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Limit of Positive Stability (LPS)

26K views 49 replies 15 participants last post by  captain jack 
#1 ·
I thought I might begin a thread to discuss this topic. I hope the knowledgeable designers amongst us will chime in with their thoughts and correct any errors I may introduce below:

A recent post inquired about the Limit of Positive Stabilty (LPS) of a particular boat model. LPS, sometimes referred to as the Angle of Vanishing Stability, is a measure of a boat's ultimate stability. The LPS figure, expressed in degrees, is supposed to approximate the point at which a particular boat will heel so much that it cannot right itself. At that point, the boat may continue to roll upside down. In theory the sailboat will eventually self-right, usually by completing the roll through 360 degrees (assuming no downflooding, etc).

The ORC (Offshore Racing Council) recommends a minimum LPS of 120 degrees for off-shore racing. An LPS of 120 degrees would mean that the boat could heel an additional 30 degrees past a perpendicular 90 degree knock down, and still right itself (again, in theory, since LPS figures are static calculations which don't reflect dynamic variables such as sea-state, downflooding, loading, etc).

In the other thread I mentioned, JeffH made the following observation:

I would say that the angle [for the Bayfield 29] is less than that, somewhere down around 105-110 degrees. You are talking about a beamy, high freeboard, heavy rigging, moderately lightly ballasted, shoal draft boat.
It struck me that JeffH essentially describes the vast majority of modern production boats.

Published stability tables for older production boats, as well as many newer ones, can be hard to come by. When I can find them (not all builders publish the LPS), it generally surprises me how low the LPS figures are. For instance, we are presently considering purchasing a larger sailboat for our family, and I was surprised to learn that the LPS of this popular 42 foot model was only in the range of 114 degrees. Our current 31 footer has an LPS of 139 degrees, which is among the lowest for all the models made by this manufacturer.

Looking at the hull form of the Bayfield 29...



...I would have thought the LPS to be higher than 105-110 -- it just looks like a more stable hull form than many of the modern production boats. So, lacking the LPS tables from a builder, how do we guage the suitability of a design for coastal or off-shore sailing? The figure of 120 degrees or better is considered desirable for off-shore sailing. What is a minimum LPS figure for coastal sailing?
 
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#2 ·
JRP—

Hull form is just one factor in LPS. The amount of ballast, the weight of the rig the beam of the boat, all contribute...
 
#3 ·
I thought I might begin a thread to discuss this topic. I hope the knowledgeable designers amongst us will chime in with their thoughts and correct any errors I may introduce below:

A recent post inquired about the Limit of Positive Stabilty (LPS) of a particular boat model. LPS, sometimes referred to as the Angle of Vanishing Stability, is a measure of a boat's ultimate stability. The LPS figure, expressed in degrees, is supposed to approximate the point at which a particular boat will heel so much that it cannot right itself. At that point, the boat may continue to roll upside down. In theory the sailboat will eventually self-right, usually by completing the roll through 360 degrees (assuming no downflooding, etc).

The ORC (Offshore Racing Council) recommends a minimum LPS of 120 degrees for off-shore racing. An LPS of 120 degrees would mean that the boat could heel an additional 30 degrees past a perpendicular 90 degree knock down, and still right itself (again, in theory, since LPS figures are static calculations which don't reflect dynamic variables such as sea-state, downflooding, loading, etc).

In the other thread I mentioned, JeffH made the following observation:

It struck me that JeffH essentially describes the vast majority of modern production boats.

Published stability tables for older production boats, as well as many newer ones, can be hard to come by. When I can find them (not all builders publish the LPS), it generally surprises me how low the LPS figures are. For instance, we are presently considering purchasing a larger sailboat for our family, and I was surprised to learn that the LPS of this popular 42 foot model was only in the range of 114 degrees. Our current 31 footer has an LPS of 139 degrees, which is among the lowest for all the models made by this manufacturer.

Looking at the hull form of the Bayfield 29...



