How to theoretically calculate the floating state and stability of a complex floating body

[BMcF]

Really, everything that is in the modern rules, and more for military vessels, could and was done by "non-computer" methods until well after I started engineering classes in middle school (1972). That is not to say we didn't use "machine tools"; slide rules, mechanical calculators, planimeters, 3-wheel integrators, pre-printed spreadsheets, etc, were all used to develop the Curves of Form, D&Os, Cross Curves of Stability, Tank Capacity Curves, etc. .

We might be about the same age. Your post prompted me to pull the largest of my three slide rules out of the bottom desk drawer and reflect on it's powerful simplicity. ;-) I recall competing in "Slide Rule Olympics" in the mid 70s.

 

[TANSL]

And who doesn't remember tables of logarithms and how tedious calculations with them were? I have used them, as well as slide rules, and, to be honest, they don't inspire nostalgia in me. They were real instruments of torture. (Imo)

 

[Skip Johnson]

And who doesn't remember tables of logarithms and how tedious calculations with them were? I have used them, as well as slide rules, and, to be honest, they don't inspire nostalgia in me. They were real instruments of torture. (Imo)

A lifetime ago I was running a survey crew and one of the workers was an exchange student working on his doctorate in math. I started leaving my book of log and trig tables in the vehicle. I could ask him for a value and he'd stare off into space for a few moments and come back with 3 or 4 significant digits and was always correct.

 

[gonzo]

You worked with rainman

 

[CDBarry]

Rooting through old papers I recently found a set of hand hydrostatic properties calculations done with a planimeter for a real job done just like the examples in the old PNA; volume, lcb, tp1, km, the whole banana, on D size vellum. I had forgotten doing it.

 

[sun]

Rooting through old papers I recently found a set of hand hydrostatic properties calculations done with a planimeter for a real job done just like the examples in the old PNA; volume, lcb, tp1, km, the whole banana, on D size vellum. I had forgotten doing it.

It's a great pity. Special thanks for the reminder. I hope others can find this method.
For some reason, I haven't been able to log in to this website for over a month.

 

[TANSL]

For a number of reasons that are irrelevant here, I'm calculating the hydrostatic and KN values of my latest design without the aid of any naval architecture software. However, I am using a 3D model created in AutoCAD. If you're interested, I could describe how I do it. It's laborious, but much faster and more accurate than doing it by hand.

 

[sun]

For a number of reasons that are irrelevant here, I'm calculating the hydrostatic and KN values of my latest design without the aid of any naval architecture software. However, I am using a 3D model created in AutoCAD. If you're interested, I could describe how I do it. It's laborious, but much faster and more accurate than doing it by hand.

Thank you!

I'm very interested in this method and hope to learn it to help more people.

Do I need to send you an email?

 

[TANSL]

Hi @sun, I have sent you a pm.

 

[rxcomposite]

I use a spreadsheet and to keep things simple I use a slab sided hull for demo purposes. To get the second moment of inertia where it all start, I multiply the area by the coefficient of waterplane. This is to correct for the area error resulting from a rectangular shape to ship shape. A standard elliptical shape formula may also be used instead of the Cwp. Of course it gets more complex as you transition from cat to tri plus the fact that at certain angles of heel, the multihull transition from a two body to a single body when one hull is out of the water. All others such as GM. KM, GZ are standard NA formula.

 

[sun]

Hi @sun, I have sent you a pm.

Hi TANSL, I have sent you a email.

 

[sun]

I use a spreadsheet and to keep things simple I use a slab sided hull for demo purposes. To get the second moment of inertia where it all start, I multiply the area by the coefficient of waterplane. This is to correct for the area error resulting from a rectangular shape to ship shape. A standard elliptical shape formula may also be used instead of the Cwp. Of course it gets more complex as you transition from cat to tri plus the fact that at certain angles of heel, the multihull transition from a two body to a single body when one hull is out of the water. All others such as GM. KM, GZ are standard NA formula.

Is this method still effective when complex-shaped floating objects are tilted at a large Angle? Capsized?

 

[rxcomposite]

Is this method still effective when complex-shaped floating objects are tilted at a large Angle? Capsized?

No. That would be putting the cart before the horse. I am not designing the hull yet but laying down the parameters that will generally satisfy my design.

To simplify, you need to work only on underwater lines, thus you have to define first the displacement which is L x B x T(draught) x Cb.

To define the waterline you need to multiply the L xB x Cwp. Cwp ranges from variuous ship types from tankers to cruisers to transom sterns. (about 0.680 to 0.780)

Thus you have the two basic which would define the Inertia and Displacement.

The hull shape center of buoyancy can also be defined by choosing an appropriate hull form such as bilged, circular, elliptical, or deep V.

If you want to investigate large angle of heel or the GZ curve, you need to apply the method discussed in previous post. That is;

1. Simpson's rule or trapezoidal method.
2. Integration- Slicing the area into more sections
3. Use of a software (more sections that you could imagine).

 

[rxcomposite]

@sun When you say complex floating body, you mean this?

 

[sun]

@sun When you say complex floating body, you mean this?

Sorry, I've been trying to log in to this website all week and it wasn't until today that I could log in normally.
Nowadays, apart from ships, there are many bionic structures on the water surface, aiming to imitate the shape of aquatic animals and the speed and stability of the structures.