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A cure for with Prop/gearbox harmonics

Waterthunder

Well-known member
I was running a three blade super wide blade on a 500HP LS1 this motor had absolutely no vibrations and would idle at 500RPM's if you noticed most all the nasty harmonics occur at idle when the power pulses are less frequent or further apart. Realize when you bring your motor off idle to around 1,500RPM's the major vibrations go away! Also realize you rarely have these vibrations on a DD car motor and here is why with these high ratio gearboxes the prop is moving much slower and the frequency or resonance is at it's worst. These power pulses are more then capable of destroying a prop or gearbox. I will keep it short but your crank is twisting from the power pulses then you have a gearbox with the gears (bouncing from loaded to unloaded then it's all conflicts with the inertia of the prop wanting to continue in it's rotation) here is where the two forces fight each other and it appears the resulting is prop damage everything in ( )'s is my theory. Anyway on this 500HP motor the customers main concerns where fuel economy and running a stock fuel injection ECU. So I was limited in the cam shaft centerline and size. See with the stock fuel injection GM uses high centerlines from 112 all the way to 116 if you run a lower centerline the computer will not work or idle. On these higher lobe centerline cams the idles very smooth. I have never seen a three blade idle at 500 or so RPM's so smooth. If you guy's really want to remove all the nasty and banging harmonics at idle, a different cam usualy will do the trick of coarse you will have to give up some low end power not much but some. However it was amazing to hear this motor idle YOU COULD NOT HEAR THE GEARBOX OR ANY RATTLE IT WAS PERFECTLY SMOOTH AS IT WAS IN A BRAN NEW LUXURY CAR. On my new boat I will be addressing these harmonics and I guarantee you there will be no sound from the gearbox and the idle will be as smooth as an Escalade. I am completely open to any theory or ideas on this I'm just starting on educating myself of these prop harmonics so I'm sure some of my statements maybe incorrect. I just filled a provisional patent and I have an invention that will isolate the prop from the gearbox, I believe this is where the problem is coming from, it's not the motor to the gearbox sure the soft drives help but their a sub par band aid to the real problem. Right now I'm working on a proto type to adress the problem between the prop and gearbox.
 
Dave will the new invention you are working make my boat idle smooth with out changing the cam ?
 
It will remove or isolate the harmonics from the gearbox and prevent them from transferring to the prop.
 
I don't think you can truly solve the problem by changing something between the motor and gearbox. I feel we need to isolate the prop from the gearbox.
 
Just a thought Dave, might it be too much backlash in the gearbox gears? I'm nowhere near up to speed on gearboxes.

Scotty
 
if you mount your monster fan on rubber like a old johnson prop i will be looking for a big tree to get behind if i see you coming my way. until you explain how u did it :)
 
Funny you should post this at this time THUNDER, I was just sittin around the racecar shop last night talkin with the guys and we were trying to eliminate things from my vibration equation and came to the same conclusion on the cams. The dead spot for lack of a better term was the obvious culprit. I went to a 4-7 cylinder swap trying to close the gap but it hasn't been all it was supposed to be either. I took my prop off this morning after spraying my engine with spray powder trying to find an oil leak, and there was no vibration. This is because there is no load to highlight the dead zone and cause the bounce back thru the engine. I split an alternator in half saturday night. It seems that the alternators on my setup are what catches the worst of it because they are at the end of the power line, sorta like the game of crack the whip when you were a kid, the last person gets it worst. The lift and duration that make thes ethings sound so sweet and make the low end power is obviously causing other problems. I have a new prop, new gear box, new engine, and same old problem. the only common denominator is the cam, while this cam is different naturally, you know that, it still is made for low end power which it really does have.

I just measured my prop last night, I never thought I would need to check sensenitch, but I did and found out that I am running an 84" three blade super wide set a little past the third mark. I only had an 80" on my last engine setup. Now that is a serious prop load for a small block to swing. I am right at my target prop rpms at WOT and I will leave to those who have ridden on it to say if it snaps or not but to me it is acceptable. However I don't race it and that is a different animal. I am contemplating several changes and will do one at a time so I can track the differences they make.

The Cam will be one of the first ones. We had pretty much settled on that last night and now it looks like a no brainer. I'll let you know, Oh yeah i found the oil leak, my pan was cracked again at the front seal area, I am gonna try a hamburger pan, just wish it had more bolts in it. LOL but they been around a while so figure they might know what they are doin when it comes to oil pans anyways.
 
You and your alternators I have never broke one yet and I know you have broke a bunch of them that's for sure. I wonder if the serpentine belt is why I have never had a problem with an alternator or bracket not even on the .800 lift camed motors. Oh well if you bump up the center line or lobe separation your idle will smoothen out.
 
Remember changing the firing order doesn't change the dwell time between when the cylinders fire or change the time between power pulses it only changes the order of the firing. When you do the firing order swap you have the exact same time between power pulses you just don't have 5 and 7 firing next to each other.
 
Yepper I certainly have broke a bunch of them. I looked at your serpentine belt system and the alternator bolted to the head looks like it cures the problem. I have looked at some other systems for earlier chevy engines and even though they use the serpentine belt when only running an alternator it doesn't have a tensioner in it, it has a turn buckle gizmo.

I will get a cam worked up and see if it helps any. I am still hunting an alternator setup that allows the alternator to be bolted to the head and uses a tensioner not a turnbuckle. If you run pwr steering and a/c they have them. thanks man
 
I made my own mounting system for these reasons when you look at the other airboat guy's mounts they use a mammoth bracket off the head that has the alt over the valve cover they have major belt throwing and alignment issues. I bolted the alt to the head for 3 reasons one simplicity less parts less weight and my whole system only requires one tolerance to be set so you have a perfectly parallel pulley system. I actually level each motor on my test stand then drop a plum bob over the serpentine groves so all the pulleys are dead on. I have found the alternator castings and block variations are anywhere from .150 to .020 thousands so I make each mount dead on for each application. Also a bracket will deflect but when your mount is 100% under compression like mine are nothing can move around. Did I mention it was light and simple. When you use a bracket it can vibrate from harmonics and develop cracks or just flat out break off.
 
Actualy I did that four years ago and now it's sitting on a shelf. Naw I got a neat idea with utilizing an output shaft of a gearbox to isolate it from the prop
 
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