...I would have thought the LPS to be higher than 105-110 -- it just looks like a more stable hull form than many of the modern production boats. So, lacking the LPS tables from a builder, how do we guage the suitability of a design for coastal or off-shore sailing? The figure of 120 degrees or better is considered desirable for off-shore sailing. What is a minimum LPS figure for coastal sailing?
Limit of positive stabiity is sometimes called the "point of no return." The boat heels over so much that righting arm shrinks to zero. At that point, once it heels further, which can happen by being tilted on a wave face at sea, the righting arm turns into an overturning arm and flips the boat upside down. A couple things to keep in mind:
The calc of this vanishing point is usually based on the hull form, but the cabintop can provide reserve buoyancy (provided the companionways is shut) and increase its value.
You'll routinely see pictures of capsized keelboats happily sitting upside down, their crew on the exposed hull bottom (or squirreled away inside). Because of hull form, a keelboat can be more stable upside down than it is right side up. That's partly because despite the weight of the keel, a keelboat's center of gravity typically is higher than the center of buoyancy when rightside up. What stops it from falling right over is that as it heels the center of buoyancy moves outboard, creating a righting arm. When a keelboat is upside down, the center of gravity is lower than the center of buoyancy, thus providing "good" stability. (Dinghies are typically the same, even moreso, as anyone who has turtled a dinghy knows.) Thus while a keelboat technically should be able to right itself, that isn't necessarily going to happen once it's upside down. Depending how much air has been captured in the hull (which affects overall buoyancy and the center of buoyancy as it rolls), it needs some external energy input to roll back rightside up. In storm conditions, waves can provide that.
The bottom line is that if hull form is exploited through form stability to provide a high vanishing angle, it typically means that it takes a lot of force should the boat become inverted to turn it rightside up again.
Steve Killing and I wrote about this in Yacht Design Explained, published by WW Norton in 1998. He could explain it all way better than I can, but that's a start.
 
#4 ·
The angle of vanishing stability and capsize screening formals can be found at:
http://www.sailingusa.info/formula.htm

They don't take into account nonstandard weight distributions of the hull or keel and they don't count the cabin house as part of the hull.

Basically, narrow, heavy and deep boats get the best numbers. 120 degrees is usually considered the minimum AVS for offshore sailing. My 1960s keel/centerboarder is considerably over the minimum: it's narrow by present standards, and heavy, but not deep.

I did own a boat that passed the formulas but I knew was too tender. I had an inclining test done (moving known heavy weights from rail to rail and measuring the change in list) the results of which alarmed the naval architect. Turns out the lead ballast on the plans (used for the formulas) was actually much lighter iron. A few hundred pounds of internal ballast helped but didn't solve the problem.

Most quoted AVS are provided by the manufacturer and. looking at typical modern designs that are light and wide, one wonders at their accuracy.
 
#13 · (Edited)
The angle of vanishing stability and capsize screening formals can be found at:
http://www.sailingusa.info/formula.htm
That is an interesting formula seemingly pretty accurate. It came back with a LPS of 128 for my 10,500 lb- 38 ft boat, which is about right.
JeffH / Nottoway / Diva,

Thanks for your commentary -- I have found it to be very enlightening.

I tried fiddling with that US Sailing AVS formula, but got some results that didn't make a lot of sense to me. For instance, the AVS value I arrived at for our boat was negative!? Even when I convert this value to a positive figure, the value is far lower than the published LPS for our boat model.

I am now wondering if this AVS formula is quantifying a value that is actually different from the LPS? In other words, is the AVS value the point on the stability curve where the righting moment begins to decrease again, and the LPS is actually the point where the stability curve crosses the x-axis, i.e. the point after which the boat capsizes?

For illustrative/discussion purposes, here is the "Intact Stability Curve" for a common production boat, the Catalina 42:



The question rephrased: Are the Angle of Vanishing Stability and Limit of Positive Stability the same points on the stability curve? If the answer is no, then this might explain why many people are surprised to learn how low the AVS is for a given vessel (like the Bayfield in the other post that prompted this thread). Thoughts?
 
