[kevlarpirate]

Jeff, the illustration I gave about the eating dinner down below
was so no one would know what the speed of either of the the boats were
so that knowledge of speed would not affect their decision as to which boat was more comfortable. The remaining inputs would be substantially reduced to what extent the dishes are sliding around and how often your shoulders were bumping the guy next to you.

As for mass, it is the biggest single factor in reducing impulse because F=ma. Impulse is defined as a very quick acceleration.

We assume mass is distributed fairly equally between different boats.
So that if one boat had a heavier hull, keel, it would also have a heavier mast and a heavier deck so those proportions would scale in a linearly between the boats.

As for a definition of what comfort really is, that is subjective.
If you know that you are going to be out there for twice as long as anyone else, well I would say that in itself is a discomfort, mental not necessarily physical.

As a note, IMO, I don't think Paulo really cares about comfort. I think he wants speed. Perhaps he wants it all.
My point is that he can't have it all. Any manufacturer of a sport boat
who adds 300 lbs of interior and calls it a cruising version should be spanked.

Yes , you can cruise in it , but to dupe someone into thinking it will be comfortable only means that that someone has never cruised any where.

Boats going upwind will be in the displacement mode, when you crack off
to 60 BAW, a sport boat will be noticibly faster

The single most important ability of a sailboat is it's ability to sail upwind
in strong winds and a developed sea without getting beat up. This is my opinion and also the opinion of many well seasoned designers. I have been in many situations where this ability became very important to the safety
of boat and crew.

Many sailors would rather sail off the wind , that's fine , we all have our preferences

kev

 

[sailingdog]

Kevlar-

No, I don't think you can assume anything about mass distribution For instance, if you take two boats of exactly the same MASS, LOA, Draft, etc, but one is built using solid laminate and the other is built using Divinylcell cored laminate, then the keel could be significantly heavier on the first boat than the second, while having a much lighter, yet stiffer hull, deck and such...

Jeff, the illustration I gave about the eating dinner down below
was so no one would know what the speed of either of the the boats were
so that knowledge of speed would not affect their decision as to which boat was more comfortable. The remaining inputs would be substantially reduced to what extent the dishes are sliding around and how often your shoulders were bumping the guy next to you.

As for mass, it is the biggest single factor in reducing impulse because F=ma. Impulse is defined as a very quick acceleration.

We assume mass is distributed fairly equally between different boats.
So that if one boat had a heavier hull, keel, it would also have a heavier mast and a heavier deck so those proportions would scale in a linearly between the boats.

As for a definition of what comfort really is, that is subjective.
If you know that you are going to be out there for twice as long as anyone else, well I would say that in itself is a discomfort, mental not necessarily physical.

As a note, IMO, I don't think Paulo really cares about comfort. I think he wants speed. Perhaps he wants it all.
My point is that he can't have it all. Any manufacturer of a sport boat
who adds 300 lbs of interior and calls it a cruising version should be spanked.

Yes , you can cruise in it , but to dupe someone into thinking it will be comfortable only means that that someone has never cruised any where.

Boats going upwind will be in the displacement mode, when you crack off
to 60 BAW, a sport boat will be noticibly faster

The single most important ability of a sailboat is it's ability to sail upwind
in strong winds and a developed sea without getting beat up. This is my opinion and also the opinion of many well seasoned designers. I have been in many situations where this ability became very important to the safety
of boat and crew.

Many sailors would rather sail off the wind , that's fine , we all have our preferences

kev

 

[kevlarpirate]

Motion behavior (as measured by accelerometers) will show us as we lower the mass, higher accelerations will result, this is both in translation and rotation.

So if you use the lighter materials as you say, accelerations will go up. for instance a carbon fiber mast will produce higher accelerations to the boat both roll and pitch.

I assume you meant you could have a heavier keel in the second boat , not the first (to keep total weight the same) ?

So are we talking comfort here or are we talking ultimate static stability?

If we are talking comfort , which would be defined as low accelerations as versus higher accelerations then the boat with the lower mass and moments of inertia (the cored boat you describe) will have higher accelerations and therefore lower comfort.

My point was that these boats have densities fairly close to each other, and therefore will behave in similar fashion
The use of words (much lighter and significantly heavier) are really in the range of a few tens of percent, and when those individual components are applied to the entire system, the desired results are less than may be expected.

If we move this conversation into the subject of ultimate static stability, we agree it is a simple experiment to show a lower CG produces a better static curve, however in the big ocean, the world of dynamics now come into play ,specifically moment of inertia.

reference: Relation Between Torque and Angular Acceleration
scroll to the bottom and see that moment of inertia for a thin rod (mast) goes by length squared, L, and for a cylinder (hull) it increases by radius squared, r

tank testing shows the importance of dynamics:

" The VCG can be lowered by a longer keel or by having more ballast (weight of the keel) at the end of the keel. However, according to Adlard Coles' "Heavy Weather Sailing" thirtieth anniversary edition, "The effects of large movements of the VCG on the propensity to capsize was surprising small".

Manufacturers claims and explanations cannot get around the laws of physics, therefore any and all claims should be carefully evaluated.

