Stern Flap Trend in Cruise Ship Design

[highlander0108]

I've worked up a post on the latest trend in cruise ship design, the stern flap, which may be of interest here.  What spurned this was seeing ship after ship with this feature and then seeing the new Royal Princess renderings with the same type of stern.



http://qe2-prideoftheclyde.blogspot.com/2011/03/shape-of-things-to-come-part-2.html

Ken

[Bob C.]

I have a video of the design and building of Royal Caribbean's Navigator, Mariner, etc. OTS.  In it they discuss the stern flap design and how it provides a 2% increase in overall propulsion efficiency.  Not sure if it takes advantage of the stern wave or the surface disturbance of the prop wash but I guess as long as it saves money, it's worth including in the design.

[skilly56]

Hi Ken,

Thank you for the link to this very interesting piece of work. And I had a good laugh at your "If it ain't Scottish it is crap" blog as well. After spending 1972 till 2010 sailing and operating Scottish-built vessels, and now sailing on "Chinese junks", I definitely know what I would rather be on!

Am I am not saying that just because my middle name is Duncan from Glasgow either!

I spent some months working on hull resistance and doing tank testing for resistance reduction in 2000, and it is not surprising to see these sorts of designs being introduced. What may have been in the 'too hard' basket during the times of cheaper bunkers suddenly becomes very 'do-able' when the fuel costs go stratospheric.

I tried to post a complimentary comment at the bottom but the cursor would not go into the text field.

Cheers

Skilly

[Peter Mugridge]

Can't they disguise these flap things by putting them slightly further forwards to hide them under the actual stern? 

[Magic Pipe]

The philosophy behind a ducktail stern is that when a ship moves forward at speed, it has to climb its own bow wave.  The more the ship's bow is elevated relative to its stern, the greater the resistance and fuel consumption (the ship is literally trying to sail up hill).  As the bow rides up on the ship's bow wave, the stern is forced underwater.  A wide, flat stern is much harder to force underwater than a tapered cruiser stern, so incorporating such a stern tends to keep the ship from having to sail uphill as much.  In addition, a flat underside to the stern creates dynamic lift, which further elevates the stern and reduces resistance from the bow wave.

[Peter Mugridge]

Yes, but my point is that surely they can put a flat underside underneath a proper looking stern?  After all, what they do above the waterline won't affect the drag in any way will it?

[Magic Pipe]

Yes, but my point is that surely they can put a flat underside underneath a proper looking stern?  After all, what they do above the waterline won't affect the drag in any way will it?

I'm not sure what your idea of a proper looking stern is, but the philosophy behind a duck tail is that there is no structure above it to add weight.  Extending the back of the ship adds hundreds of tons of weight to the very stern, defeating the purpose of a feature that is intended to prevent the stern from being forced downward.

[Peter Mugridge]

There is a way you could do that, but it'll have to wait until I am at home so I can draw a sketch and upload it.....

[skilly56]

Without having studied the subject at all, I believe that little 'duck tail' appendage adds considerable positive buoyancy at the tail end when compared to a 'cruiser stern' ( e,g,. QE2). Would welcome some other thoughts on this.

When a ship accelerates up to speed, it develops a trim by the stern known as 'squat'. When the ship trims by the stern, it's 'angle of attack' changes as the bow lifts, and this adds to the hull drag and fuel consumption (as stated by Magic Pipe), and also the aft draft (the depth of lowest aft part of the hull immersed in water). This 'squat' resulted in the QE2 running aground and incurring considerable damage - I seem to recall being told that she would squat by some 3+ metres when doing 30 knots, and, when this was added to the 'suction' effect experienced in shallow waters, the squat value would be even greater, hence the grounding.

By adding the 'duck tail', when the ship tries to 'squat' at speed, the additional buoyancy provided by the duck tail when it becomes immersed would act to counteract the amount of squat, and thus the aft draft increase is reduced. The effect of the wake pushing against the underside of the ducktail would also add dynamic lift as well.
Between the 'duck tail' at the stern and the bulbous bow at the other end, the two appendages combined would also reduce the ship's pitching moment.

Some ro/ro ferries have 'sponsons' added to their sides to give additional buoyancy - effectively giving the same result as the 'duck tail', but in this case to assist the heeling tanks in trying to reduce the list when rakes of heavy wagons are rolled on and off the ship from the outermost rail lines.

I was born in Christchurch, NZ. My relatives down there tell me they have been doing a lot of 'involuntary' squatting recently.

Cheers
Skilly

[Peter Mugridge]

This is how you could hide the duck tail...

You basically bring the stern down in the traditional way, but then have a "slot" underneath it just above the water line.  Because the stern does not contact the hull until the front of the slot, the weight of the stern acts on that point, not the very back of the ship.  Thus the platform would still act on top of the water and not be pushed lower.

Yes, it'll still look funny, but only if you're actually at a low side on angle from which you could see through the slot under the stern.  From most angles it'll look traditional.

More so because in order to avoid over-stressing the front of the slot area and to reduce the downwards force in that area it would more or less force a return to the traditional tiered stern decks - but without losing any capacity because without this they would have to stop the stern where the front of the slot is.

So really this would effectively be a traditional shaped extension that would be beyond the limits of the present vertical ferry type sterns.

Win-win!

PS Yes, I know - I forgot to sketch in the rudder!

[highlander0108]

I'm sorry Peter, but all that structure above the "flap" would be a considerable amount of mass and would negate the effect as described by Magic Pipe.  The location of the mass is important in the hydrodynamic calculation.  THe weight is still there, but suspended further back.  She'd still squat and be trying to get out of the hole and above the bow wave I think.  I wonder, though, if the concept of a trim tab, as used on small craft, has ever been applied to ships.  Picture a conventional stern, but with your flap on hydraulic rams to push the plane down.   

[Peter Mugridge]

You mean a bit like the wing on a hydrofoil but designed only to raise the stern a little bit rather than lift the whole hull out of the water?

Going back to my sketch, you could always run a huge girder like frame structure mounted midships and effectively suspend the stern from it; 90% of the weight would then bear down midships.  It should be technically possible to do that without inducing excessive flexibility, but it would have the disadvantage that it would increase construction costs because it would mean having to build in a traditional fabricated manner rather than by welding a lot of pre-constructed boxes together.

On the other hand, if you did that, would having all the weight bearing down midships induce excessive pitching motions?



Well... if we keep thinking about it there must be some way of making ships more fuel efficient without ruining their appearance with these sticky out things at the back.

[Mauretania1907]

  I sometimes think todays newbuilds are SO ugly anyway that the duck tail stern does not detract from their inherant gross and ugly looks. But then I was built in 1907, when four funnels were considered quite the thing to have.
That gripe aside, I am quite interested in the whys and wherefores of the design. One has only to ride a speedboat to see squatting in action.
And as for Christchurch yes squatting is an issue there, the sewer system is needing a total refix. (altho not so bad as Japan)

[pete cain]

Am I right in thinking, that  Athena, ex Stockholme, has had some sort of addition at her stern, to do the same job as one of those there Duck tails?. Images 1, 2 & 29 show what I'm on about, nice site by the way, worth a read.

     http://www.cruise-australia.net/MV-Athena.htm

[Matteo 91]

Am I right in thinking, that  Athena, ex Stockholme, has had some sort of addition at her stern, to do the same job as one of those there Duck tails?. Images 1, 2 & 29 show what I'm on about, nice site by the way, worth a read.

     http://www.cruise-australia.net/MV-Athena.htm

A small OT note:

And that ship is still happily afloat... 

while another one lies on the ocean floor because of her.