#5 · (Edited)
That is an interesting formula seemingly pretty accurate. It came back with a LPS of 128 for my 10,500 lb- 38 ft boat, which is about right.

To comment on two points in Notoway's post, "Most quoted AVS are provided by the manufacturer and. looking at typical modern designs that are light and wide, one wonders at their accuracy." One of the great things about modern design software is that it is much easier to accurately model LPS and get a reasonably accurate LPS for a boat while it is being designed. Of course manufacturers of boats can tilt the numbers a bit by assuming full tanks and empty lockers and the like.

For what it is worth when this technology has been applied to 1960's era keel/CB boats they generally have LPS angles well under 120 degrees, often down around 110 or so. Although narrow, they carried a larger percentage of their beam to the ends of the boat, and had lower freeboard both fators that results in a smaller limits of positive stability.

As seen in the US Sailing estimate of stability, displacement itself has a comparatively small impact on angle of vanishing stability, and because heavy boats of the 1960's tend to have comparatively high vertical center of gravities, their weight works against them in dynamic resistance to capsize as well.

Jeff
 
#6 ·
It might be noted that the point of maximum righting moment, where the couple GZ is largest, is at deck edge immersion. Further heeling results in a decrease in the righting moment, which is the displacement times GZ. While the vessel is still stable and possesses righting moment, that tendency to right is diminished with every degree past deck edge immersion. Obviously the size and shape of deck structures as well as the flooding of cockpits, etc...only come in to play at these large angles of inclination.

Initial stability, as measured with an inclining experiment, is a measure of the vessel's stability at small angles of heel where hull form and freeboard are much less a factor in ultimate stability. Initial stability will give us the sense of whether the vessel is tender or stiff as well as the resultant motion in a seaway.

High freeboard, whatever it's other detractions, does impart a large angle of heel prior to deck edge immersion and the resultant decrease in stability. Significant dead-rise to the underwater hull form raises the center of buoyancy as well which will provide a greater righting moment at large angles of heel. Flared bows will also increase stability at large angles of inclination. To the extent that beam plays a role in stability, it is perhaps best understood to be one of more rapidly vanishing stability after the point of deck edge immersion. A beamy, high initial stability boat that feels quite stiff may fail to offer significant stability much past the angle of deck edge immersion or, perhaps better stated, after deck edge immersion the beamy vessel may experience rapid decreases in positive righting moment and start to feel very tender rather quickly.

As a result, I am somewhat puzzled with the fascination with beamy, plumb bowed boats, although open to enlightenment.
 
#7 · (Edited)
Without getting into a lot of detail but, there is a number of mistaken assumptions in your posting. First of all, on most ballasted keel boats, the maximum stability is around 90 degees of heel, at which point there is the maximum spread between the vertical center of gravity and the instantanteous center of bouyancy. Depending on the specifics of the design, the deck hits the water somewhere between 45-55 degrees of heel.

Deepening deadrise lowers the vertical center of buoyancy at low heel angles reducing inititial stability, but within normal, second half of the twentieth century, designs has little impact on stability at high angles of heel. At high angles of heel, the boat pretty much floats on its topsides and so the portion of the hull where the deadrise occurs is located is typically out of the water.

Adding buoayancy in the form of beam increases the amount of force required to get the boat to its limit of positive stability, but it also increases the amount of force required to bring the boat back up again once its passes its its limit of positive stability.

When a boat carries its beam towards its ends there is more buoyancy outboard and so it has more form stability and as a result it takes greater force to right than a boat with a identical beam which occurs only at a single point.