Newtonian physics has served us now for 400 years and will continue serving us

 

[PCP]

Kevlarpirate, it is very difficult to have a meaningful discussion with you. Besides the arrogance you seem to put very little discipline and methodology in the way you think. You think more like a pirate and less like a scientist .

As an example look at this:

I say on post 67: "In what regards motion and speed, the importance of the LWL is…noticeable."

On post 69 you imply (disagreeably) that I don't know of what I am talking about.

On post 76 you agree with what I was saying on post 67: " I am nor disagreeing that bigger boats are not faster and more comfortable".

It is not possible to discuss anything this way .

Five years ago, on this forum I have posted a thread about this subject (seaworthiness, boat design and comfort) and as you can see, I was saying about the same things you are saying now.

Of course in 5 years I have learned a lot more and even if what I was saying is basically correct I have found that there were other points to consider, some technical others that have to do with living in the real world. People have a certain amount of money (not much, unfortunately) and they want to buy the more comfortable, seaworthy and fast boat they can get, and that is what really matters to the vast majority of people.

I suspected that I was basically wrong on the way I was seeing things because the best world Architects were not designing (not even for their personal use) the kind of boats I thought they should be designing and, because I don't have your Ego, I started to suspect that I was not understanding something. Since then I have a much better understanding of what they are doing and why and that has permitted me to have a much better understanding of boat design, the advantages and disadvantages of each type, even in what motion comfort is relevant.

A quote of some of the things I have said on that thread:

"....And about sailing, there are many differences in the kind of sailing (traveling). There are the ones that want to go as fast as possible with a full crew, others want good speed but a boat that can be easily solo sailed, others want maximum comfort in a seaway others an optimized safety, for the size of the boat. There are a lot of compromises to be made (in hull shape and rig), originating completely different boats, depending on the assumed different priorities…..

It seems to me that you think that weight (mass), besides the one needed to give the boat stability) is always a bad factor in a sail boat.Although I agree that mass is always a bad factor in a racer or even in a cruiser-racer, it is not (in my opinion) in a pure cruising boat with priorities aimed to have an easy motion, maximum safety and lots of autonomy.

…. I believe older boats, even old racing boats, are normally "less radical" than most of modern boats and have a smoother sea movement. They have a less flat hull and have more ballast, depending less on form stability……"

http://www.sailnet.com/forums/general-discussion-sailing-related/11338-how-heavy-too-heavy-ii.html

Regarding this thread and your posts, instead of discussing each point at a time you make a lot of claims and statements as if you knew everything about this topic and other opinions should not even be considered. I would say that, if not for Jeff (he asked me), I would not be around this discussion. I like to learn and I have learnt a lot in the last years, but I doubt I can learn something that way.

Let's see if I can answer some of those somewhat disconnected statements that you have made:

…, just trying to save Paulo some headaches and money.
Anyway, it sounds like Paulo knows which way he would like to go, ... …..As a note, IMO, I don't think Paulo really cares about comfort. I think he wants speed. Perhaps he wants it all. My point is that he can't have it all.

Any manufacturer of a sport boat who adds 300 lbs of interior and calls it a cruising version should be spanked.Yes , you can cruise in it , but to dupe someone into thinking it will be comfortable only means that that someone has never cruised any where.
kev

It seems that you know more than myself, regarding what I want . Fact is that I am quite sure that I can live happily with different types of boats (each has its own advantages and disadvantages) from broad sterns to canoe type boats (small beam). It all depends on what appears on the market at the (very) right price.

What I really know is what I don't want and that is an old designed boat. I want a modern design, a fast seaworthy boat, adapted to solo sailing with an interior suited to cruising and a boat that is fun and enjoyable to sail. I also want a boat that can go better than my older one against the wind on short period waves.

Comfort is one of the variables. Certainly I would not have a slow boat or a boat that is not agreeable to sail, but to be more or less comfortable is a question of tradeoffs. What do I lose on comfort, what do I gain on seaworthiness or speed? Each (kind of) boat is a case, to be studied separately, as the occasion arises and anyway, this is not relevant to this thread.

…,
If he wants a flat boat, he will most likely have a bad "computer generated" ahem.... static curve to sleep with. That's his choosing, sweet dreams
If he wants high tech thin materials , he will have a noisy interior,
and be serenaded by slapping waves against his hull all night long while tucked into one of those nice coves there in the Med.
kev

I am pretty sure that I know more than you about stability curves, that, as Bob Perry has said (or any other boat designer will say), are the tool that you use to access boat stability, while designing it.

You seem to have a comprehension of what is the goal in designing a certain type of stability curve, but wrongly assumed that a certain type of Stability curve is the model from all other types of sailboats. Different sailing boat types have different kinds of stability curves that show the advantages and shortcomings of each type of boat, in what regards stability. In the last years I have looked over many different boat stability curves, trying to understand the options the boat designers have intended in designing a boat with that kind of stability.

You are not going to learn much if you assume that there is a correct type of stability curve and that all the others are wrong.

