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How fast can I really turn a propeller?

A

Anonymous

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Ok, please skip the dissertations about reducing prop speed to reduce noise. I know that is true, but that is not what this thread is about.

My question is, how is a propeller's max RPM fugured?

According to this prop speed calculator http://www.altimizer.com/propspd.html max prop RPM is dictated by diameter and temperature (speed of sound).


Is that true? In other words, a smaller prop can turn higher RPM before it comes apart than a larger prop?

Or do they all pretty much come apart at a given speed?

Am I really OK turning a 70" propeller at 3200 RPM as the calculator would lead you to believe?
 
Ok red here you go .. the type of prop such as wood vs. composite is what will make it come apart at differnt rpm's. The tip speed is important because if you break the sound barrier with the tips you actually lose thrust and it take as much as 1/3 more horsepower to get it back. The airforce did major tests on all this because during wwII they were trying to be superior in the air against the german planes.

I have always been told that a wood prop will explode a little above 3000 rpm's I have explode one and don't really want to do it again. A lot of things can affect that statement such as moisture in the prop nicks on the prop and lord knows what else.

composite props are a lot tougher and can spin faster without exploding however breaking the sound barrier is a bad thing. so calculate your tip speed allow for the humidity and elevation of your site and have at it.
 
Red,

Why don't you contact the manufacturer, they know. Be careful, breaking a prop can do a lot of damage to the boat and the engine.

Jim
 
I have spun my 64" wood prop 3500 rpm when I had a turbo on my motor, and my dads old caddy boat used to hit 3400 with a 74" wide blade wood prop (that was loud). the smaller prop the faster you can spin it. A while back they had these props called speed props, they were short( 54-60 inch) props with no metal leading edges designed to spin 4500 rpm. They were popular on the smaller displacment auto engines around here.

Larry
 
I start by saying I am conservative on running boats. Not meaning I will not dog that 540 hard, and when my CHT gauge was working, I would dog WOT as long as the CHT stayed below what I thought was hot.

And while I play around with a few things on that 540 I am not a mechanic.

But when I play with that calculator, lots of standard engines says it is not turning up the propeller fast enough and you need to turn it more RPMs. This is true for Lycomings and ZZ4 chevys. Now I know you can play with the timing on the 540 to get couple hundred extra RPMs but is possible over reving worth the price of a propeller exploding or trashing an engine.

Sensenich narrow composite blades are rated for 2900 RPMs and I believe the Power Shift Plus is 2750 and the Power shift Signiture is for 2900. I believe that Sensenich says this is what it is rated for and Power Shift says this is the maximum optium RPMs.

With that said, I would agree with cntry.
 
The first myth is there is a magic RPM for all props. There is no magic RPM for all props. The longer the prop the less RPM it can turn before the tip’s reach the speed of sound. The larger the diameter of the prop the faster the tip moves. I made a simple excel program to give tip speeds, engine rpm’s, gearbox reductions and other useful info. See you obviously can’t turn a 74� or larger prop the same RPM as a 70 the tips will become super sonic and the prop will loose efficiency. This is one reason why a prop plane can’t fly the speed of sound at level flight the prop will loose thrust efficiency. When a prop tip



Prop Diameter, inches Prop RPM Rotational Velocity, radians/sec Tip Speed, inches/sec Tip Speed, miles/hr
70 3200 335.1 11728.3 666.4
72 3200 335.1 12063.4 685.4
74 3200 335.1 12398.5 704.5
76 3200 335.1 12733.5 723.5
78 3200 335.1 13068.6 742.5

When a prop tip reaches the speed of sound the air around it becomes turbulent and inconsistent thus causing a pulsing or an uneven load on the prop and this will cause a weaker (wood prop) to come apart. I hear this sound all the time because I guess a lot of air boaters want to turn more RPM’s and brag about how much they swing their props. I laugh when I hear that screaming loud prop that tells me the owner is an idiot and doesn’t know how to set up a boat. A lot of people do this around the houses in my area, fortunately all my neighbors are airboaters and laugh. They know another newbee is on an airboat and is too scared to ride the marsh that’s why they drive around a housing area for hours. Sometimes I can see why there is such an anti airboat community. Just a handful of idiots turning their props past the speed of sound can make living near the water painful.
 
