2-blade propellers "normal" vibrations

[cmedsailor]

2-blade propellers \"normal\" vibrations

If I have understood it well from various discussions a 2-blade propeller, no matter if it is perfectly sized, will always cause some vibrations in comparison to an equivalent 3-blade (perfectly sized) propeller.
Why is this so? Is it due to the greater slippage?
And could these “normal” vibrations damage the engine / shaft in the long run?
Thanks

 

[sailorman]

Re: 2-blade propellers \"normal\" vibrations

my 2 blade MaxProp doesnt vibrate.
it could be down 2 worn cutlass bearing

 

[TigaWave]

Re: 2-blade propellers \"normal\" vibrations

Vibration only comes from shafts and bearings or misalignment.

A prop that is balanced will not vibrate, you will get pressure waves as the tips pass under the hull which could be heard and will change with revs, but this is not vibration.

Most vibration is from lack of adequate shaft support or worn bearings.

 

[cmedsailor]

Re: 2-blade propellers \"normal\" vibrations

Cutlass bearing definately needs replacement. I have bought a new one already and it's the first thing to do when lift the boat out of the water (together with volvo seal replacement).
Thanks for the answers.

 

[Nigel_Ward]

Re: 2-blade propellers \"normal\" vibrations

Vibration from 2 blade props can arise on some boats as both blades pass the stern deadwood at the same time. As most modern yachts do not have deadwood ahead of the prop like the old wood boats this is less of an issue.

However vibration can be caused by bad combinations of blade numbers/gearbox ratio/ engine cylinder numbers.

 

[Marsupial]

Re: 2-blade propellers \"normal\" vibrations

Yes ANY engineer will know about cyclic irregularity, simply the fewer bits that drive the more vibration. In engines its easy to understand and experience, 1 cylinder engines make more vibration than a 2,3,4,5,6, . . . 12. Designers try to reduce the effects with heavy flywheels and balance shafts, multi cylinder engines over 6 dont need them, thay are "in balance".
The same with props, the more blades the less vibration, change a two for a three and what you thought was smooth - wasnt! MOBOers know 4 and 5 blades are best.

 

[Vamoose]

Re: 2-blade propellers \"normal\" vibrations

My 2 bladed Bruntons Varifold is just as smooth (if not smoother) than my old 3 bladed prop.. But I did renew the cutlass bearing