About hi-tech materials and the noisy interior while at anchor, you are wrong.
Old boats (and some modern ones) are built with a single skin (solid laminate) while hi-tech boats used a double skin with a core at the center. The double cored skin will insulate the boat from sound and temperature far better than the solid laminate, no matter its thickness.

…,
If he doesn't mind having to ride 6 feet above the water on the high side rail to steer..If he doesn't mind having to steer perfectly in the slot 100% of the time and be constantly playing the main, that's great, good exercise too.
by the way, heel is a good point , and to that, I have a variety of headsails and typically use smaller ones and never go over much over 20 degrees…..
kev

That is a very confusing statement. You mean that if in your boat you can go safely faster but have a heel superior to 20 degrees you would change for a smaller headsail, going slower? I am not that kind of sailor and I don't think you are either.

Different boats are designed to sail better and faster at a determinate angle of heel and almost all racing boats (older and new) and all narrow beam boats are designed to sail better (close to the wind) around 25 or 30 degrees of heel. My boat worked that way and yours too.

The only boats that are designed to sail with not much heel are multihulls and modern boats based on the design of Open racing boats, boats with a big stern and two rudders. These boats are designed to perform between 10 and 18 degrees of heel. You are not going faster if you push the boat and can even go slower.

If this kind of boats heels very little, how can I go 6ft over the water?
And if this kind of boats are designed for max stability and solo sailing why do you say that I would have to "to steer perfectly in the slot 100% of the time and be constantly playing the main"? That is quite the opposite, these are the most forgiving boats, boats that are made to go on autopilot on bad weather while their solo sailor is sleeping or resting in the interior.

… The single most important ability of a sailboat is it's ability to sail upwind in strong winds and a developed sea without getting beat up. …
Many sailors would rather sail off the wind , that's fine , we all have our preferences kev

Certainly a sailboat should have the ability to go upwind but to make that its most important ability, I disagree. Even in what concerns racing boats, particularly Open formula boats where you have the freedom to design more or less beamier boats, the boats designed for Open races are big transom boats, boats that can go well upwind but that are optimized to sail downwind.

That means that even on circumnavigation races going close upwind is far from the prevalent conditions. That is a lot more relevant if we consider cruising. Cruisers only go against bad upwind conditions if they cannot chose otherwise and in 99% of the cases they can and wait for the right wind.

Englishmen have a say: "Gentlemen don't go upwind".

I would not be so adamant, but going close to the wind in bad weather certainly is not the first choice for cruisers.

Of course, I agree that, for safety reasons, a boat should have the ability to "sail upwind in strong winds and a developed sea" but making a priority into being able to do that and moreover in a relative comfort, it seems a vast exaggeration. Anyway your term "beat up" is imprecise. If it means what I think (pounding?).

Most modern big production sailboats have that ability, depending on the wing angle. A boat optimized for those conditions can go at 30 degrees or slightly less and a typical mass production cruiser can go at 45 degrees.
Optimizing a cruising boat on the ability to go upwind makes no sense to me and I would say, no sense to the boat builders and naval architects and that's why you don't find them on the market, unless they are cruiser racers and therefore intended to race upwind.
….

As for a definition of what comfort really is, that is subjective. ...My Ericson 46 is very observably more comfortable than the SC50.
…but again, if you were to interview crew after the SC sailed a circle around us I would be willing to bet the SC crew never even considered discomfort as an issue.
kev

Well, it seems that I am not the only one that considers the relative comfort as a tradeoff and not an absolute item . And if you will let those guys chose between both boats for going on extensive cruising (providing both have similar interiors) I believe most of them would chose the fast and more modern boat.

I agree with you (finally ) on the subjectivity of the definition of comfort and what I have said in the beginning of this thread seems to go accordingly:

"The tastes of different sailors about the kind of sea motion they prefer, varies a lot. What is comfortable to you can be just boring and devoid of any sailing pleasure to another cruising sailor.

The type of comfort motion you like is a very personal thing, and varies as much as the different kind of cruising boats that are suitable to do Ocean passages, from the relatively heavy boats that you favor to the fast cruising racers, or Open type boats

I have owned a heavy displacement boat and I have owned a light displacement boat. I do prefer the motion of the light boat, not in all situations but on almost all of them. You have seen that on this thread I am not the only one that thinks that way. That doesn't mean I am right. There is not right or wrong here. Sailors should try both kinds of boats to see the difference in type of motion and to make a personal choice."

Saying otherwise, and going with your mass theory as the absolute panacea for comfort, would be implying that multihulls are less comfortable than monohulls and that seems a quite extraordinary statement considering the huge growth in popularity of monohulls among cruisers.

Ted Brewer, the one that created that comfort ratio formula also agrees with you on the subjectivity of comfort (and sea motion). He says:

"Nor will one human stomach keep down what another stomach will handle with relish, or with mustard and pickles for that matter!

It is all relative."

