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  #31  
Old 03-21-2014
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Re: Limit of Positive Stability (LPS)

Quote:
Originally Posted by bobperry View Post
It's all about the location of ballast, VCG and hull shape It's as simple as that. Ballast to disp ratio does not tell you enough to arrive at any stability conclusions. If you have a B/D of 50% and that ballast is all in your bilge you may have a very tender boat. If you have a B/D of 35% and that ballast is all at the bottom of a deep fin you may have a very stiff boat.
refer to my previous post about the findings about changing the depth of the ballast. i will have to look for the quote, on that, to post. i don't know if i saved the link for that, or not.

anyhow, the ballast/displacement ration, as in most such numbers and ratings, is not an absolute. it's a guide. there are exceptions. however, it does give a very good idea about the stiffness of a boat. if you have a low ballast to displacement ratio, it means that there is a greater percentage of weight that is near or above the water line. as you noted, placement of weight is very important. weight not below the water line is a liability.

for instance, the B/D ratio of the average dinghy is much lower than that of the average keel boat. and, as that suggests, the average dinghy is far more tender than the average keel boat.

having ballast deep in the water does effect a boat's resistance to being heeled, but not as much as it might be assumed and at certain degrees of heel more than others.

stiffness usually means initial stability. if you have two boats with 35% of their weight as ballast and one with 50% of it's weight as ballast and you compare these boats in similar situations, you will see what i mean.

let's say that one of the 35% boats and the 50% boat have a common shallow fin and one of the 35% boats has a deep fin with a bulb.

stiffness, being initial stability, applys to the first....say 10 to 20 degrees of heel. after that, you are talking about ultimate stability. does she settle in and resist heeling forces or does she just keep on going over?

admitttedly, your deep finned 35% boat will tend to resist the heeling force of the wind to a greater degree than the other two, as it heels farther. that's because it has a longer righting arm.

however, at small degrees of heel ( where one refers to stiffness ) it doesn't have a longer righting arm than either of the other two boats because their ballast will be right below the CB. in that situation, the 50% boat will have a greater resistance to heeling because it will have more of it's weight below the CB.

now all of that is assuming that all three boats are the same in all aspects. if you change something, it will change things. ratios, like that, are kind of relative. regardless of the B/D differences of these three boats, if the 35% boat with the shallower fin was very beamy and flat bottomed, it would be stiffer than either of the other boats. that's form stability. however, that boat would have much less ultimate stability and would tend to stay upside down if it capsized. that would even be the case if the beamy 35% boat had a deep keel. the form stability would make it stiffer but it would also make it easier to capsize, once it reached a certain degree of heel, even though it's long fin would give it a longer righting arm. of course, that has to do with the shift of the CB as a boat heels.

all such ratios and numbers are useful in comparing the qualities of different boats but are only useful within a certain framework. there are more elements of boat design that effect the performance of the boat than just the single element covered by one of these numbers. that always has to be kept in mind. however, for the purpose of my posts, the B/D ratio is completely satisfactory in accurately making my point and it is a good reference to help understand the capabilities of a boat.

there is a tendency, i notice, for people to try to isolate one element of design....keel shape, rig type, etc....and say that any boat with that element will behave the same as any other boat with that same element, regardless of the other elements of vessel design that may be present. that kind of thinking is very misleading. as with anything else in the world, a sailboat is a culmination of ALL of it's parts and not just defined by one part.
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  #32  
Old 03-21-2014
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Re: Limit of Positive Stability (LPS)

this might be of interest:


"This (the test data presented in the chapter) suggests that alterations in form (of a sailboat) that improves capsize resistance may be rendered ineffective by a relatively small increase in breaking wave height."----Andrew Claughton in Heavy Weather Sailing 30th ed. p 21


further more, this should be kept in mind:The 1998 Sydney to Hobart Race Review Committee report, in reference to the tragic Sydney-Hobart Race, notes the following:

"There is no evidence that any particular style or design of boat fared better or worse in the conditions. The age of yacht, age of design, construction method, construction material, high or low stability, heavy or light displacement, or rig type were not determining factors. Whether or not a yacht was hit by an extreme wave was a matter of chance."
----Rob Mundle in Fatal Storm, Publisher's Afterward p 249. International Marine/McGraw-Hill Camden, Maine.
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Last edited by captain jack; 03-21-2014 at 12:56 PM.
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  #33  
Old 03-22-2014
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Re: Limit of Positive Stability (LPS)

Quote:
Originally Posted by captain jack View Post
your original premise is incorrect. the difference is not tender vs stiff. it is form stability vs ballast. there are two ways to achieve a stiff vessel. one is to have a wide beam. this is form stability. the extreme version of this is the catamaran.

the problem with this kind of stability is that the more of it you have, the more your boat will tend to be stable when upside down. also, wide beam creates a situation where a boat reaches a sudden tipping point. it's very stable, up to a certain degree of heel. however, after that degree, the righting arm rapidly decreases and even passes by the CB.

and, finally, a beamy boat that manages to survive the first beam wave is easier capsized by the following wave, than the boat with a narrower beam, because it gives more surface for the next wave to push on....like a lever for the wave to use. the narrower boat has less surface for the following wave to grip and will tend to allow the wave to pass over.

however, there is another kind of stability: stability through ballast. this does not rely on form. if your ballast is a higher percentage of the over all displacement, your boat will be stiffer.

the capsize formula is set up to favor the narrower boat to the broader boat. that's why it compares displacement to beam.

now, there is a ballast displacement formula that figures how stiff your boat will be based on how much of it's displacement is ballast. the more of the displacement that is ballast, the stiffer the boat: ie the more it can stand up to it's sails. 35% is average stiffness. my boat has around 2 on the capsize formula and 125 AVS but it alo has a displacement ballast number of 50%. that means that, although it has good ultimate stability, it is pretty stiff, too.
This is an amazing post. It makes a lot of very strong statements and almost none of them are true. Top to bottom, as defined within English speaking ship and boat design community, by definition tender and stiff only refer to form stability. The center of gravity of the boat (what you refer to as ballast stability) does not come into play in the definition of stiff or tender, at least as these terms are properly applied.

Neither is it necessarily true that a boat with a lot of form stability will automatically be any more prone to remain inverted or that it may be prone to suddenly lose stability and capsize. There certainly are cases where this is true, and there was a point in time when there were more of those designs out there than there seem to be today. But there are a lot more factors involved in determining the limit of positive stability and the shape of the righting moment curve than simply form stability, topsides and cabin configuration, and vertical center of gravity being two of them, and those factors can easily negate the general statement being quoted above.

The majority of modern research concludes that a beamy boat is actually less prone to capsize by the action of a wave than a narrow one, so much so, that the height of a "dangerous Wave" is defined as being "a breaking wave that is twice the beam of the boat"

And while it is true that a boat with a lot of form stability will tend more closely track the angle of the face of a wave, we now understand that to be a good thing from a capsize standpoint. A boat which does not track with the face of the wave due to having too much roll moment of inertia, inadequate dampening, or too little form stability, is likely to get out of sync with the wave train encouraging 'excitation' rolling and once a rail is dipped, a greater likelihood of a capsize.

The percentage of ballast is only a very small piece of the puzzle. Far more critical is its impact on the vertical center of gravity. Large ballast ratios located high in the boat as might be the case with a shoal keel, have far less impact on stability, than a smaller ballast ratio placed deep beneath the boat.

Both the Capsize Ratio, and the Motion Comfort Index have pretty much been discredited in that the neither contain the factors now known to be primary determinants in the likelihood of a capsize or motion comfort. In other word, they tell you nothing useful about your boat.