(edit shown in Italics) In hindsight, as I thought about yesterday's post, I thought that this matter of the increase in inverted form stability that results from carrying beam towards the end needs more explanation. If we think about the plan form of a 1960's era boat such as the keel/ centerboarder mentioned earlier in this thread, they carried their beam very far towards their ends compared to more modern IMS/IRC derived designs. If you looked at these boats from above, the 1960's era boats are closer in form to a rectangle and IMS/IRC derived modern boats are more triangular in form. So while the more modern design may have a greater beam, it rarely has as much deck area as the same length 1960's era design.

If you think about calculating form stability, (assuming similar amounts of flare in the topsides which is reasonable since both 1960's era and IMS/IRC derived designs have very little flare) in its simpliest form, the force to over turn is proportionate to the deck area times the lever arm. So for the sake of simplifying our example, we can assume that equal length boats of both eras have similar deck areas (which is not really a fair assumption since modern designs of equal length typically have smaller deck areas) and the modern boat has 20% more beam which is pretty typically the case, and by way of simplifying things the 1960's boat is a rectangle and the modern boat is a triangle. The center of the area of the rectangle form would be at a point that is half its width, while the center of area of the triangle would be at a point one third of its width. In effect, in this simple model, the rectanglar form of the 1960's boat would have 20% more form stability than the triangular form of the modern hull form [1.2= (1 divided by 2)/ (1.2 divided by3)]

Of course this is a bit of an over simplification, since 1960's boats are not literally rectangles and modern boats are not triangles, but what it does show is that modern IMS/IRC derived boats, while somewhat beamier than 1960's era boats do not necessarily have greater inverted form stability,and it fact, they often are carefully modeled to have poor inverted form stability inorder to achieve CE open ocean classifications.

The fascination with current crop of plumb bow, moderate beam, ultra low vertical center of gravity, carefully modelled hull forms is substantially better seakeeping, higher stability forces required to achieve any given heel angle, often higher angles of positive stability, lower resistance through the water permitting smaller sail plans, much more comfortable motions than similar length older style boats and of course greater speed.

Feeling enlightened......Gotta go
 
#9 ·
Especially boats like mine... takes a lot to get it to heel past 15˚, but things get really hairy after that... :)
 
#11 ·
Yup... but I doubt it'd sink... ;) A problem that many monohulls have... :D
 
#12 ·
It seems to me that inverted form stability is important--and morbidly fascinating--but only relevant in a small portion of the cases of boats that are knocked down past 90 degrees.

Anecdotal evidence (is there any other kind in matters like this where the sample is so small?) is that most boats knocked down below horizontal come up one way or the other--if they come up at all--and right themselves. Often without masts and deck gear including deck structures.

I think the more interesting question is how to balance in a useful sailboat the attributes of initial stability, ultimate stability, ballast, beam, draft and hull volume. And for the boat owner/buyer, how to make a reasonable judgment of stability for themselves. While the screening formulas for stability are very simple they do give amateurs (most of whom won't be getting an IMS certificate) some notion of the factors involved. If the present screening formulas are inadequate maybe someone could come up with one that could be done with ordinary lines plans, boat specs and a pocket calculator. Or is there already such a formula?
 
#14 ·
LPS and AVS should be the same value, since they are describing the angle at which positive stability equals zero. The US Sailing number is only a rough approximation, which for my boat is pretty close to right. A proper LPS calculation calculates the vertical, lateral and longitudinal center of gravity and then models the center of buoyancy as the boat heels until it locates the angle at which the center of gravity is directly over the center of buoyancy which only occurs at three points, LPS, and level at rightside-up and upside down. There is also an IMS AVS which is calculated for all boats that are rated under the IMS rule. This number does not add for the cabin structure or deduct for rigging weight and so typically yields a low number for most racer-cruisers.

Jeff
 
#16 ·
Keelhaulin'

Thanks for pointing that out -- yes, I was using total draft, not hull draft. I was actually working from the formula that Nottoway had linked to -- I didn't realize there was an interactive site that would do the work for me. Very handy!