Regards

Paulo

 

[seabreeze_97]

I have to ask. Any of you ever seen an old-style metronome? It has this metal rod, weighted at the bottom. It also has a weight on the rod and it slides the length of the rod. The rod wiggles back-and-forth rapidly if the weight is low on the rod, but as you move that weight up, the rod moves more slowly. Hmmmm. Isn't it neat how that so neatly parallels a sailboat? Maybe you should re-think that whole 500lb mast example Jeff.

 

[PCP]

I have to ask. Any of you ever seen an old-style metronome? It has this metal rod, weighted at the bottom. It also has a weight on the rod and it slides the length of the rod. The rod wiggles back-and-forth rapidly if the weight is low on the rod, but as you move that weight up, the rod moves more slowly. Hmmmm. Isn't it neat how that so neatly parallels a sailboat? Maybe you should re-think that whole 500lb mast example Jeff.

That's a good example. I have found one on line that is similar to a boat with a bulbed keel. You cam make it longer or shorter

FREE METRONOME SOFTWARE - Best Metronome .com

But you have to consider that you have two weights. One on the top, other on the bottom, like a boat with a fin bulbed keel. You are not only putting the weight up but you are also increasing the lenght of the keel.

Regards

Paulo

 

[kevlarpirate]

Paulo, I am not here to converse with you . These long winded exchanges are not productive. Anyone reading my statements has the right to do their own due diligence to and I suggest they do.

I am here only to:
debunk any statements which do not follow laws of physics
debunk false marketing claims.
debunk false opinions

To support my positions I reference facts, statistics and personal experience.
Anyone who has questions can ask for further explanation.
It has nothing to do with ego, just knowledge.

I read your very long post and several conclusions you made were exactly the opposite of what I stated. I will not comment as to how you may have drawn the wrong conclusion.
One thing I will clarify is in reference to interior noise is that created by flat “zero dead rise transoms slapping and slamming all night long at anchor. It is more a design issue , and as for cored vs: solid hulls and noise,
It is a thickness issue. However, high modulus materials have higher acoustic velocities , no magic there, and the thinner they are , the more sound transmitted.

My posts are all consistent, there is absolutely nothing confusing about my statements , they are can all backed by references, and vetted by any and all readers willing to do some researching of their own.

Good luck with your future boat

 

[kevlarpirate]

I have just come across some very well researched reading
Technical Articles

the main page has been taken down , but the individuals are still available.
still looking but the author may be John Holtrop.

judging by his style and terminology he is obviously technically well versed.
He uses the correct words in the proper flow.

I strongly suggest reading "Dynamic Stability" to the letter and making a hard copy in case the page gets taken down. Pay particular attention to moment of inertia varies as radius squared , not just radius. this is why moment of inertia
is dominant over CG by lengthening the "roll over" response time.

 

[Jeff_H]

Sorry to have been missing in action here. I am also sorry but this is a long one.

Let me see if I can address some of the points which have been raised so far. To begin with, I thought that the technical articles posted by Kev were interesting, but in many ways struck me as an oversimplification. That said, sometimes simplification is necessary to get key points across.

Some things that we do appear to agree on, is that static stability in and of itself only tells part of the story, and frankly a pretty small part when it comes to motion comfort or predicting whether a particular boat will be likely to capsize. I think that we agree that dynamic stability is a critical predictor of the likelihood of a capsize and of the motion comfort of the boat.

I think we all agree that mass is a major component in the behavior of a boat, but we still seem to have a bit of a disagreement in sole or major significance. I think that we all agree with the idea that in terms of motion comfort that it is the range of motion (wider angles or larger amounts of movement), and change in the rate of speed of that motion (the accelleration and deaccelleration of a boat through all ranges of motion) that really dictates the motion comfort of any specific boat.

From a physics standpoint, I don't think any of us disagree with the previously posted equation A= F/m. And so I believe that we all agree that mass plays an important role here.

But where we seem to still have a gulf is the issue of how much of a role mitigating the accellerating- deaccelelrating forces play and and the signifigance of hull form and weight distribution on the amount of force that an individual boat encounters. And it is here that I think that Paulo and I are in agreement, that through careful decisions about weight distribution and hull form the forces experienced by the boat can be reduced so that a boat of lesser mass may still have a better motion characteristics. This is not to say all lighter weight boats will have a better motion, just that it is possible for a properly designed boat to have better seakeeping and seakindliness behavior than a heavier boat.

I still think that making the case will still come down to the physics so I would like to take a specific case, which is the one that Seabreeze 97 refers to, which has to do with the impact of an increased moment of inertia on the speed of harmonic motion. I would think that we are all in agreemeent that as the weight on a metronome is moved away from the access of rotation, the speed of rotation will be slower for a given fixed input force (The spring on the escapement mechanism). This in effect is a simple pendulum turned upside down.

