How is the LWL achieved in design?

There will always be a midship drawing that is called for in the plans submission. The drawing is to present the majority of the scantlings. What if the midship frame falls beyond the exact mid of the boat/ship as in odd spacing or if the greatest transverse area falls beyond the mid, which one do we present?

 

Doesn't matter if it does not fall exactly at amidships.

The drawing is to show the principal scantlings for global and location strength requirements. So whether the frame is in the exact location matters not. But the correct scantlings that are indicative throughout the vessel, is!

 

I completely agree with that practice. I always start with LWL to find if the design guide boundaries (Cb, Cx, entrance angle, ect) would satisfy the SOR.

I do all the number crunching (LCB, LCG, Displacement) before I start the hull lines drawings.

Picking a hull design and working backwards to make the SOR fit is counterintuitive to the design spiral.

 

That it is called a design "spiral" means that many variables will change to produce the best fit to the primary goal, whatever that may be. I find that everything changes, at least a bit, and that having firmly fixed several of the design factors may limit achieving the best fit. In general, I agree with your last statement but don't use it as gospel. Very often the idea of a hull design comes first and must be either verified, rejected or significantly modified by the spiral. The LWL or WL location seldom remains permanently fixed in my hulls as other variables often force some revision. Station locations are often changed and don't remain fixed until the last pass through the spiral.

Of course, I don't use any computer software which might make things quite different in that regard. I wonder how the better designers before computers did it. Certainly working from a carved model would not allow using your method in starting the design.

Just rambling thoughts.

 

Tom,

As stated, it is my way of doing things, not as a gospel. As you can see, I ask questions from the expert to learn how they do it their way.

I agree that many things may change during the design stage. That is what the design spiral is for. Sometimes, the SOR itself may need changing if the design proves it cannot be met or the design may be impractical. Design is always a compromise although we always aim for the optimum.

 

Ad Hoc,

I have long used the PNA book design guide based on Fn. (You have posted it here somewhere in the forum).

Is this a good guide for a "parent hull" for hull design? I have not used paramarine software so I am not aware of what it uses as a parent hull. Any other source for parent hulls that I can use?

 

RXC..

A 'parent hullform', in this context, is one that achieves the desired goal. You then know all the particulars of that hull form.

So, when designing another vessel, you can use that parent hullform, as your basis, assuming the new hull does not fulfill, as a guide to establish how much better or worse the new hull is compared to the original. You must also recognise that the new hull form may be an improvement of the parent hullform. In which case the parent hullform, is just your starting block. It does not mean it is the 'best' or 'optimal' etc...each hull is, or should be, unique to its own SOR.

Many people tend to view hull design or a lines plan as a "black art" and a near 'fixed' commodity or the "be all, end all". To us, it is just a part, one part of many that becomes the finished design. We chop and change hulls like we change socks. Each hull is modified to suit. One can do this once you understand that the main driver (at least in the high speed field) is the length displacement ratio. Hull shape per se plays a very minor role.

Software helps you to quickly alter an existing lines, or create a new one. But the hull must satsify certain design attributes, whether it is the parent hullform or another.

Since why use a parent hullform that has an LCB of say 8% aft..when the layout, from the SOR yields an LCG of midships?...you cant use it..thus, use a different hull. If one doesn't exist in your armoury, modify the one you have to get the corect LCB at level trim, assuming it still satisfies your other objectives. By stretching, or lowering the transom draft etc etc. It is these "quick methods" of manipulation you need to be aware of, of what change does what, to a hull form. Simply put...the trend. Trends are far more important than absolutes.

 

Rhino and I expect most other "CAD" software which calculate displacement, center of buoyancy, waterplane center, moments, etc directly from the 3D math do not use explicit stations for the calculations. Other algorithms are used which should be more accurate than Simpson's rule applied over 10 or 20 intervals. Create the hull shape in 3D math using any methodology and then run the analysis. It does not depend on how the shape was created nor on the design strategy.

Station locations for line drawings and frame locations do not have to be selected initially.

 

Yes, but determination of hull shape factors depends on definitions of stations, even in CAD. For instance, CP and CM in Rhino.

 

True; I always wonder why some want 'proven hull'; we can often read this nonsence in government tenders and hear from private customers. For naval architect it is quite easy task to adjust the hull to design considerations; use of 'proven hull' term makes no sense because it is likely to be 'proven' for different conditions.

 

Exactly.

It also means the client (in these cases the Govt.'s) is buying "old technology". There is no impovement or evoloution allowed for, as one would do from inservice experience and differing conditions...so why don't they all use Turbinia??

Man walks into car show room.
Salesman...what would you like sir
customer..can i have one from 1990 please?
Salesman...er....why have an old one sir, we have up to date new improved models.
Client..i want a "proven car".

 

They treat it as they are buying magic, not technology.
Common problems with customers without understanding of hull design basics.

Those prefer to copy the mould from 'well proven boat', overweight it or overpower, and finally have poor result...

 

A primer--The Design Ratios

Just a year ago, there was another thread on this site regarding Center of Flotation which prompted my giving a bunch of "lectures" on the design ratios that we use in naval architecture. At the end of the series, I published all of the lectures in a single PDF document which you can download on that thread, on my website, and since I am writing this, I attach it here. This got very good reviews at the time--a lot of people thought it was very helpful to understanding the basics of naval architecture. Enjoy.

Eric

 

Only one station needs to be defined to calculate CP and CM using Rhino and an Excel spreadsheet, the midships. And the midships location can be defined after the hull shape is determined. The area calculation function is used to calculate the midships area. The displacement volume is calculated by Rhino without using any defined stations. LWL, Beam at the waterline, draft, etc are either returned directly by the Rhino Hydrostatics command or are simple measurements. Simple multiplication and division yields CP, CM, etc.

 

Not really, those coefficients are related to biggest section, not to middle. So one needs to define range of sections.

From RhinoMarine help:
In order for the sectional area curve, Cp, and Cx to be computed, RhinoMarine must compute stations in the model (these are the only values that need stations; all other calculations are carried out on the analysis mesh). In the "Station Locations" section of the Calculate tab of the Hydrostatics dialog, click on Add to define a range of stations. Note that you must define enough stations to properly describe the hull