Hull Speed

That's why I used a naval architecture example from a SNAME paper of the use of "Dynamic Stability". But perhaps Blount should be disregarded because he has a degree in mechanical engineering, not naval architecture.

 

You didn't upset me at all.

 

Perhaps my previous attempt to clarify my comment which started this was insufficient. I was refering to whether the vessel would stay upright and on the surface when travelling at high speed. That is a question of dynamic stability, not static stability which is described above.

 

You are refering to a different use of the term "dynamic stability" in naval architecture than that which for example Blount and Codega used. The work under the curve use is frequently used as a criteria for resistance of a vessel to capsize from a wave or a wind gust. It doesn't not consider the change in pressure distribution about a vessel due to dynamic pressure.

 

Terribly sorry, but this is just nonsense.

What you’re attempting to explain is simply the effect of the pressure distribution around a hulls surface whilst underway to explain how/why trim occurs. It has absolutely nothing to do with stability in the terms you mentioned. Only a non-naval architect would attempt such a connection, so I accept you are somewhat confused or misguided by trying to link events which are not related.

No, stability has a very clear definition. Your understanding of it will not change the known and accepted definitions used by naval architects nor change the definitions used in universities and text books just because you’re unclear of its meaning.

No, this is because you have not understood what is written in the paper not what is the difference between stability and dynamic stability when related to high-speed planning craft only. I shall refer to it here for everyone’s clarity:



As others may read for themselves, Blount is defining it as speed dependent and relating to dynamic instabilities. Nothing to do with it does it float and float up right as you cited. Floating is a hydrostatic condition, nothing to do with dynamic forces at all. Thus your understanding is unclear.

You did not state at high speed nor did you state only relating to planning hulls. Again, your understanding, from reading reference is clouding your understanding because you are unaware of the several forms of stability and dynamic stability definitions that are used in naval architecture. You’re not a naval architect thus I would not expect you to know them, nor just by reading a few references.

No he isn’t. You are!

 

This thread has got way off topic.

But I would suggest that a study of the ISO Stability and Buoyancy standards (in all three parts) would help understand the different types of stability and how to estimate them

Naval architects get very confused when the term "dynamic stability" is not used in its proper context, which is when discussing the area under a static stability curve, or the work done in heeling a vessel to a specific angle. It has nothing to do with forward motion. So static stability is never "real". it can be likened to the load on a crane when heeling a boat to a certain angle. However it does give a good indicator of the probable behavior at sea and can be used to compare different designs

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

But this what comes in mind when talking about 'dynamic stability', it is used in rulebooks. To add, classic definition of static stability is that inclination is considered slow thus 'quasi static'. In contrast, dynamic stability can be everything that is not static.

 

What is nonsense, that the pressure distibution around a vessel underway differs from hydrostatic? Or that it's possible for the hydrodynamics to be such that an intial small disturbance is amplified and may grow very large?

Wrong. Read what I wrote. Trim is an example of hydrodynamic forces affecting the attitude of a vessel but generally a relatively benign one. Another example which is not benign is a vessel which can heel abruptly to a large heel angle when traveling above a certain speed. This behaviour has been described multiple times. Other types of undesirable behaviour have also been described.

"Stability" by itself has a clear meaning in naval architecture when taken as the short form of "static stability". "Dynamic stability" has several meanings as previously discussed, and each is different than "static stability". "Dynamic" is used together with "stabilty" as "Dynamic stabilty" to differentiate it from "static stability".

Agree it is speed dependent. That's what I've said.

As I clarified earlier my post which started this was intended to raise the issue that some vessels when traveling at sufficiently high speeds do not always stay upright. What term would you suggest for that issue?

I clarified that I was refering to high speed. I understand the intial confusion about what I meant.

When you say "only relating to planning hulls." do you mean that the effects of dynamic pressure only are relevant to certain types of hull forms. If so I disagree.

Unfortunately in your zeal to demonstrate that I don't know what I'm talking about because I'm not a naval architect (though I do have a degree in naval architecture) you continue to ignore that clarification. Is ignoring the clarification deliberate?

I disagree.

 

This is nothing to do with trim. You’re diverging off definition for some unknown reason. You made a stamenet about "does it float...and up right".

You clarified because the terms you used did not describe what you either thought it did, or wanted to convey correctly.

Not at all…your definitions are incorrect. You use broad brush statements, which actually have very defined meanings and when picked up by others, you then clarify, which is fine. But you attempt to maintain your initial assertion was correct when it is the definitions you use are incorrect (or your understanding) without clarification to begin with.

When a vessel floats and up right, is just hydrostatics. Yet you later clarified you were referring to dynamic conditions, fine, but and again later to planning hulls. These ‘later’ definitions are very far removed from simple hydrostatics of “does it float”.

Communication is all about saying what you mean, otherwise you won’t mean what you say. Hence the endless clarification statements. If you stated from the outset, what you clarified at the end, there would not be this constant interlocutor.

 

I deliberately did not use the word "float". Please do not claim that I used words which I did not use.

Always interesting to hear someone else tell me that they know what I was thinking and I don't.

I never asked "does it float". Unfortunately you read that into what I said and used that as the basis for what I said.

Why didn't you drop it once I clarified what I meant?

 

Correct you did not use the word “float”.

So other than floating how else will a vessel deliberately stay upright and on the surface if it is not floating???

Because you digressed into different definitions, differing from the implication of hydrostatics (floating) into dynamic forces on planing craft, not related to the initial one as justification, like trim etc, for some reason? And you kept referring to it, again, unsure why. It was clear afterwards your initial comment meant something else, but you raised other aspects as if in justification?? You were mixing up definitions...and each out of context from the previous.

 

Apparently we're talking past each other. I could speculate on the reasons but won't.

 

HEY ROBERT (OP), are you going to respond to this?

-Tom

 

Robert, this is your thread, will you come back? This forum needs new ideas, some of them can lead us to unexplored fields, no matter how strange they could seem.

 

I think it is possible to treat static stability as dynamic stability where speed=zero. It may help some understand how, as the boat speeds up, the ratio of forces on the hull change.