If we apply this principle to motion comfort, we can look at two cases and see how the placement of the weight impacts the rotational motion of the boat. Lets look at two boats with equal height masts and equal depth and profile keels, and equal hull shapes. But one of these boats is constructed conventionally and the other with higher tech construction techniques. For the sake of discussion let us assume that that the low tech boat has the same 40 mast as the high tech boat, and for the sake of discussion, we put a 100 lb weight at the top of its mast. That weight would incease the roll moment of interia by 160,000 pound-feet squared (100 lbs x 40 ft x 40ft) and as Seabreeze points out, reduce the roll rate of the boat. The other boat does not have the weight at the top of the mast but neigher does it have the teak decks, or heavy teak interior, or a hull liner, or marble counter tops. Instead it it has a 3,300 lb bulb in the bottom of its keel, the center of gravity of that bulb being 7 feet below the roll center of that boat. In this case we have 3,300lb x 7 ft x 7 ft or an increase in roll moment of inertia of 161,700 pound-feet squared, which is similar to the increase in the moment of inertia for the 100 lb weight at the mast head.

I think that we would all agree that both should have a slower harmonic motion than an equal weight boat that did not have either the weight at the mast or bulb at the bottom of the keel. But despite the fact that these boats have equal roll moments of inertia, I suggest that the behavior of the boat with the weight at its masthead as compared to the boat with the bulb will be extremely different in a seaway.

If we look at mechanics of a boat lying beam to the waves and rolling in steep seaway, 1. there will be a rotational moment that is generated by the change in angle of plane of the wave face acting against the form stability of the boat, and 2. another rotational moment that comes from sheer of the surface of the wave acting on the underbody of the boat that results from the difference in the speed of the water at the surface which is moving faster relative to the water deeper in the wave, and 3. a third rotational moment that comes from gravity acting on the boat causing it to want to slide down the inclined plane of the wave and but that slide is being resisted by the keel creating a moment between the center of actual resistance and the center of the momentum of the boat.

In our example of the two boats, since they have the same hull, weight and underbody profile, it would suggest that they will feel a similar roll inducing moment due to moment #1 and moment #2.

But when we look at roll inducing moment #3, in this case the boat with the weight at the top of its mast wil have a higher vertical center of gravity and so the lever arm between the force couple (lateral resistance and center of gravity) will be longer creating a greater rotational force, and since both boats have an equal roll moment of inertia, there will be a faster roll rate for the boat with the mast weight than the keel weight.

But also, if we continue to look at the rotational moments acting on the boats, in the case of the boat with the keel weight, the weight of the keel is acting in couple with the center of buoyancy creating a moment trying to right the boat vs. the weight at the top of the mast, which is also creating a couple with the center of buoyancy but one trying to overturn the boat, and so with equal inertia would tend to further increase the roll rate of the boat with mast weight.

If we think then of these two boats sliding down the face of the wave an into the trough, when they hit the bottom of the wave, the boats being of equal weight and profile will hit the bottom of the wave moving at the same lateral velocity, but the boat with the mast weight will hit with a greater rotational speed.

If we look at the forces felt by the crew on the boat, we would need to look at the rate of deacceleration. As I assume that we would agree upon, the force of deaccelleration will be proportionate to the change in speed, and the distance/time over, which that change of speed occurs. To some extent we do not have enough information, but in a general sense, boats in principle would experience an equal form stability righting moment at the bottom of the wave if they were at an equal heel angle.

But since the boat with the mast weight has a greater rotational speed, it will actually have a greater heel angle at the bottom of the wave. In theory this greater heel angle would be result in a higher rate of side motion since the keel will have rotated out of the water flow but we can ignore that for a moment.

Since the boat with the mast weight has a greater rotational speed at the trough and it will need experience a greater change in speed as it flattens out to begin its climb. That greater speed will create a greater momentum. Assuming an equal form drive righting moment for both boats, this greater momentum relative to righting moment will result in the boat with the mast weight having a higher heel angle at the time that the boat stops moving in the trough and begins rotate back to level. This greater momentum would also generate a greater impact force which would be felt by the crew as well. In other words the boat with the mast weight would likely experience a less comfortable motion having some mix of greater impact force and a larger roll angle experienced by the crew.

By the same token, looking at likelihood of a capsize, as has been discussed earlier, as boats heel there is a point of maximum righting moment atfer which righting moment decreases. The amount of that maximum righting moment and the speed at which it decreases is mostly dependant on the hull/cabin shape and weight distribution.

Continuing our example, the boat with the mast weight will be at a higher angle of heel when it impacts the trough of the wave. it will also have a reduced limit of positive stability that would result from having a higher vertical center of gravity as compared to the boat with the keel weight. So you have a boat that is rotating at a higher speed, hitting the bottom of the wave at a larger heel angle, that has a smaller LPS, and a more rapid loss in righting moment as it approaches its LPS.

I would think that we would all agree that assuming the two boats in our example have equal moments of inertia, the boat with the mast weight would be more likely to capsize than the boat with the keel weight.

I would think that this example provides one small case where weight distribution can be shown to be a significant determinant in both motion comfort and the likelihood of capsize.

Getting back to the original topic, I hope that we can agree that at least in this case, since the Capsize Screen Formula and the Motion Comfort Index lack any information about the weight distribution of these boats, it would seem to provide no useful information about which of these two boats would be more comfortable or less likely to capsize.

And assuming that we agree on the physics of the above example, I would assume that this satisfactorily explains the physics of the example that I cited earlier regarding placing a weight on the mast of a boat relative to these two dangerously useless formulas.

