Asymmetric Propellers

[Legend]

I came across the term Asymmetric Propellers in relation to a new class of SSN's... the Chinese Type 093 if I must be specific... and I didnt completely know what to make of it. They mentioned it was seven bladed, but then why not just say that and not include the "Asymmetrical" part?

What would having an asymmetric propeller have going for it, besides exessive vibration?

[Bgile]

I can only assume they are referring to the odd number of blades. 3, 4, and 6 all divide evenly into 360 degrees. It obviously still has to be balanced, and the USN has been using 7 blades for many years.

[lwd]

Perhaps less likely to produce harmonics?

[Bgile]

Maybe, and probably harder to get a shaft rpm count. They are hard to machine, and there was a furor when a Japanese company sold the necessary milling machines to the USSR a number of years ago.

[ontheslipway]

"producing harmonics" is something that every propeller does, though the (first) harmonic frequency is shaft rpm times number of blades and thus goes up with more blades. With an increase in blade number, the loading per blade is less and you induce lower pressure fluctuations in the far field. Still, this effect is much larger going from 2 to 3 or 3 to 4, but it helps. You try to avoid an even number of blades because the wake of a sub has the vertical and horizontal rudder wakes spaced out at 90 degrees. This wake leads to a local increase in blade loading, so having more blades reduces both the loading per blade as well as loading variation per blade (noise). You can image you'd prefer not to have this loading variation of all blades in phase, hence the odd number. Frigates and cruise lines (and such) usually have 5 blades. Though 7-bladed controllable-pitch hubs have been made, they are very large (loosing efficiency). When you have space to fit any propeller you want, your efficiency typically goes down with an increase in blade numbers and for surface ships the effect of the propeller tip thickness is an inboard noise source as well. A sub doesn't have this problem as there's no hull nearby the propeller tip. By the way, most surface ships cannot place a propeller with an optimum diameter in this case 5 blades are often more efficient that 4 (more or less)

[Bgile]

Thanks foeth,

Your posts on this sort of thing are always interesting and informative. :)

[Legend]

Yes thank you. That was extremely informative and explains exactly what I was looking for!