# calculate by bojean

DELFTship forum Hydrostatics and stability calculate by bojean

• Author
Posts
• #36987
kedson
Participant

how to calculate by bojean?

• #37462
Peter Edmonds
Participant

Bonjean curves were very useful many years ago when I was a student, and onwards; pre-computer for hull shaping and calculations. (I guess that we are talking about the same thing, with spelling error. I don’t think I actually used them after graduation (53 years ago), as launching calculations were one of the earlier applications of computers into Naval Architecture. At this stage the programs were written in Extended Mercury Autocode (EMA), a functional precursor to Fortran.

The Bonjean curve had as its ordinate the cross sectional area at that section, up to the waterline concerned. Each curve was usually plotted with its axis The vertical axis was traditionally at ship scale.

Their main uses were for launching (end launching) and longitudinal strength.

For launching, prior to stern lift, the progressive waterlines would be set up on the ship profile with the Bonjean curves. Where this waterline cut the vertical axis was the local draft at that section. Bonjean area was read at each such intersection, and then integrated longitudinally for both volume and moment. These gave the volume of buoyancy and its longitudinal moment about the fore poppet for each successive waterline, to match against moment of weight force about the fore poppet, defining the point of stern lift.

Longitudinal strength involved superimposing a wave profile on the profile of the ship, then again calculating volumes and moments of volumes. This is much too long for a posting here.

I don’t see any applications for Bonjean curves these days, where we can interrogate our hull model for a given waterplane, and can have the software balance the hull for draft and pitch to meet a nominated volume and longitudinal moment of volume.

I hope this is enlightening, if not particularly useful, given the effective technical obsolescence of the Bonjean curves.

Naval Architecture was the home and developing ground of many tidy and elegant numerical calculation procedures in the era before calculators. We can think of the traditional displacement sheet as the precursor to the (Excel) spreadsheet. It would be very easy to implement the displacement sheet (cell by cell multiplications and additions) into Excel.

Peter Edmonds
Naval Architect
Perth, Western Australia

• #37463
Marven
Keymaster

That’s totally correct.
To be precise, in the days when lines plan were still manually calculated the calculations could only be performed on the stations (or waterlines), using planimeters or even manual methods of calculation.
Nowadays with 3D models it’s fairly simple to lift calculations directly from a 3D model and bonjean curves have become more or less obsolete.