How is the LWL achieved in design?

I was talking about Rhino, not RhinoMarine. Rhino has commands to directly calculate the volumes, areas, centroids, moments, etc needed for all or virtually all hydrostatic calculations. Then "Simple multiplication and division yields CP, CM, etc" as well as other desired hydrostatic properties.

I've seen CP, CM, etc defined based on largest section and also defined based on mid-ships station. Rawson & Tupper, Basic Ship Theory, fourth edition, volume 1, pp 1314, for example define CP and CM using "midship sectional area". The use of midship sectional area is probably a carryover from doing calculations with a lines drawing and only having a discrete number of stations to work from. The section(s) to be used for CP, CM, etc can be defined where the user thinks they should be.

The calculation of the volumes, centroids, etc in Rhino is totally independent of the selection of the station used for CP, CM, etc.

 

Yes, but then one needs to calculate elements of metacentre and submersed volume using formulas from ship theory, such as r=Ix/V, sectional areas, etc. - hell a lot of job to present it in form suitable for naval architect/classification society. Also stability can't be effectively calculated that way.

For commercial vessels discussed in 'Basic Ship Theory' usually midship section is the largest one so this definition of CP and CM is traditional. For small craft largest section might not match with middle, so CP and CM are defined based on largest section. We do remember that area under curve of sectional areas Ai=f(LWL) divided by area of rectangle LWL*Amax gives us CP; it works only if biggest section is used.

 

Perhaps the point I tried to make back in post #23 was missed. Rhino and other software can calculate areas, volumes, moments, centroids, etc without specifying station locations in advance.

 

Yes, they triangulate the surface, calculate normal vectors for each triangle, define pessure and summarize it over the surface.

 

A simplier and perhaps easier to understand definition of CP is the volumetric displacement divided by the product of Amax and LWL.

I suspect the origins of the definition in terms of areas of curves of areas was directly related to the use of planimeters which could directly measure areas but not volumes. So one way to calculate a volume is to first create a curve of cross sectional areas, and then find the area of that curve. Keep the units straight and the result is the volume.

 

Yes of course, but it needs maximum section anyway, not middle. And to find max section one needs to check range of sections, that can be done on area curve.

 

Assuming "pressure" was meant by "pessure", I don't see why pressure is needed for calculations of geometric properties such as area, volume, moments, centroids, etc.

 

Looks like two different discussions.

One is that some software does not require specification of a set of stations to calculate areas, volumes, etc.

The other is that the maximum area needs to be defined for CP, CM, etc.

I haven't seen any disagreement with either.

 

Area curve was a tool to design hull shape. In classic naval architecture displacement and LCB were defined first, after area curve is drawn and lines plan is drawn based on areas available, for each station. Thus the designers kept target displacement and trim.

There are recommendations on area curve shapes - fullness of bow and stern, etc. For sailing yachts there were graphs from Archer on area curve design, later used to develop lines plan.

Integration of volume can be done without graphical area curve, of course use of planimeter helps but we were taught to precess it in form of tables.

 

Yes, there is no disagreement. Software does not need sections to check volume but naval architecture needs sections for standard presentation and other purposes.

 

Thanks Ad Hoc. I feel more confident now.

Interesting this thread. It has attracted the most senior ship/boat designer.