0l 150 Merc. With this prop it will only turn 5000rpm. Normally turns the rated 5600rpm with a 25" prop. other than not getting the rated max rpm the boat still takes off really well and runs goodn ice low rpms at cruising speed. Just don't want to hurt the engine by possibly "lugging it"? This is just our family cruising boat, not used for going fast...
opinions???

[Roos]

No problem.
The reason why people want to run max rpms are that the engine is often stronger at their max rpm or even above that if you make som adjustments. The wear increases as the rpm increases and actually squared. So if you run your engine at 5000 rpm that is in fictal wear 25000. If you run it at 7000 rpm it is 49000, almost twice as much.
The less you rev your engine the longer it will live!

Cheers!
Mike

[Raceman]

I'm sorry, but I disagree STRONGLY with Roos' opinion above. Lugging a two stroke can cause a lot of problems, and depending on the engine can cause total destruction. Over gearing a two stroke is a whole different ball game from doing the same to a 4 stroke. For instance if you were talking about an inline 6 Merc pitching it to only turn 5K at wide open throttle would absolutely assure burned pistons from detonation. Since you're talking about a V6, with it's flat top pistons, it's much less prone to detonation failure, but it's still a probability when lugged. 5K at WOT just isn't enough. When you cause detonation in an outboard you can't hear it as you would in a car that's "spark knocking" as it's frequently called, either from inferior octane or too much load, or even too much timing in older stuff that wasn't computer controlled.

In the case of a Merc, RPM toward the high end of the rated operating range, or even above is always preferred to at or ESPECIALLY below the Mfg's suggested operating range.

[Slider]

This is just a theory, but wouldn't turning higher RPMs keep an engine cooler and better lubricated? More RPMs=More Gas=More Oil over a certain period of time. Like I said... just a theory.

[The Big Al]

I'm sorry, but I disagree STRONGLY with Roos' opinion above. Lugging a two stroke can cause a lot of problems, and depending on the engine can cause total destruction. Over gearing a two stroke is a whole different ball game from doing the same to a 4 stroke. For instance if you were talking about an inline 6 Merc pitching it to only turn 5K at wide open throttle would absolutely assure burned pistons from detonation. Since you're talking about a V6, with it's flat top pistons, it's much less prone to detonation failure, but it's still a probability when lugged. 5K at WOT just isn't enough. When you cause detonation in an outboard you can't hear it as you would in a car that's "spark knocking" as it's frequently called, either from inferior octane or too much load, or even too much timing in older stuff that wasn't computer controlled.

In the case of a Merc, RPM toward the high end of the rated operating range, or even above is always preferred to at or ESPECIALLY below the Mfg's suggested operating range.

I have to agree, you will find the weak link very fast with a engine under constant heavy load. Heat is death! And if it get's hot, the load will not let it cool.

[kingsbiship]

The eng. will be under a strain the whole RPM range. This together with
running hotter is for sure a recipe for problems!

[bmax]

Over propping and under propping will most certainly cause MELTDOWN :eek:

Assuming this is a carbed motor ?

The most important thing to remember on a carbed motor , is that the TIMING and JETTING are fixed .

Max timing is set approx 25 to match fuel/air flow at 5500 rpm (eg)

At 5000 rpm , not enough fuel to match 25 timing = lean

At 6000 rpm , too much air flow to match 25 timing = lean

Hope you get the idea . Basically the motor is an air pump (2.4lt) will pump 2.4 litres of air through its full cycles , the carbs are fixed to match at the rated 5500rpm . Timing is mechanically set also to match .

:D Now , after saying that mouthfull , 500 rpm may not be enough to lean out enough to meltdown . You be the judge !

[pyro]

This motor is a 2.0 L, but whatever. Prop it for 5800-6000 RPM, that's where it belongs, PERIOD. You'll enjoy even better holeshot and acceleration.

[Raceman]

Over propping and under propping will most certainly cause MELTDOWN :eek:

Assuming this is a carbed motor ?

The most important thing to remember on a carbed motor , is that the TIMING and JETTING are fixed .

Max timing is set approx 25 to match fuel/air flow at 5500 rpm (eg)

At 5000 rpm , not enough fuel to match 25 timing = lean

At 6000 rpm , too much air flow to match 25 timing = lean

Hope you get the idea . Basically the motor is an air pump (2.4lt) will pump 2.4 litres of air through its full cycles , the carbs are fixed to match at the rated 5500rpm . Timing is mechanically set also to match .

:D Now , after saying that mouthfull , 500 rpm may not be enough to lean out enough to meltdown . You be the judge !

It's not really true that the engine will be lean at 6K RPM. That's really not a bad RPM range for the V6's to operate in, although some of the lower horse motors will be off their powerband a little there.;)

[Markus]

Is it ok to use a prop that has too much pitch and therefore will not reach max rpm? Tried putting my 28" RE4 on our Vegas V with 2.0l 150 Merc. With this prop it will only turn 5000rpm.

If the recommended WOT operating range is 5000-5500 rpm or 5000-6000 rpm, you will be fine. If the recommended WOT operating range is higher, e.g. 5500-6000 rpm, you need to prop it to run at higher rpm. Also, you want to have some margin if the boat is loaded down.

The most important reason why you don't want to lug the engine is because it has so much torque in the mid-range that the strain on components gets very high. As an example, if Yamaha V4s and V6s are run at too low WOT rpm, the wrist pin will eat into the piston.