Respectfully,
Jeff
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  #34  
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Re: Limit of Positive Stability (LPS)

As always I find it highly entertaining when a forum member decides to "educate" folks like the great designer / NA Bob Perry or other NA's such as Jeff H.. Always fun to watch.....
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  #35  
Old 03-22-2014
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Re: Limit of Positive Stability (LPS)

Jeff knows his stuff and explains it very well. He has a knack for that. What does bother me though is that if I had responded to Paulo is such a manner as Jeff responded to Jack I would have been castigated and called names. So I take from this that it's OK to correct Jack but please do not try to correct Paulo. There is some hypocrisy there me thinks.

My attitude is this:
If we participate here we should all muck in and contribute so as a group we are in the constant learning mode. For me it's not about being right. It's about knowing what is right. I hate not knowing. I like to be corrected it it leads to a better understanding of the subject. Sometimes I find that in correcting an error and explaining my correction I actually learn more about the subject myself.

There was an interesting post yesterday attacking me that was unfortunately, very quickly deleted. It said, in one part, it's not what you say but how you say it that is important. That is the opposite attitude of mine. I don't care are how you/I say it but I do care what you/I say.
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  #36  
Old 03-22-2014
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Re: Limit of Positive Stability (LPS)

Quote:
Originally Posted by Jeff_H View Post
This is an amazing post. It makes a lot of very strong statements and almost none of them are true. Top to bottom, as defined within English speaking ship and boat design community, by definition tender and stiff only refer to form stability. The center of gravity of the boat (what you refer to as ballast stability) does not come into play in the definition of stiff or tender, at least as these terms are properly applied.
i only go by what i have read. i posted that because, based on what i read, i wouldn't have used tender and stiff to convey that idea. i would have used, for instance, form stability instead of stiff. you can have a boat with good ultimate stability without having a very tender boat....as i have heard most people use the word tender; that being to indicate a boat which heels very easily as soon as wind is in the sails.

from everything i have read, if you have two boats of the same hull shape, the one with more of it's weight below the waterline will have greater initial stability than the one with less of it's weight below the waterline. that was the basis of that post.


Quote:
Neither is it necessarily true that a boat with a lot of form stability will automatically be any more prone to remain inverted or that it may be prone to suddenly lose stability and capsize. There certainly are cases where this is true, and there was a point in time when there were more of those designs out there than there seem to be today. But there are a lot more factors involved in determining the limit of positive stability and the shape of the righting moment curve than simply form stability, topsides and cabin configuration, and vertical center of gravity being two of them, and those factors can easily negate the general statement being quoted above.

The majority of modern research concludes that a beamy boat is actually less prone to capsize by the action of a wave than a narrow one, so much so, that the height of a "dangerous Wave" is defined as being "a breaking wave that is twice the beam of the boat"

And while it is true that a boat with a lot of form stability will tend more closely track the angle of the face of a wave, we now understand that to be a good thing from a capsize standpoint. A boat which does not track with the face of the wave due to having too much roll moment of inertia, inadequate dampening, or too little form stability, is likely to get out of sync with the wave train encouraging 'excitation' rolling and once a rail is dipped, a greater likelihood of a capsize.

The percentage of ballast is only a very small piece of the puzzle. Far more critical is its impact on the vertical center of gravity. Large ballast ratios located high in the boat as might be the case with a shoal keel, have far less impact on stability, than a smaller ballast ratio placed deep beneath the boat.

Both the Capsize Ratio, and the Motion Comfort Index have pretty much been discredited in that the neither contain the factors now known to be primary determinants in the likelihood of a capsize or motion comfort. In other word, they tell you nothing useful about your boat.

