# How to analyze the buoyancy and stability of the hull with multiple seated points?

Discussion in 'Boat Design' started by sun, Jun 13, 2023.

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### sunSenior Member

The hull is seated in the water, and there are multiple seated points. How to analyze the buoyancy and stability of the hull ? This situation is completely different from a single sit-in point. Is there a self-study reference ?

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### IkeSenior Member

need more information on the boat. Type of hull. length. Beam draft etc.

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### sunSenior Member

Thank you! Are there any conventional theories and calculation methods for general ships?

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### Alan CattelliotSenior Member

Good question. Makes me think off issues with the calculation of th inverted stability of multihull. We have discussed how to do these calculations with naval architects many times ; no conventionnal or documented methods were proposed. In the end, it has been decided to make iterations "by hand", based on approximation of the density distribution of geometrical shapes.

Solutions do exist, though. I use DualSPHysics – DualSPHysics: A combined CUDA and OpenMP implementation of the Smoothed Particle Hydrodynamics method based on the advanced SPHysics code. https://dual.sphysics.org/ to make these calculations. A Boltzmann approach with "smooth particules" modelization. Very well documented, CUDA optimized - requires a lot of computing power -. Also perfect to handle shloshing effects & damaged stability. Because nothing is perfect, the calculation post-processing has a flaw in the calculation of Momentum around the X-axis - Momentum of forces are well calculated during the simulation itself, rotation angles and rotation velocity are OK, but the post-processing of calculations is not done properly -. Please see my post Moment computation on floatings 2 https://forums.dual.sphysics.org/discussion/2328/moment-computation-on-floatings-2#latest. to handle this small issue.

Cheers,

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### DCockeySenior Member

You use a hydrodynamic tool for hydrostatic calculations? Interesting.

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### Alan CattelliotSenior Member

Thanks for the interest. I've been testing this code for almost 15 years now. No successes with the first attempts to use it on real cases though. I had to wait for the V4.0 and for a real CUDA machine to begin making some dynamic simulations of 8m motoryachts, then 12m sailing yachts. In the last two years, I've realized, for my clients, power prediction and sea handling simulations on 18m and 24m motor trimaran, working in both design phase and testing phase. Finally, I was able to compare prevision with full scale testing. As a result, I gain confidence in the capabilities of this approach for dynamic simulations of watercrafts. It is at this point that I've started thinking : "who can do more can do less".

At the Certification Institute where I worked previously during a few years, hydrostatic calculations were daily performed, but we were facing issues to solve specific cases of great importance for the security and life saving at sea : The inverted stability of multihulls. I must say that it was also a personnal concern, because I've encounter a near-dramatic situation with the broken up of a sailing trimaran during a round the world record attempt, of the coast of Tasmania. The risk of capsize is very high in multihull, and the question of whether or not we are able to bring safety to the crew of a capsized multihull have been posed many times. Where do the air pockets forms ? Where should be put the emergency hatches ?

Commonly used stability softwares could bring some answers to these questions, but I was not fully satisfied by the very rough approximations made. A contrario, the DSP simulations are able to handle very complex shapes, taking into account multiple material densities, which is critical in the case of inverted stability.This is why I've started developping simulation loops for stability calculations using this hydrodynamic code. Calculating stability curves that way requires a very high computing power, and very long computation times, compared with approximative methods, which could probably be seen as time waste in a vast majority of cases. But
• a consistent and unified approach is a efficient way to get comparable and justifiable results
• the cost of calculation power is diminishing with the time, following an negative exponential law
• the value of the experience of a "calculus" is raised when efforts are not diluted into a great number of specific approaches, using several different codes. But focused on a deeper and deeper knowledge of a small number of mastered refined tools.

Just my opinion, though...

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Are you aware of the excellent research conducted by the Wolfson Unit, HERE and HERE on this subject?

Indeed, since they are focused upon "static" situations as per any stability code.

It is more about "design" in the naval architecture sense, and the ergonomics of design, than a pure stability analysis..as that will only get you so far.
And of course good seamanship...the often overlooked factor.

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### Alan CattelliotSenior Member

Many thanks for these publications, Ad Hoc. They are of a great quality. Multihull stability poses some great issues. Numerous discussions during ISO meeting have unfortunately given some very erroneous requirements for diagonal stability of multihulls in the 12217 standard, and I will surely have a deep dive into these publications.
I should say that the use the DSP code for hydrostatics allows also to compute with a minimum of approximation the way multihulls are floating, after they capsized., which is an essential questions, regarding the high risk of capsizing that goes with these kind of boats.
For instance, being able to decipher where emergency hatches should be placed is fundamental, and the same goes with being able to identify where, on an inverted multihulls, a crew member can find air pocket, allowing him to reach by the safer path, these emergency exits.