Respectfully,
Jeff

 

[kevlarpirate]

Jeff. These issues are not that convoluted

One issue I see is perhaps you are not treating statics and dynamics as separate problems , you seem to be comingeling them in one unduly complex thought.
Now don’t misunderstand me, What I am saying is that we need to analyze them individually. then combine later.

The static curve is a trivial concept . Everyone reading this can very easily find articles which describe it thoroughly. And of course , we want to have a high LPS. and minimum P/S ratios. The table of LPS minimums for all boat lengths to qualify for offshore racing categories are also readily available.

Dealing with moment of inertia , the way you need to think of it is in the transient sense.
In the case of a capsizing wave , that event is over in a just few seconds.
Refer to my prior post for that link. the section on dynamics It makes it very clear.

Also, the rotational acceleration rate is a function of the systems “total”moment of inertia, therefore it does not matter where the individual moments comes from.

Let me illustrate. Think of a holding a wagon wheel with your hand and your arm is out stretched horizontally Then rotate the wheel forward and reverse.
The torque needed to overcome the moment of inertia is independent of whether
There are 3 heavier spokes or 10 lighter weight spokes, any number of spokes for that matter , as long as the individual moments combine to equal the total moment of the original system.

When a wave rotates your boat, think of it as a two spoke wheel,
The next step is to write a mathematical expression to describe that. , ( you don’t have to, it has been done a long time ago)


Probably the best point I can make is that all this work has been done and is very well referenced and explained. It is there for free on many websites and in books. I have pointed out several.

The book; Desirable and Undesirable Characteristics for Offshore Yachts
Is very thorough and easy to follow. Start around page 70.


One last note , you continue to stress CG position. We all want our CG’s to be low
But the point is , and this has been proven in the real world and tank testing that
substantial vertical movements in vertical position of CG do not effect dynamic behavior that much.. The most substantial change we can make is removing the mast, and we find rollover frequency increases, not decreases.

Counter intuitive , don’t you agree? Anyway , those old boys actually did know what they were talking about with the capsize screening ratios.
It's in the physics

kev

 

[kevlarpirate]

ignore min P/S ratio . we want minimum inverted area (vs: positive area)

 

[Jeff_H]

Kev,

As I see it, we have agreed that we would discuss the physics of motion comfort and likelihood of capsize as a way of exploring the original topic. Like any other discussion based in science on the behavior of a vessel under way, the only way to fairly analyze what is happening is to break the forces into their various components and from those component parts build "the wheel" that explains how a given boat is likely to behave in a given condition.

I also suggest that there is no way to comprehensively discuss the physics behind the behavior of a boat without looking at both the dynamic and static forces at play.

For example, we cannot discuss something seemingly as simple as drag without considering the dynamic forces (wave making, induced drag due to tip vortices, inter-planar turbulence, surface wave affects and so on), as well as, the more static conditions such as frictional drag.

I do not see how we can have a meaningful discussion of the mechanics of motion and capsize with breaking various components that impact motion or capsize screening into its basic parts. Even the model testing that suggested that a boat is more likely to capsize without its mast is understandable when broken into its various parts. But it is also important to look at the full range of testing. If you look at the plots in the test repost, starting with no mast and adding rigging weight initially reduced the likelihood of a capsize, but at some point adding mast weight actually increases the likelihood of a capsize.

Just for the record, I am very familiar with "Desirable and Undesirable Characteristics for Offshore Yachts". It does a very nice job of explaning the thinking of that era. But even John Rousmaniere, has told me and posted here in prior discussions that this was the best thinking of that era, but that we have a much better understanding of these issues than we had when Desirable and Undesirable Characteristics for Offshore Yachts was published nearly 30 years ago, and that modern designer have learned the lessons of that era and have designed boats that have responded well to those lessons.

Similarly, in conversations that I have had a SNAME and American Yacht and Small boat symposia, some of the folks who worked on the Capsize Screen Formula have have told me that in the wake of the Fastnet, the capsize screen formula was written quickly in order to get into place some kind of rule of thumb about the likelihood of a capsize. It was meant as a quick first screening based on the types of boats that existed at the time.

Most yacht designers that I have spoken to about the CSF and MCI dismiss them as being a very poor indicators of how a yacht will perform in rough conditions and useless in evaluating designs that have taken the lessons of the Post Fastnet era and applied them to newer designs. Even Brewer has said he developed the MCI as a tongue in cheek index and was surprised that it caught on.

But of course, those kinds of statements do not address the physics of the situation any more than "those old boys actually did know what they were talking about with the capsize screening ratios."

Respectfully,
Jeff

 

[cormeum]

But when we look at roll inducing moment #3, in this case the boat with the weight at the top of its mast wil have a higher vertical center of gravity and so the lever arm between the force couple (lateral resistance and center of gravity) will be longer creating a greater rotational force, and since both boats have an equal roll moment of inertia, there will be a faster roll rate for the boat with the mast weight than the keel weight.