Respectfully,
Jeff
i acknowledged, in a previous post, that there are a lot of different elements that effect stability. however, i have seen a lot of internet arguments, some of them pretty heated, on both sides of that. however, most things i have read about seaworthiness do not favor the modern, beamy cruisers. i have read reports of studies supporting that but none of the people arguing in favor of the seaworthiness of beamy boats have ever backed up their ideas with source information and i haven't seen information supporting those ideas.

i am always up for learning, though. ideas do change as new information is gained. do you have links or sources that you can post regarding this modern research? i think seaworthiness is of importance to every sailor. it is to me. i'd like to know all i can. everything i have read is from older sources. if there are new ideas, i'd like to read up on it. thanks.
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  #37  
Old 03-22-2014
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Re: Limit of Positive Stability (LPS)

Jack:
We are all learning here. Can't see a day coming when I won't learn something. I applaud you for diving head first into the deep subject of stability.
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  #38  
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Re: Limit of Positive Stability (LPS)

Quote:
Originally Posted by bobperry View Post
Jeff knows his stuff and explains it very well. He has a knack for that. What does bother me though is that if I had responded to Paulo is such a manner as Jeff responded to Jack I would have been castigated and called names. So I take from this that it's OK to correct Jack but please do not try to correct Paulo. There is some hypocrisy there me thinks.

My attitude is this:
If we participate here we should all muck in and contribute so as a group we are in the constant learning mode. For me it's not about being right. It's about knowing what is right. I hate not knowing. I like to be corrected it it leads to a better understanding of the subject. Sometimes I find that in correcting an error and explaining my correction I actually learn more about the subject myself.

There was an interesting post yesterday attacking me that was unfortunately, very quickly deleted. It said, in one part, it's not what you say but how you say it that is important. That is the opposite attitude of mine. I don't care are how you/I say it but I do care what you/I say.
most definately. i want to know. if i post something, it is based on what i have learned. however, that doesn't mean that the sources i have read are up to date or even correct. if there are other sources that are better or more up to date, i want to learn from them. you should consider all the facts and ideas when you form your own views.

i am familiar with Paulo and i don't want to be like that or come off that way. human understanding of the world around us is forever changing as new information becomes available. sharing information and expanding the knowledge of the group is a great benefit of the internet. there is much easier access to new ideas and information than there was previously.

of course, not all of those ideas or information are good. there are down sides to everything

in some respects, though, i do think how you come off is important; although not as important as what you have to say. if you come off as attacking or insulting, people tend to get defensive and put up walls against you. then, even though you may have valuable information to share, they won't listen to you.

i won't deny, it is easy to forget and have an emotion based defensive response to someone who has...less tact...when they correct your errors. i try to fight those natural responses. if a person just outright says, " that's BS" without anything backing that up, i think that their input is useless even if it is grounded on real facts. the way that Jeff ( i don't know him so i hope he doesn't mind if i use his actual name, as you did )initially stated i was wrong was the sort of response that makes some people defensive, but the body of his post was of an informative nature. he backed up his initial statement.

some people wouldn't have overlooked the feeling of that initial statement, i suppose, and would have missed the value of the post. but, if he knows something i don't, i want to learn it from him.

i'd rather ask for sources that support a person's ideas, so i can learn from them, than continue on in a misinformed manner.
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  #39  
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Re: Limit of Positive Stability (LPS)

Quote:
Originally Posted by Maine Sail View Post
As always I find it highly entertaining when a forum member decides to "educate" folks like the great designer / NA Bob Perry or other NA's such as Jeff H.. Always fun to watch.....
...........well.....i must admit, i didn't know they were experts....if i had, my post would have been questions, to understand, rather than statements, to 'educate'....don't i feel silly, now. however, it happens to everyone from time to time. foot in mouth disease is the most common human malady.
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Old 03-22-2014
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Re: Limit of Positive Stability (LPS)

"as defined within English speaking ship and boat design community, by definition tender and stiff only refer to form stability."
So, where do dynamic stability and inherent stability factor into form stability?

As best I can recall, tender and stiff have always been used to refer to the overall stability of a vessel, without regard to when or how the vessel was heeling. Have I just been reading/hearing too many sloppy authors?
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