Stability is then to be calculated with "multiple seated points", each of which represents a different buyoancy. Add to that the complexity of the shapes of the superstructure and deck, and you get a very complex problem, that is really hard do solve with the conventionnal tools I know. We even have the idea to capsize a 24m multihull, to see what happens in reality, and compare it with the approximations we were performing, in this Certification Institute. But we never had the chance to do that, unfortunately.

Do you know if such test have been performed ?

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ISO is somewhat of a waste of space. It is merely a bureaucratic "tool" to fill what is/was perceived as a loop hole in legislation between simple leisure craft and commercial vessels. It has grown way beyond its remit and bares little to reality and there is no centralised body to seek clarifications or dispensations etc where one finds clauses that are at variance with others or common sense, like one can do with Statutory and Class rules.

That is Design per se, and nothing to do with stability software analysis.
This is where the knowledge and experience of said designer plays a critical role, and not a set of prescriptive rules and software.
The designer must be aware of all implications of the ergonomics of their layout in all loading scenarios and endless what if's to ensure as much safety as possible.

But you'll find parallels in the commercial world.
Some design "just" to meet the rules..which is fine. Some go beyond the set of prescriptive rules and add features to enhance safety etc.
The issue is one of cost. The just pass the minimum cost less, inevitably. And owners like cheap(er) boats. As such most accidents you see/read do not have features that "would have been nice" - in hindsight. But are omitted owing to ... yup..cost implications. Not every client is willing to pay for that bit extra, when the bare minimum will do the same job.

Thus it comes down to...would you buy a Lada or an Audi?...both are cars both do the same job.
And yet .. one is quality build and ergo, costs more. The other is not.

Sounds like you're trying to perform a dynamic analysis.
Whilst being an admirable task, many have tried in the past, it has way way too many variables and way way too many..what if's...and becomes an analysis for the sake of analysis.
One will never be able to investigate all of them, as they by nature, unknown. The wave you decide to use for the knock down... in reality may be 20% more energy, or a greater amplitude for the same period etc etc than 'assumed; in the model.
All one can do...is mitigate as much as possible through quality design and build.

Last edited: Jun 17, 2023
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### Alan CattelliotSenior Member

Part design, part stability...

#edit# when a multihull is in capsized position, emergency hatches should open such as to open to a safe path, for people to reach a location that maximize their chance of survival

In this situation, what if the emergency hatch is 1m below water ? What will happen when you will broke the glass ? Are you 100% sure that your software gives the exact flottation plane when a multihull is inverted ? What is the effect of a certain wave spectrum when the boat is in this position ? Does your software manage also this ?

#additionnals# I fully understand your position, and would not necessarily reommend a naval architect to use a DSP code. But for engineers, like me, it is the opportunity to have a versatile numerical tank test, dynamic VPP and hydrostatics software all in one. Its use is also not limited to naval architecture, which is good, since 100% of my incomes do not come from boat projects. I would also recommend to hire anyone capable of using this kind of software, to lower the risk for people's life associated with capsized offshore multihulls.

Last edited: Jun 17, 2023
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Software is not a panacea and one should not conflate it with Design or an improvement, just because one has used it.
Software is just a tool..all it will help you do, is the task faster...it does not automatically equate to being "better".

It is all 'design'....otherwise the tail is wagging the dog!

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### sunSenior Member

After the ship is grounded, the stability of the hull is affected by a single grounding point, which can be derived from the basic knowledge of the ship principle. How multiple grounding points affect the stability of the hull is always a difficult problem, especially the change of the number and position of the grounding points will significantly affect the stability of the hull.
This requires a theoretical method. Many literatures use the original displacement to subtract the displacement after the stranding to obtain the grounding force. However, when the stranded ship is in a complex environment, it is impossible to quickly obtain the displacement after the hull is grounded. In addition, this method cannot obtain the influence of multiple grounding points on the hull structure.
I wonder if we can regard each seated point as a local point of the hull immersed in a special large density liquid, and the resultant force of all the seated points is a force applied to one point of the hull, and further analyze the influence of a single seated point on the stability of the hull.

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### Alan CattelliotSenior Member

This is what is underlined by classical theories. In fact, the whole Newton's mechanical Principles give formal mathematics to détermine this point of application. To my opinion, the difficulty lies in capturing geometries, volume and densities for this single point to bé compute. It's not an easy task. The scientists developping the DSP code that I use have even made errors doing so for complex shapes.
Not a trivial issue.

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Indeed it is not.
All one can do, is use ones experience and knowledge to provide a "best guess" and analysis the effects of the grounding(s).

Design is a very methodical process...one cannot introduce too many steps at once.
It becomes very very complex very very quickly.

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### sunSenior Member

Since it is difficult to solve problems only by relying on Newtonian mechanics, it may be necessary to take multiple measures. For example, on the basis of establishing a mechanical model, it is necessary to excavate the relationship between ship Statics parameters and find appropriate mathematical methods.

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