• If you log in, the ads disappear in the forum and gallery. If you need help logging in or getting registered, send request to: webmaster@southernairboat.com

5000 RPMs

roberti53

roberti53

Well-known member
I'm wondering why everyone says to set the pitch on your prop so that the engine runs at 5000 RPMs?
Seems to me that if you want more low end performance you would want more pitch on the prop causing the engine RPMs to be more like 4300-4500.
Isn't running the engine at these high RPMs hard on it over time?
I run a ZZ4 with a 2:1 belt drive on a 8X15' heavy hull.
I want unstuck power for snow.slush. and dry ground.
Please help me understand..................Thanks
 
It's best to check the Optiamal operating range for your engine and prop. You also have to consider the load (stress) you are putting on your engine. If you max out the pitch and limit your engine where you can only turn 4200 RPM's you are putting a greater strain on your engine. You are also not in the best power range for your props design.

Most chevy engines are designed to peak out around 5000 rpm's and the props are designed to peak at 2500 rpm's (not including the super wide blades.)

The super wide blades are supposed to peak at around 2100. Thats why people use the 2.3 or 2.6 ratio.

I hope this answers your question. If not I'm sure some of the other guys can elaborate.
 
I personally believe there is no magic RPM to set a prop at. Here is why and how everybody has their favorite number! First off the prop manufactures will tell you to turn it at the exact RPM that their prop is most efficient or generates the most thrust. I disagree with this because each motor is different just as is each boat sure their prop will be the most efficient at that RP[M but the motor may not ever make it there or have enough torque to swing any pitch. Their saying a 12 ft boat should run the same pitch and RPM as a 20ft to me that's wrong! Second all the boat manufactures will tell you to swing way less RPM's then the boat and prop and motor will perform best at because the less the RPM's the less warranty issues! I can't say what RPM to turn any prop because each driver likes something different just as each boat performs differently. All I can say is set your prop to where you like how your boat performs! Plus a gearbox ratio will change how much pitch you can run. A 2.68 box can perform better with less tip speed because you can swing so much more pitch. How ever a weak motor with a 2.38 or 2to1 will be a stone when you run a lot of pitch!
 
When I had my blade pitched at 5200 the boat was pretty wicked and would flat out fly on dry so I decided to throw some more pitch to it for fuel economy, I now have it set at 4700 the difference is amazing. Yes my cruise is about 2700rpm and it still has good power but no where near the performance. I agree with Thunder you pretty much have to throw those numbers out and play with the pitch to find the best spot for your combo and type of riding. I think I'm gonna bump it back up to 4900 and that should do her :D
 
Man you said it best, the best pitch for performance is not the best for fuel economy or cruise so to say a specific RPM is the best to turn a prop is crazy. Thank you for stating that. Here is a good way to put if there is a best RPM for props to turn then why do planes have variable pitch props. Answer because one pitch can not do it all!
 
DAVE IS RITE ! MY TOUR BOAT LIKES AT ALMOST AT THREE PITCH ! BUT I ONLY TURN 4300 BUT WHAT PUSH ! CANT STOP IT ! AND ITS HUSH ! TURNING LESS THAN 1700 !AND LIKE THE MAN SAID GO BACK TO 2 MARK SORT OF RACE BOAT !
 
Dave, I gotta say I agree with you. That is why we water test every boat before it leaves our factory. We might reset the prop 3 or 4 times to get it exactly where it feels best. Every engine is different. Every Boat is different. As far as I know we are the only manufacturer on a waterfront location that test every boat before it leaves the factory.
 
I went with an 82" super Wide because I have seen several people with mild motors turning an 80" 3B 2.68 set on #2. I am going to set my 82" at the #3 setting and I hope it holds my motor around 4800 - 5000 RPM. I have no desire to go 200mph, I just want to be able to cruise at 2800rpm and hold a speed of 35mph. My boat ran dry with a 1.7 and a 3B 80" falcon set dead center. The problem I had was my cruise RPM was between 3000-3400 depending on how ruff the bay got. Once my motor went over 3500RPM my fuel consumption dropped to 2 gallons a mile loaded with gear. This is why I ran two fuel tanks and carried 80 gals of fuel. During the off season on smooth water with no load I could cruise at 2800 rpm and I made about 3 miles per gal.
 
thats the best thing about an ajustable prop is you can tune it to what ya want for any given ride and with your hull and the hp and torge range driving style and on and on carbon ajustable props an reduction units no matter what kind the best thing thats happened to air boats. they make 300 hp turbine engine about 150 pounds my brother wants to put one on a small boat with a variable pitch prop flaten em out spool it up twist em and see what happens
 
So would his hold true also? Pitch your prop for 4500 WOT and cylinder pressures and load higher with less performance than pitching WOT 5700? First example like running up the mountain in Overdrive TQ locked up with a big trailer behind compared to 3rd TQ free? :?
 
thats an interesting way to put it......maybe thats why thunder has had such a rough go of it trying to change peoples way of thinking. to make it all work you need the steeper gear.
more torque at the same rpm, because of the bigger gear, i imagine its only more of a load on the engine if you pitch it so it cant get back to the same(target) rpm, or where that particular prop works best on your boat.
 