One problem, though, I know the total draft (hull and keel) of our boat, but not our hull draft without the keel. :( I could guess, but that wouldn't be very scientific.

Actually, I'm surprised that the formula does not want the keel draft -- seems like that would be a major component in stability.

Also, I posted the Intact Stability Curve for the Catalina 42 just as an example. We don't actually own one. That curve was generated by Catalina Yachts and the designer, so I have to assume it's a more comprehensive calculation (such as JeffH described above) and reasonably accurate.
 
#17 · (Edited)
Jeff,
I think I was unclear in my post and, perhaps, in your's as well what terms we are talking about.

The righting arm GZ is the horizontal measurement between the vertcal vectors of G and B. I'll leave out any discussion of GM as we're discussing stability at large angles of heel. As the boat is heeled, B moves outboard towards the immersed side. This intuitively makes sense as we are immersing more hull to one side. The more hull we immerse the further outboard B will swing and the larger GZ will become. For most vessels, at or near deck edge immersion, GZ or righting arm is the greatest, but not all vessels. At that point, we have the maximum hull volume immersed to the heeled side and have thus moved B as far outboard as it can go. As the deck edge immerses, we lose buoyancy to the heeled side, and B begins to move inboard. For sailboats of certain design deckhouse structure can obviate this and will certainly influence it as heeling continues. We are not at LPS, but we are at the point of diminishing stability.

It can be seen by this why increased freeboard adds more positive stability after the angle of deck edge immersion and how tumble home will reduce potential stability. Increased freeboard also increases the range of stability. Tumble home reduces stability at all angles after immersion of the tumbled home portion of the side. Conversely, hull flare will generate a larger righting moment at large angles of heel despite it's other drawbacks. Deadrise is desirable only to the extent that B is initially high. If G is comensurately high, there is little advantage in terms of ultimate stability. If G is kept low, as in a traditional full keel boat, the effective raising of B by deadrise is desirable.

A traditional full keel boat may in fact achieve maximum righting moment at 90 degrees of heel. In fact, due to deckhouse shape as well as the position of G they will maintain a GZ, or righting arm well past deck edge immersion and perhaps have only 30 degrees or so in the danger zone with negative GZ. To your point though, I think it is more common for modern boats to at minimum begin to rapid lose GZ after deck edge immersion, and for most the maximu is developed at about that point. The problem with more modern designs dependant on form stability (read IOR) is that G has risen, thus at deck edge immersion, or soon thereafter, righting arm begins to dissipate rapidly. Due to their large danger zone of negative stability they will remain inverted for dangerously long periods of time. Whereas the traditional full keel boat will be almost impossible to keep inverted. Designs that rely on form stability too much show deceptive GM numbers which are not indicative of the boat's performance ar large angles of heel, particularly after deck edge immersion.

Just looking at the graph above in JohnRPollard's post illustrates my point (wish I'd looked closer earlier) and is perhaps a source of much confusion. Maximum positive stability is not at 114 degrees as labeled, it is at approximately 50-55 degrees. Maximum righting moment is being developed at 50-55 degrees and it decreases from there. 114 degrees is the point of vanishing stability or negative GZ. This Catalina has about a 65 degree danger zone and, once capsized, will have difficulty getting back on her feet. The most likely reason being that G is at or near the waterline.
 
#18 ·
I see your point about maximum righting force. Certainly in the case of the Catalina 42 which is loosely an IOR hull form with a shallow keel depth, it would appear that maximum righting moment occurs near the point that the rail hits the water. This would seemingly be true for any boat where the center of gravity is near or above the waterline.

