Asymmetric Propellers

Propulsion systems, machinery, turbines, boilers, propellers, fuel consumption, etc.
User avatar
Legend
Senior Member
Posts: 325
Joined: Tue Oct 21, 2008 12:46 am
Location: Tomahawk, Wisconsin

Asymmetric Propellers

Post by 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?
AND THE SEA SHALL GRANT EACH MAN NEW HOPE, AS SLEEP BRINGS DREAMS.
Bgile
Senior Member
Posts: 3658
Joined: Wed Mar 09, 2005 7:33 pm
Location: Portland, OR, USA

Re: Asymmetric Propellers

Post by 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
Senior Member
Posts: 3822
Joined: Sat Jun 17, 2006 2:15 am
Location: Southfield, USA

Re: Asymmetric Propellers

Post by lwd »

Perhaps less likely to produce harmonics?
Bgile
Senior Member
Posts: 3658
Joined: Wed Mar 09, 2005 7:33 pm
Location: Portland, OR, USA

Re: Asymmetric Propellers

Post by 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.
User avatar
ontheslipway
Supporter
Posts: 233
Joined: Mon Oct 18, 2004 8:19 am

Re: Asymmetric Propellers

Post by 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) :whistle:
Bgile
Senior Member
Posts: 3658
Joined: Wed Mar 09, 2005 7:33 pm
Location: Portland, OR, USA

Re: Asymmetric Propellers

Post by Bgile »

Thanks foeth,

Your posts on this sort of thing are always interesting and informative. :)
User avatar
Legend
Senior Member
Posts: 325
Joined: Tue Oct 21, 2008 12:46 am
Location: Tomahawk, Wisconsin

Re: Asymmetric Propellers

Post by Legend »

Yes thank you. That was extremely informative and explains exactly what I was looking for!
AND THE SEA SHALL GRANT EACH MAN NEW HOPE, AS SLEEP BRINGS DREAMS.
chuckfan3@gmail.com
Member
Posts: 28
Joined: Fri Nov 24, 2023 6:56 pm

Re: Asymmetric Propellers

Post by chuckfan3@gmail.com »

Legend wrote: Wed Dec 30, 2009 5:48 am 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?
I believe the “asymmetric” in this context means the individual blades are sickle shaped, not more or less symmetrical when viewed down the axis. This reduces the characteristic noises generated when each blade enters the wake sheet of the rudders and diving planes during propeller rotation.
OpanaPointer
Senior Member
Posts: 553
Joined: Wed Jul 06, 2011 1:00 pm

Re: Asymmetric Propellers

Post by OpanaPointer »

Bill Jurens
Moderator
Posts: 878
Joined: Mon Oct 18, 2004 4:21 am
Location: USA

Re: Asymmetric Propellers

Post by Bill Jurens »

The terminology can be confusing.

Usually, an asymmetric propeller is considered to be one that has blades of different sizes and shapes. The ultimate example is a one-bladed propeller, essentially a two-bladed propeller with one blade cut off and the hub somewhat reworked and counter-balanced so as to prevent undue vibration. Believe it or not, this actually works, and can have some advantages. In specialized situations.

The other situation might describe a propeller where only one or two blades differ from the rest, e.g. a five-bladed propeller where only two of the blades can change pitch in real time, or where all of the blades are of slightly different, but constant pitch. There are some applications -- and variants -- on this sort of theme, but the operational 'windows' where these sorts of designs are useful are fairly small, so they are very rarely seen.

Bill Jurens
Post Reply