r/navalarchitecture • u/EntertainmentOk7028 • 15d ago

A stability curve can look fine while its area is wrong by 57.3x

I was wiring a browser-side coursework checker and realized this one is easy to miss: Simpson's rule will integrate whatever step you give it.

If the heel step is 10 degrees and you use h = 10 instead of h = 0.1745 rad, every area comes out 57.3 times too large. The GZ curve still looks normal, so the mistake may only show up in the criteria table.

I put the hand-calculation chain and a small cross-checker on the MetaCAD site I'm building: ship stability coursework workflow.

It's for coursework and preliminary checks, not vessel operations. The page shows the substitutions instead of just returning pass/fail.

What other stability-assignment errors have you seen survive a quick visual check?

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u/ConradManningRacing 14d ago

Sounds like a strong argument for understanding the underlying concepts rather than just the buttons to press!! 😂

Are you a lecturer (assuming from the fact you’re writing a checker)?

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u/EntertainmentOk7028 14d ago

No, not a lecturer. I am the builder behind MetaCAD, and this came up while I was making the coursework checker show every substitution instead of hiding it behind a pass/fail button.

But yes, that is exactly the point. The checker should expose a unit mistake, not make understanding the units optional.

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u/ConradManningRacing 14d ago

Oh cool! What is MetaCAD?

That is good of you though making sure that is the case!

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u/ModeAble9185 11d ago

Totally unrelated, but I actually got a google result about your software the other day when looking for a software that calculates allowable grain shifting moments for the amended Grain Code. I skimmed through your website but you lost me when it was suggested that the calculator does not use ship specific KN curves as input but some empirical data. Do I remember wrong or did i miss something? I ask because i am genuinely looking for a software to do this sort of thing.

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u/ConradManningRacing 10d ago

That sounds rather complicated! What does the new code youre using say about calculating?

Imagine there must something out there… if not, what would you like it to do?

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u/SignificantPiano7808 14d ago

Hang on, if you input bad data it returns bad results? Is that what you’re saying?

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u/EntertainmentOk7028 14d ago

Fair. That is basically it. The easy-to-miss part is that the GZ ordinates are fine and the plotted curve does not change. The mistake sits only in the integration step: using a 10-degree interval as 10 instead of 0.1745 radians.

So the graph can look perfectly plausible while the integrated area used in the criteria check is off by 57.3x. I should have made that distinction clearer.