I somewhat disagree with the implication of your comments that "a traditional full keel boat may have its maximum righting moment at 90 degrees", "the traditional full keel boat will be almost impossible to keep inverted" and "modern designs....(read IOR)". The problem that I have with the two full keel statements is that keel type has little to nothing to do with the limit of positive stability, or the point at which maximum stability is achieved. Since the angles of positive stability and maximum stability are solely dependent on the location of the center of gravity and hull form, keels of almost all designs can achieve very large angles of positive stability and maximum stability. In fact as most 20th century full keel boats are actually constructed, they tend to have comparatively very heavy construction, heavy rigs, shoaler draft, and comparatively low ballast to displacement ratios, all of which would suggest a very high vertical center of gravity relative to the truly modern IMS/IRC derived designs, which tend to have deeper draft, light hulls and rigs, little flare amidships and no tumblehome and carry their ballast in bulbs deep below the rail. As a result, while traditional hull forms (which coincidently also tended to have full keels) typically had greater stability than IOR boats (which by no stretch of the imagination could still be called modern since the rule peaked in popularity over 25 years ago and died altogether roughly 20 years) these boats do poorly in terms of the angle of heel at which they achieve maximum positive righting moment or limit of positive stability as compared to truly modern IMS/IRC derived designs.

Historically, capsizes were very common in traditional full keeled vessels. Even discounting for the pre-20th century use of internal ballast, by any reasonable metric one could not assume that a full keel boat is any less likely to capsize, and once capsized that it will remain inverted for any less time than a properly designed fin keel boat.

Respectfully,
Jeff
 
#19 · (Edited)
Jeff, Sailaway, or anyone else with an answer,

Why does the US Sailing Screening Stability Value (SSV) formula use hull draft rather than total draft (i.e., including keel) for purposes of calculating the AVS? It seems counterintuitive to me.

Also, where did you get that "hull draft" value for your own boats? Did you actually go out and physically take a measurement (keel span - total draft = hull draft)? Or are these values available in a database somewhere?

Great discussion. Thanks.

Addendum:

I see your point about maximum righting force. Certainly in the case of the Catalina 42 which is loosely an IOR hull form with a shallow keel depth, it would appear that maximum righting moment occurs near the point that the rail hits the water. This would seemingly be true for any boat where the center of gravity is near or above the waterline.
Respectfully,Jeff
Not sure what you consider to be a "shallow keel depth", but the stability curve that I embedded above is for the standard draft Catalina 42, with a published draft of 6'8".
 
#21 · (Edited)
I can accept that. But I would have thought that the US Sailing AVS formula generates distorted results because it does not factor keel/ballast depth.

Still, when I guestimate our hull draft, I get AVS results that range from 132 degrees (1.5 foot hull draft) to 152 degrees (2 foot hull draft). I'm fairly confident our hull draft is within that range (we draw 4'10"). The designer's puplished LPS for our boat is 139 degrees.

So I am impressd that this formula seems to work anyway, given that both JeffH and I have tried it out and produced results within the expected range. What I'm struggling with is WHY it does work given that it seems to ignore keel draft?
 
#24 ·
Sailaway, I find that proposition intriguing, and counterintuitive. If your suggestion is correct, that might explain why the LPS/AVS for the Catalina 42, and other similar relatively flat-bottomed modern designs, is so low.

But, with that in mind and looking at some photos of JeffH's boat hauled out (that he has posted in the photo gallery), I am surprised his LPS number came out as high as it did. The hull form of his boat seems to emphasize low wetted surface -- very different from our boat. Our boat is more old school -- the hull is down deep IN the water, rather than floating atop it like the wider, flatter modern designs. [This is not intended as a criticism of Jeff's boat, just an observation.]

JohnR- I couldn't find a hull draft for our boat either. Using the actual design drawings I was able to measure fairly accurately the hull draft. I came up with SSV of 26.73 and AVS of 133.9. I am going to call Ted Brewer and ask if he has the actual values tomorrow. This is for a Brewer 40 PH.
JRD, that's interesting. I have some drawings too of our boat, but it's somewhat of a guess as to where the hull ends and the keel begins. Viewed in athwartship cross section, ours has the fairly traditional wine glass profile, so the transition from hull to keel is very gradual. And the ballast is bolted externally to a fairly deep keel stub. So I am left wondering where US Sailing gets that value? From the owners? If so, there could be a large fudge factor on some boats.