Thank you TRex I have seen way more motors blown up from 2to1 or 2.38 gearbox's being pitched to only turn 4,500 to 4,800 then I ever have with a 2.68 turning 5,000RPM to 5,400RPM. The prof is the burnt holes in the pistons I haven't seen a burnt valve or piston with a 2.68 but man I have seen a ton of 2to1 with burnt up parts from cylinder pressure. That is one of many major reasons I try and sell only 2.68 ratios with my motors. Number one they will perform better and cruise at less RPM's and two the motor is under less of a load and is less likely to burn up parts! I just drove a 18ft boat with one of my 500HP LS2's and a 2.68 that run's like a beast four people and gear and it ran the hill like childs play. I know someone with the same exact motor with a 2.38 and for some reason they stuck a four blade on it and they wonder why it don't perform! I don't just sell motors I try and sell the whole combination. Trust me I know what works better on my motors then anybody else! And there are many more reasons to run a 2.68! It's been ten years and it's still the same arguments over a 2.68 I figure another 10 years and we will start talking about cylinder pressures and piston temps when a motor is under geared and over loaded!
 
The biggest reasons we tune for 5000 is that the cam, intake and all are done right at 5000 and with a hydrauliuc cam, you see valve float on a Cadillac at 5200 or so. It doesn't mean you have to turn 5 grand, but we work from peak power and go backwards through the band. In most cases , you have a 3000 rpm "band" in which the engine works it's best. As long as the "low" side of the band is within your working area, then you are fine. In other words if you have a radical cam in something that works from 3500 to 6500, it'd really suck with a stock converter. The engine would have trouble getting to the rpm needed to make power. Like a boat that can't get on plane easily. Thats one advantage of the Cadillac. At 2500, were making a lot of torque, alot! It makes the boat seem as if it has more power than a 502 or even a 572... at that rpm. It may not make the peak power and even the hp of the later, but the power it has is in the best useable spot. One advantage would be be smaller more efficient (at that rpm) ports on intake and heads. To me, it seems as if you could get a small runner dual plane and make the ports smaller on a 572, it'd be a bad mother on an airboat because of inches, stroke and everything else a boat likes. This to me is more important than cam selection. If your cross sectional area is geared to the rpm range of the engine operating range and size of the engine, you already have 3/4 of the battle won. The proper cam is simply the icing on the cake.
 
It's all a fine balance you start making too much cylinder pressure then things go wrong quickly! I like port velocity too but I have learned not to base everything on port velocity alone!
 
I agree, but it is a crucial part of the formula. The way I look at it is that in a perfect world, you can make "X" amount of power at a given rpm with a given amount of cubic inches. Compression ratio/octane are also factors. What you don't do to enhance the maximum power simply deducts from your overall performance. Like a credit score or on a math quiz. I look for the cross sectional area for a peak rpm, even if it's only 3000. I would then design a cam to complement that particular build, keeping detonation in mind also. They claim an engine will look for .62 of MACH on port velocity and that's where peak power occurs. I don't know 100% if that's true, but that's what we base our math on and it seems to work. If I'm not mistaken, that 28" H2O will only yield that airspeed maximum.
 
Ok I need a decoder ring because yawl lost me. I guess that is why we pay you guys to build us motors because I could not figure all that out. Hell I can't figure out how to turn the clock on my truck radio back an hour for daylight savings. :lol: :lol: :lol: :lol:
 
Crow, I think what they said was put gas in the thing and drive it ........ :? . If it pops, it pops.
 
CROW THEY ARE SAYING LET THAT MOTOR RUN ! 4800 IS COOL FOR US A LITTLE 49 OR 5000 WONT HURT LESS STRAIN ! JUST DONT PROP IT DOWN ! THATS WHY WE ARE BROKE AND THEY GOT MONEY ! THEM GENTELMAN DO A LOT FOR THE CAUSE ! THAT WHY THEY CALL IT CUBIC DOLLARS ! GLAD TO SEE YOU PROGRESSING ! COOL DEAL !
 
You must log in or register to reply here.
Back
Top