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.