I'll be curious to hear Mr. Brewer's thoughts. Maybe you could direct him to this thread and see if he might be willing to post some comments? If not, I look forward to hearing from you what you learn.

In the meanwhile, I'm going to run the numbers on our boat again, then I will post the figures and how they were derived. Maybe others would be willing to do the same with their own boats and perhaps the comparisons will make for interesting discussion...
 
#23 ·
JohnR- I couldn't find a hull draft for our boat either. Using the actual design drawings I was able to measure fairly accurately the hull draft. I came up with SSV of 26.73 and AVS of 133.9. I am going to call Ted Brewer and ask if he has the actual values tomorrow. This is for a Brewer 40 PH.

John
 
#25 · (Edited)
I am not sure why the formula is written the way that it is. When I tried to enter a deeper canoe body the AVS initially got smaller, but when I put my boat's draft in, the number came back at 358 degrees, obviously not a realistic. I also experimented with changing displacement, entering my boat's displacement when fully loaded, which I have actually measured when it was in the travelift. By increasing my displacement by 3000 lbs, my stability dropped by 5 degrees.

If I think about it, I can rationalize it this way, on a boat with a shallow canoe body, as the boat heels the center of buoyancy moves up the topsides of the boat more quickly than a boat with a deep canoe body. Therefore the CG is would be over the center of buoyancy at a smaller angle of heel. Thinking about why increasing displacement reduces the AVS, similar to above, the deeper a boat floats on its side the closer to its keel the center of buoyancy is likely to be and so the sooner that the CG will be above the CB. The reason that the formula suggests that a shallower canoe body than mine would have less stability is that a similar boat with a shallower canoe body would float deeper for its displacement and so would also capsize at a smaller heel angle. What the formula does not consider is freeboard. My boat has pretty low freeboard, and so would tend to have a lower AVS than a boat with higher freeboard.

What ever else you can say, the 128 degree AVS number is pretty close to the actual number for my boat, which may simply be a coincidence since my boat is such an anomaly in the design world.

With regards to John Pollard's comments on my boat, it is what I was saying earlier. Light, shallow canoe body boats do not necessaily have small Limit of Positive Stability angles. That misconception goes back to the days of the IOR when light weight was associated with boats that had very high vertical centers of gravity. My boat actually has pretty high ballast to displacement ratio and a very low center of gravity compared to heavier boats of that same era. Newer IMS/IRC influenced designs have even smaller LPS angles still, way narrower than my boat and most traditional cruising designs.

Jeff
 
#26 ·
I was unable to call Ted (work sure gets in the way of important business!), might have a chance tomorrow. I'll ask him if he would be interested in reading the thread and shedding some light on the subject.

John
 
#27 ·
I will confess to approaching this subject from the experience of a ship's officer. That is not all to the bad, with exception granted to the notion of stinkpotters. What is apparent to me though is that, within our discussion, we are to some extent approaching things somewhat backwards. We should first approach such matters from the theoretical and then consider the practical.

Consider two opposing ideas. One holds that a stiff ship is desirable, the other that a tender ship is desirable. The stiff ship responds well to a press of canvas and heels much less under that load. She also has a very fast rolling period. The tender vessel will heel more readily and may well dip her rail much sooner under much less press of sail. So the vote goes to the stiff ship right? Not necessarily. We don't sail generally upon calm seas and the ship's motion cannot be ignored. This is particularly so when running. The stiff ship is much more prone to synchronous rolling given her already short rolloing period while the tender vessel will be far less likely to engage in such potentially catastrophic behaviour. All of which merely points out that things are not quite as simple as they may appear.

