# Distribution of rocker.

Discussion in 'Hydrodynamics and Aerodynamics' started by Collin, Nov 3, 2011.

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### CollinSenior Member

I'm having a hard time understanding how a designer decides where the rocker is distributed in a hull and what effect it has on performance.

Some boats have a straight run forward, then the rocker curves up towards the stern. (bad example, this one is more balanced)

Some other boats have the opposite.

Once you have the amount of rocker you want determined, how do you go about deciding how to distribute it for the best efficiency?

Would a sharper run forward be more efficient, or is rocker that looks like the shape of a foil more efficient?

Is there a reason why it isn't symmetrical fore and aft?

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### PARYacht Designer/Builder

Yacht design 101 . . .

Your questions seem quite simple, but are actually a fairly complex set of calculations and design decisions, based on many mitigating and competing factors.

The only real way to answer your questions is, to improve your understanding of yacht design basics.

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### CollinSenior Member

It must be too simple for anyone to mention, since this is something I've been trying to find an answer to for a while now. Is there somewhere on the net I can learn about this?

Why is a whitehall's rocker a certain way while a guideboat is entirely different? I'm clueless.

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### Leo LazauskasSenior Member

No, it is more complex than anyone can explain in a few lines.
PAR gave you the best advice.

Good luck!
Leo.

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### Mr EfficiencySenior Member

I think you'll find these things have evolved through trial and error and settle where they are, because they work well in practical use, rather than being the direct offspring of some theory or other.

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### fastwaveSenior Member

I think what all the people above just said is that they do not know the answer.
So welcome to a very big club

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### daiquiriEngineering and Design

Hi Collin,

I wouldn't dismiss the replies given so far with just a simple statment "they do not know the answer".
As PAR and Leo have told you, you are asking a few-line (or few pages, not important) explanation of the essence of naval architecture - the design of the hull form. It is a science which has been evolving over the last several hundred years. It was continuously evolving even before, since the beginning of navigation on rivers, lakes and seas, but in a less scientific way (as Mr Efficiency has mentioned).

The fact is, a hull form is very seldom shaped for the optimization of a single performance indicator (speed, for example), but is rather a synergistic process which runs under a range of constraints, like:
- type of boat (passenger, cargo, leisure etc - with all the sub-cases)
- type of propulsion (sail, power, etc - with all the sub-cases)
- boat size
- target resistance or power
- target speed
- required minimum stability
- expected average meteorogical conditions
- waterway-related constraints (channel depth and width, bridges clearance etc)
- wavemaking constraints
- minimum seakeeping characteristics
- maneuverability and handling characteristics
- deck fittings, equipment and machinery layout
- internal accommodation layout
- internal structure and powerplant layout
- port operations
- available building materials and tools
- building procedures and costs
- etc etc etc etc
Each one of these design steps can require a book or two for a complete explanation.

Of course, not every type of boat will require all the above-mentioned steps. For example, the sailboat in your pic has likely been designed with considerations regarding speed, stability, wind & sea conditions, seating position of the skipper, and boatbuilding materials and method.

If you are interested in these topics, I suggest you to get a copy of Dave Gerr's "The Nature of Boats" or "Principles of Yacht Design" by Larsson & Eliasson, which will give you a good introductory knowledge. For a more in-depth discussion of these things, one single book will definitely not do. You can, for example, see the "Introduction to Naval Architecture" by E. Tupper.

Again, for the type of boat in your picture - a classic wooden dinghy - I would suggest you to read Skene's "Elements of Yacht Design", as many of these classic boat's shapes were determined with the use of trochoidal-wave theory, which is explained in that book.

Cheers, and keep looking for answers!

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### fastwaveSenior Member

My comment wa not meant to undermine any of the replies above, but just a joke that supports the complexity of the problem and that no one knows the correct answer, let alone a simple one (I included myself in that club as well).

The distribution of rocker depends on many interlinked parameters, but one of the 1st order ones is speed and how the boat behaves in pitch attitude through the speed range.
Distributing the rocker fwd or rwd will have a pronounced effect on the bow up characteristics, and also the resistance. Like everything else it is a compromise of many factors and can only be answered by complex studies (Tank-CFD etc)
Hope that helps a little more

One last comment.
Science and techonology might have evolved over decades, but the right answer never changed. The laws of physics are the same as they where before. What we do is evolve our interpretation of the right answer.