3200rpms in to hard to turn a prop.. Your not getting any push out of it like that, all you doing it making noise.. Get it to turn 2850-2950 max!
 
Good thread, thanks for the replies.

When a prop tip reaches the speed of sound the air around it becomes turbulent and inconsistent thus causing a pulsing or an uneven load on the prop and this will cause a weaker (wood prop) to come apart
Click to expand...

That is the key to what we are talking about here.

What I'm really trying to find out is, if you stay at .9 mach, is that ok? That would be 3250 RPM with a 70" propeller....
 
Man you guys are freaking me out :shock: I'm running my prop at 2750 and I hope that it don't explode. I do have a wood prop. I didnt have to worry at first . With no mufflers I ran 2400 RPm's. Now I'm at 2750, with mufflers.
 
check out pponk.com they have a propeller calculator for maximum prop rpm for diameter of prop being turned.
 
It don't think there is a magic # for the tip speed. Each hull and motor set up will probably use a different pitch setting, depending on the operator's desires for how the airboat performs.

I know that 13 Kline with a 540 ran really nice with the original style PowerShift 2 blade (a cut down power shift) at 2950. I also know that with the Signature series (2 blade) Power Shift doesn't seem to have the same thrust at that RPM. And that propeller did lose power when the pitch was decreased allowing higher RPMs. We originally set it and hit the field. It was turning at least 3200 RPMs and it just didn't move the boat well on grass with 4 people aboard. We parked her in the marsh, repitched her and ran the dog out of it. With that setting, I could stand on the throttle and she wouldn't turn up more than just under 2800. We ran all night in marsh grass with 4 people. We have dropped the pitch once to try to save gas and set it where the maximum RPMs would sit at about 2500. She ran at a lower cruise RPM and used less gas, but wouldn't get moving in marsh grass without almost everyone jumping out. Empty she would get moving, so you could load up Navy Seal style and she kept moving, but that sucked. Set at a Maximum RPM of about 2800, near what Power Shift recommended.

I also
 
I think the best RPM to swing a prop is the RPM that the boat performs best at what you want it to do. The best prop RPM for running the hill is not the same RPM that run’s the best in the water and then some people like a lower cruise RPM which I like personally love there is nothing like cruising the river @ 1,900RPM and having 5,000rpm left if you need it. Oh ya I like fuel efficiency also. So what application do you want the best tip speed for.
 
Wade - We turned Sensenich woodens for years on 0-540 at 2900 with never an issue. Aside for a cracked propeller or prop strike you should be fine. I even know a couple of boats with 540 that have the time altered (advanced ?) to turn a up to about 3050. Now the operators don't ever do it unless needed, but running heavy marsh grass, sometimes you need that extra to get the boat moving. Neither of them have ever had a prop issue, but they also wax the wood every time that boat goes out. The old man trained them the hard and rough way about airboat maintenence and it stuck.
 
Turning a wood prop 3050-3100 will be fine.. Tim Wagman at Florida Airboat Propellers would rather you turn a wood prop 3100 than a Composite.. But I think 2900 at prop is perfect!
 
Thanks , That's was what I need to know. I will wax mine up also. Sounds like a good idea.
 
Now he dont reccomend turning it 3100, but he would rather you turn a wood prop 3100 thann a composite.. Like i said 2900-2950 is perfect!
 
Wade - Waxing the prop kept it alot easier to clean each trip, but mainly it made sure you rubbed every inch of that bugger. I was surprised at the nicks we would find and several cracks.

Good Luck
 
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