The amplitude would be greater but the rate would be slower. The moments of inertia are resistive to changes in motion. So a high I(r) would reduce the rotational acceleration along the vector of the force applied. You are correct in assuming that having a weight at the end of the mast would induce a greater F(g) but that would only go as the sine of the angle between the mast and g. For a same total I(r) the acceleration is the same along an axis but the amplitudes will be different in the situation you describe since the contribution of the weight at the top of the mast is above the axis of rotation and so adds to the force (g) displacing the object from vertical.
In a single impulse situation (t of the applied force is small compared to the oscillation period), a higher I(r) will induce less amplitude and a slower acceleration than low I(r)- all other things being equal.

 

[Jeff_H]

The amplitude would be greater but the rate would be slower. The moments of inertia are resistive to changes in motion. So a high I(r) would reduce the rotational acceleration along the vector of the force applied.

You are correct in assuming that having a weight at the end of the mast would induce a greater F(g) but that would only go as the sine of the angle between the mast and g. For a same total I(r) the acceleration is the same along an axis but the amplitudes will be different in the situation you describe since the contribution of the weight at the top of the mast is above the axis of rotation and so adds to the force (g) displacing the object from vertical.
In a single impulse situation (t of the applied force is small compared to the oscillation period), a higher I(r) will induce less amplitude and a slower acceleration than low I(r)- all other things being equal.

I think that we are basically in agreement. I am not sure whether you had noticed that in the example, both boats had a roughly equal roll moment of inertia (Ir), which was based on one boat having a smaller weight at its masthead as compared to the other boat with a big weight at the bottom of the keel. In essense, they would both resist acceleration due to an equal roll moment equally, but the weight at the top of the mast would mean a greater roll moment on the mast weight boat than on the keel weight boat and so would also have a faster roll rate.

What I have not touched on in this discussion is that in short to medium duration repetative waves, increasing roll moment of inertia will tend to cause the roll of the vessel to be out of phase with the wave train. In larger waves, this can cause the vessel to have a greater impact at the crest and troughs which can mean more rotational force being applied to the vessel. In the case of the mast weighted boat, this would be exacerbated by the higher rotation velocity relative to keel weight.

Jeff

 

[cormeum]

I think that we are in agreement. In the example, both boats had a roughly equal roll moment of inertia (Ir), which was based on one boat having a smaller weight at its masthead as compared to the other boat with a big weight at the bottom of the keel. In essense, they would both resist acceleration due to an equal roll moment equally, but the weight at the top of the mast would mean a greater roll moment on the mast weight boat than on the keel weight boat and so would also have a faster roll rate.

Jeff

Since the moments of inertia are the same, the accelerations would be the same for both. Intuitively, I'd think the "roll rate" would be slower since the masthead boat would swing through a larger arc (bigger theta)- since the a's are the same, t would have to be larger to account for the larger amplitude.. The amplitude of the roll would be different due to the location of the mass being below the roll axis in the keel situation, thereby resisting gravity trying to tilt the boat and greater for the masthead example since the mass would be applying a force in the same direction as the displacement from vertical when the boat is heeled.

I resist using "moments" for anything else than inertial properties of objects because it's easy to confuse things when using words instead of math. Both these situations, of course, ignore damping.

 

[PCP]

...Catamarans when caught in a storm are notoriously dangerous by design as are light weight sport boats.

....I am writing here for other readers who may be new to this and are doing internet searches to educate themselves. I have plenty of non biased knowledge to share. I will be monitoring posts to detect bias and misinformation .

About non biased and misinformation, I found this statment particularly...odd? Strange?, or should I say biased?

About this :"Catamarans when caught in a storm are notoriously dangerous by design" - let me tell you that one of the things tank testing have showed is that catamarans, particularly cruising cats, are many times harder to capsize than a monohull. They have a huge stability and that statement of yours is not supported by reality.

About this: ..."(light weight sport boats) when caught in a storm are notoriously dangerous by design". I suppose you consider a Class 40 cruiser/racer a light boat. He weighs only 4500 kg, for 12m length. Everybody knows that this boat, for its size, is one of the safest boats around. After many Transats (with lots of boats, solo crewed), including circumnavigation races, they have an impeccable safety record. And you can bet they have already got their share of storms.

Of course I am not even talking about the Minis, even with more Transats on their log (thousands), sailing sometimes with winds over 50k and huge seas, also with an impeccable safety record (these are 26ft with a 1000kg displacement).

Your statment is so obviously biased that I refuse to discuss this issue further, and I have to say that I am a little worried when you say: "I will be monitoring posts to detect bias and misinformation."

Regards

Paulo

 

[kevlarpirate]

To understand moment of inertia , it will be very helpful to disassociate it from any other system with other forces acting upon it.

Think of the system as masses on the end of weightless sticks. Then when understood, and using proper terminology, and understanding the math apply it to a more complex system.

Paulo, a catamarans stability curve has more area under the curve than above the curve, due to having a mast. Therefore the design is more happy to be upside down than right side up, meaning the system requires more energy to right it than the original energy to invert it. The only feature which makes a cat safe is it's size relative to waves of the ocean state it is sailing in, providing of course it does not break up.