One area Jeff and I will surely agree upon is the perverting nature of racing boat rule making. The notion that racing derived hull forms has improved the breed is open to question, with the debacle of IOR coming readily to mind. Readers of this forum are undoubtably concerned more with seaworthiness than ultimate speed, although the stating of such is in no way meant to obviate the interaction of speed, comfort, and seaworthiness. The only reason for the mentioning of it is the persistant advertizing of the race it/cruise it sort that may imply there are no trade offs. In my opinion, seaworthiness may be the factor that goes by the wayside.

To truly discuss the subject, a grasp of basic principles of naval architecture is required. You may purchase PNA, Principles of Naval Architecture, but I'm afraid that most of us are not really up for any refreshers in calculus. I'd recommend a seemingly odd choice that will provide a more than rudimentary knowledge of the terms required, that at the same time, will not require more than a pleasant couple of hours reading to plainly explain the factors and forces we're dealing with here. "Stability and Trim for the Ship's Officer" by John LaDage and Lee Van Gemert, the second edition of 1956 is a wonderful book of some only 200 hundred pages. It's published by Cornell Maritime Press. I recommend you purchase the second edition off Amazon for a mere pittance rather than the new edition, updated by my late fellow instructor at the USMMA, Bill George. This in no way should serve to diminish Bill's efforts, it merely reflects the fact that CMP is getting the princely sum of $50 for the latest edition and our purposes can be well served by the older editions.

Within it's covers you'll find the terms of CG, CB, and the metacenter well and clearly explained. More importantly, the understanding of righting arm, GZ, and righting moment, GZ x Displ., carefully explained. There are many common misunderstandings of stability that are easily and understandably eradicated by a quick reading of this book.

Along with that book I'd recommend C.A. Marchaj's "Seaworthiness, the forgotten factor". While updated in 1996 it is somewhat dated, the author's prejudices are perhaps a bit heavy-handed, but coupled with the knowledge gained from LaDage it gives an excellent jumping off point for the fascinating discussion of sailboat design. A discussion which has prompted me to pull out my long neglected copy of Chappelle's "The Search for Speed Under Sail".

In answer to John Pollard's question I can only mention that displacement means little without reference to the locations of CG and CB. As in Jeff's case, it is of utmost importance as to where the added weight is loaded. A factor not mentioned yet, in the discussion of stability at large angles of heel, is the waterplane. As the boat is loaded, or heeled, the waterplane changes affecting possibly both CB and CG. If the boat is loaded deeper the effects of tumble home may obviate even a lower CG. Tumble home will also reduce righting arm at large angles of heel compared to a slab sided or even flared hull boat. The shift in buoyancy on a slab sided boat when heeled is not proportional due to the consideration of deadrise. Imagining a ballasted log might help with the understanding of the concept.

I should confess also that i have not gone into the AVS link provided and must do so soonest if I am going to participate further here. Hopefully tonight's Christmas party and a compliant wife will make that possible yet this evening.

Great thread in what promises to be a great forum.
 
#29 · (Edited)
I will confess to approaching this subject from the experience of a ship's officer. That is not all to the bad, with exception granted to the notion of stinkpotters. What is apparent to me though is that, within our discussion, we are to some extent approaching things somewhat backwards. We should first approach such matters from the theoretical and then consider the practical.

Consider two opposing ideas. One holds that a stiff ship is desirable, the other that a tender ship is desirable. The stiff ship responds well to a press of canvas and heels much less under that load. She also has a very fast rolling period. The tender vessel will heel more readily and may well dip her rail much sooner under much less press of sail. So the vote goes to the stiff ship right? Not necessarily. We don't sail generally upon calm seas and the ship's motion cannot be ignored. This is particularly so when running. The stiff ship is much more prone to synchronous rolling given her already short rolloing period while the tender vessel will be far less likely to engage in such potentially catastrophic behaviour. All of which merely points out that things are not quite as simple as they may appear.

.
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.
 
#30 ·
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.
 
#31 ·
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.
 
#32 · (Edited)
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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#39 ·
...........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.
 
#35 ·
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.
 
#38 · (Edited)
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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