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### DCockeySenior Member

Depends on how rocker is defined and how the shape of the boat changes with a change in "rocker". Rocker is frequently defined as the curvature of the centerline profile. If the centerline profile is changed but the buttocks are not changed (shape of the sections near the centerline changes) then there will not be the same effect as if the entire hull is "bent" with the section shapes being kept constant.

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the worlds best designers keep changing them in the F18 class so there is no real answer, yacht design is an art
PS you also need a different rocker for different water conditions

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### PARYacht Designer/Builder

Each application will have an ideal arrangement and then will be adjusted to suit the SOR. In some very limited SOR's, such as racers, you can toss out many of the constraints and get a reasonably pure design element, but again it's a complex set of subjects, included in the original poster's question and even my usual verbosity couldn't scratch it in a few thousand words. Simply put Collin, you need to establish a working relationship with hydrodynamic theory and practicality. Armed with this, you'll have a better understanding of not only rocker, but the convoluted collection of discontinuous compromise that make up every design's physical attributes (damn, I should copy write that last line . . .)

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### CollinSenior Member

Thanks guys. Your answers are a lot more in-depth than I expected.

I've learned a lot about aerodynamics and hydrodynamics in terms of yacht design, but there's no mention at all about how people come to design the rocker in the hull in the books I have.

Daiquiri, I'll look in to those books

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### Mr EfficiencySenior Member

Just look out for those 'convoluted collections of discontinous compromise', which I suspect is a technical term for a "can of worms".

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### JRDSenior Member

Just one view on the subject....

Gidday Colin,

All of the above is of course correct, but I'll stick my neck out and try and desribe what influence the rocker has with the sail boats that interest me most. (small, light and fast ones) Your question is a simple one, but in fact very pertinent. There is no one answer but a collection of desisions, some based on real numbers and a few more that are understood by science but were developed very much "by the seat of the pants engineering over many centuries'

Just think of the rocker in the keel line as one vertical cut through the hull elevation, a "section, or single buttock line" if you will. What it does do is defines where the volume of the hull is located, but only when considered together with the waterlines and body plan.

The volume distribution then contributes to the displacement volume, prismatic coefficient (CP), longitudinal centre of bouancy (LCB) and to the transverse stability.

If you desire a certain CP and LCB for a set displacement, the rocker, along with the water line and bodyplan "fullness of curve" must be adjusted until the volume is distributed as you require.
Overall these volume adjustments will influence the following dynamics:
Surface area - which relates to frictional drag
Form or viscous drag relating to behaviour of the bourdary layer
Wavemaking drag which relates to the CP and LCB position
Separation - if one of your curves or corners is trying to make the water flow where it dont want to go...
Attitude - curvature of the buttock lines affects the bow up / bow down attitude of the hull particularly when surfing in waves or planing

If you compare some of the high performance racing dinghys that sail in dispacement and planing mode, you will see that the rocker differs in spite of similar performance eg.
The I14s and 49ers have more pronounced rocker further faward - but their fwd sections are quited Vee'd
The 12, 18 foot skiffs and moths have less rocker and it is distributed further aft, but the fwd sections are U shaped.

You would find that their CP and LCB to be similar, but the respective development has driven designers to use differing shapes to achieve this. All however have rather straight lines aft for fast and controlled planing.

These are far from the only influences, but will hopefully provide a small snapshot of the subject. If I'm wrong here.... well Im happy for one of the pros to jump in at this point and provide some meaningfull corrections.

P.S.
I had a look through the index in "Principles of Yacht Design" Rocker is not mentioned. So I skim read through the section on hull design again, still no mention or rocker, or the keel line shape. Should you still read it? - sure, but expect to draw your own conclusion on the rocker, but you will better understand it is only one minor parameter making up the whole topic of hull shape and design.

Good luck

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### daiquiriEngineering and Design

Not directly - but it is implicitely there. As you have correctly noted:
"If you desire a certain CP and LCB for a set displacement, the rocker, along with the water line and bodyplan "fullness of curve" must be adjusted until the volume is distributed as you require."​

Both Cp and Lcb determination for a given design Froude number are explained in the PYD.

Cheers

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