Please reference appropriate literature which explains this.

as for other literature available , I suggest you read Principles of Offshore Design Larrson and Eliasson . chapters on stability, it is all there , fully explained , no need to fight here

 

[PCP]

Paulo, a catamarans stability curve has more area under the curve than above the curve, due to having a mast. Therefore the design is more happy to be upside down than right side up, meaning the system requires more energy to right it than the original energy to invert it. The only feature which makes a cat safe is it's size relative to waves of the ocean state it is sailing in, providing of course it does not break up.

Please reference appropriate literature which explains this.

as for other literature available , I suggest you read Principles of Offshore Design Larrson and Eliasson . chapters on stability, it is all there , fully explained , no need to fight here

Kev, Jeff had posted this about that book, I mean Priciples of Offshore Design. I agree with him.

...Just for the record, I am very familiar with "Desirable and Undesirable Characteristics for Offshore Yachts". It does a very nice job of explaning the thinking of that era. But even John Rousmaniere, has told me and posted here in prior discussions that this was the best thinking of that era, but that we have a much better understanding of these issues than we had when Desirable and Undesirable Characteristics for Offshore Yachts was published nearly 30 years ago, and that modern designer have learned the lessons of that era and have designed boats that have responded well to those lessons.
Jeff

You should read more up to date information on stability. The information on that book is not specific to Cats and anyway, Cruising cats were just new born babies when the book was written. At the time there was no sufficient or reliable information about them.

You are right, a Cat has about the same stability in the normal position or upside down. The mast is of no relevance, because it breaks' when the boat capsizes. But you are missing the point, in what concerns cat's stability:

...tank testing have showed that catamarans, particularly cruising cats, are many times harder to capsize than a monohull. Paulo

Of course, this could not be true, if that theory of yours, that puts as critical factors to prevent capsizing, mass and a big moment of inertia, was true. Cats are the sailboats with less mass and sailboats with a low moment of inertia (but with a huge stability). But as this is true, as has been demonstrated on extensive tank testing, than, your theory.....

Regarding Cats, stability and Capsizing I would recommend to you," Model tests to study capsize and stability of sailing multihulls" by Barry Deakin. I believe it is from 2003.

I can quote a small part to you:

"Only one reliable account has been found of a catamaran capsize due to a beam encounter with a breaking wave. This was a 9m yacht wich encountered a wave about 9m high, with a breaking crest. The low incidence of such casualties is perhaps due to the low probability of encountering a wave of sufficient size, on the beam…when it breaks.

The much greater incidence of monohuls capsizes may be because they…require smaller waves."

Regards

Paulo

 

[kevlarpirate]

Paulo, To me, you don't sound like you read that book and chapters on stability.
How long ago did Jeff make reference to it?

More specifically, catamaran stability is nothing new. One problem you are having is thinking new knowledge is replacing laws of physics. this is simply not true.
As for catamarans, I have capsized countless times not by a beam sea but
by getting in the danger zone and not being able to recover because
you can't let the main out fast enough. Another scenario is digging the leeward bow in , which swings the apparent wind rearward (as the boat slows and also turns down) and then over you go.

My point about stability was that once you are over , there is no way to re invert to upright. Even the worst static curve monohull has a chance of inverting back to upright. The point is that cats are vulnerable to capsize. However that capsize occurs is not the issue, the issue is they do.

In 1999 Transpac i recall Double Bullet , 76 feet long, capsized in 8-10 foot seas,
Also included here is a link to another disaster, this was a 50 foot cat.

Perhaps with your knowledge, you could forward a theory on how these events happened. I will be interested to read it, thank you in advance

TRANSPAC 99 - Press Release 7-07-99 - Double Bullet II capsizes, 6 crew rescued by helicopter

Catamaran washes ashore, crew missing | KATU.com - Breaking News, Sports, Traffic and Weather - Portland, Oregon | News

 

[seabreeze_97]

That's a good example. I have found one on line that is similar to a boat with a bulbed keel. You cam make it longer or shorter

FREE METRONOME SOFTWARE - Best Metronome .com

But you have to consider that you have two weights. One on the top, other on the bottom, like a boat with a fin bulbed keel. You are not only putting the weight up but you are also increasing the lenght of the keel.

Regards

Paulo

It was my attempt to address the basic motion comfort as handled by Jeff where he draws on the example of two identical boats, one with, and one without a 500lb mast-end weight. He states the motion comfort would be adversely affected. A simple metronome disproves that statement.
Having identical boats, but one with more weight aloft, the other with considerably more weight down under can yield two boats that have the same motion comfort. Sure, but not everyone wants a 6-foot 4-ton bulb keel. Obviously, different construction and weight of the hulls would require different keel and rig dimensions to achieve the same motion comfort numbers, if that were the goal. In the racer's world, with unlimited funding, I can see where some high-tech construction and low slung, heavy bulb would be preferred, but then is motion comfort really the priority? In the cruiser world, why have an inconveniently long keel loaded with expensive lead when a few extra feet of mast (or a slightly heavier and I would wager considerably less costly mast) would achieve the same goal?