But, if Mercury says that the WOT operating range is 5000-5500 rpm, and you are at 5050 rpm, you should be OK. After all, Mercury is selling into a market where most outboard owners only have one propeller.

[bmax]

Guy's, i think you missed the point , please read again , i was only trying to explain what over /under propping acually does to a two stroke carbed motor . The example that i used was juat that , an example . 500 rpm is not really an issue , i was only trying to explain why .

MXZ , sorry if i have confused you or anyone else , it was not my intention .

I agree with Raceman and Markus's first statement , prop it high , but that still does not answer your original question does it .

Personally i would switch to a lower pitch prop to run in the upper rpm range if you were running by yourself at 5,000 rpm .

Bmax .

[mxz]

Thanks for all the info guys, I am just using the 28" until I buy a smaller prop. I was running a 25" prop on the boat but sold that prop when I sold my Vector.

[Raceman]

But, if Mercury says that the WOT operating range is 5000-5500 rpm, and you are at 5050 rpm, you should be OK. After all, Mercury is selling into a market where most outboard owners only have one propeller.

Rated RPM or not, if you pitch a Merc to ONLY turn 5K at wide open throttle you're running a BIG risk, and it's not worth it to keep from buying a used prop somewhere. I've just seen too many detonated engines over the years not to think it's downright foolishness to pitch to low 5's. Things change over time, and the gas here gets worse by the day as the US oil companies greed makes them continue to try and squeeze every penny they can by lowering the standards farther. This crap won't hardly last until you get it home now before it starts going stale.:mad:

[pyro]

Do not hold the throttle wide open running with that prop, or you WILL melt your motor down. It belongs at 5800-6000 rpm.

This will end up being another sad story, someone asks a question but already has their mind made up, the experts speak up against it, then they go do it anyway, and blow up their motor. blah blah...

Why do we waste time even answering these posts?

[skidoo29]

pyro


Do not hold the throttle wide open running with that prop, or you WILL melt your motor down. It belongs at 5800-6000 rpm.

This will end up being another sad story, someone asks a question but already has their mind made up, the experts speak up against it, then they go do it anyway, and blow up their motor. blah blah...

Why do we waste time even answering these posts?

hey Pyro what makes you think hes not listening to you and the other guys.
I own this boat with mxz, it is a familly boat that has seen wide open throttle maybe 2 times since we rebuilt it and they were both with a different prop.
read his last post again, we are searching for another prop but what do we do until then, not run at all, give me a break.
I/ we appreaciate the help from all of ya,
but "I" could do without the crying, get off your high horse and if you don't want to help then don't respond, I don't care either way.

warren

[mxz]

Pyro, I asked the question because I obviously wanted to know the answer and am concerned about doing damage to the motor, nowhere did I say that I was not going to take the advise given and run the crap out of the motor with the wrong prop until it blows....Until I asked the question I did not know that using a prop that was too big could damage a motor..
As for running the motor at 5800-6000rpm, well acording to the manual it is only rated to run at 5600rpm.

[Raceman]

As for running the motor at 5800-6000rpm, well acording to the manual it is only rated to run at 5600rpm.

Just to be clear on the 5600, the engine is structurally capable of far exceeding that RPM, but Merc figures they've got it choked where turning it any harder puts it above it's peak power. Given a choice of 400 over or 400 under, the 400 over is night and day safer.

[mr_velocity]

pyro


Do not hold the throttle wide open running with that prop, or you WILL melt your motor down. It belongs at 5800-6000 rpm.

This will end up being another sad story, someone asks a question but already has their mind made up, the experts speak up against it, then they go do it anyway, and blow up their motor. blah blah...

Why do we waste time even answering these posts?

hey Pyro what makes you think hes not listening to you and the other guys.
I own this boat with mxz, it is a familly boat that has seen wide open throttle maybe 2 times since we rebuilt it and they were both with a different prop.
read his last post again, we are searching for another prop but what do we do until then, not run at all, give me a break.
I/ we appreaciate the help from all of ya,
but "I" could do without the crying, get off your high horse and if you don't want to help then don't respond, I don't care either way.

warren

If it were me I wouldn't run it. Just because you can't reach the max rpm doesn't mean its only a problem at max rpm. The motor is overloaded through the entire RPM range. No one can tell you what to do since its your boat, but you're going to do more $$ damage then it will cost you to buy a brand new prop and have it shipped overnight.

[HighVelocity]

On the late model Mercs the anti-knock sensor helps prevent detonation.

That's why everyone is concerned about your older motor's saftey.

When it comes to ovedrive gearing/pitch in boats my Dad always said, "Son, There Is No Free Lunch" Boats don't ride on ball bearings.

I think if your proping up to save a few dollars you may pay more on the back end with a costly repair. Since I was a kid, I knew the name Mercury and high RPMs were
synonymous.

Good Luck,
HV

[chris_lacey]

I have a 24 chopper you can use until you get somehing else, if you want. Let me know. Better safe than sorry.

Chris

[Jimboat]

I agree with Raceman & mr. velocity - by over-propping, the engine will be more heavily loaded throughout the full RPM range. I suggest re-propping to get the right "gear ratio".

[mxz]

Thanks for the offer Chris.
Bought a 24" prop, this will put it at the right rpm. Thanks for all the info guys.