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  #11  
Old 10-11-2013
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Re: Stability question

Size matters. A LOT.
The wave that would put a 20 foot boat right over might just annoy a 40 foot boat. You need to have a holistic view of the various numbers, they do not exist in a vacuum.
That said, for offshore I would tend to avoid the modern fat-butt wing keel design so loved by coastal cruisers because of all the room you get and wonderful initial stability. My own boat is old-school with a length of 35 feet and max beam of 10.5' amidships tapering to a narrow stern. I have been laid over almost 90 degrees by breaking waves and the boat pops right back up. My positive stability goes out to about 125 degrees or so IIRC. The modern boat when layed over has the very wide stern holding the rudder out. Capsize issues aside, just steering the boat becomes a major PITA.

So.....look at some of these boats for an idea of more moderate designs.
Older C&Cs. The old C&C 30 was the most stable boat C&C ever made.
Older Tartans. From what I hear the 34s and 37s are fine boats offshore.
Valiant. They aren't cheap but good to study even if you can't afford one.
Pacific Seacraft: Ditto.
For low budget operations the various Alberg designs are slow, but certainly are stable.
See Alberg 30, Alberg 37, Cape Dories, Pearson Triton, etc. etc............

One more thing - the traditional "meter boat" design that is very narrow with a very deep keel is amazingly stable. I think their positive stability essentially goes to 180 degrees. They also are very cramped below, very wet, horrible rollers going downwind and get very hard to control when pushed beyond hull speed off the wind. BTDT. There is a reason boat designs evolved away from max capsize resistance after WW II


BTW, there is a LOT more to it than just not getting stuck inverted. Are you willing to trade comfort for speed? Speed for storage? Ease of steering or responsive steering? Keep in mind for every hour of storm conditions you'll likely have 10 or 100 of light air conditions. Would you give up light air ability for huge fuel tanks? If you have neither are you in a hurry?
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Last edited by Coquina; 10-11-2013 at 10:02 AM.
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  #12  
Old 10-11-2013
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Re: Stability question

Quote:
Originally Posted by svHyLyte View Post
Ted Brewer's "Comfort Ratio" seems to be a useful indicator of yacht performance in our experience. For more information see (click on) Crunching Numbers.
(I apologize that this is cut and paste from an earlier piece I had written but it applies here.) Its seems that as soon as someone posts a question about stability or the seaworthiness of some particular boat, that a well meaning responder mentions the Capsize Screen Formula and the Motion Comfort Index and sends them to Carl's Sail Calculator. And no sooner than poster questions the seaworthiness of some boat, that someone cites the Capsize Screen Formula and the Motion Comfort Index in that vessel's defense or prosecution. But as I have explained many times in the past, (and I am about to explain yet again) these surrogate formulas tell almost nothing about how the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them in any way borders on the dangerous.

Both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.

I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 1000 pound weight at the top of the mast. (Yes, I know that no one would install a 1000 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.

And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, if you had two boats, one with a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a very heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of low density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.

There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):
Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Hull Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return, nor does it predict how likely a boat is to downflood at a comparatively small heel angle.

At this point there are no simple formulas for these things. Bob Perry's key points are exactly right, that to really understand the seaworthiness of a boat, you need to understand the likely behavior of its hull form and its buoyancy and weight distributions. And even then, you also need to understand that no matter how seaworthy a boat may seem, when things get extremely bad out there, the biggest factors affecting seaworthiness can quickly become skill and luck.

Respectfully,
Jeff
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Last edited by Jeff_H; 10-11-2013 at 01:45 PM. Reason: typos resulting silly statements
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Re: Stability question

Quote:
Originally Posted by Coquina View Post
... The modern boat when layed over has the very wide stern holding the rudder out. Capsize issues aside, just steering the boat becomes a major PITA.

So.....look at some of these boats for an idea of more moderate designs.
Older C&Cs. The old C&C 30 was the most stable boat C&C ever made.
Older Tartans. From what I hear the 34s and 37s are fine boats offshore.
Valiant. ...
You mean one of the rudders out while the other is full buried in the water?

Your advise seems to be that older designers are better for offshore work. That does not make sense. Boat evolution generally has provided safer faster and better boats even if each case is a case.

And then there is that idea of "moderate designs"!!!! What the hell is that? You mean modern designs are immoderate as a all? The older ones are more moderate and therefore better, generally speaking? Or are they just old and outdated and outperformed?

If I had the money I would buy a new boat if not the more recent my money could afford, provided it has the characteristics of a voyage boat or at least one that could be adapted for that.

Regards

Paulo
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Re: Stability question

Quote:
Originally Posted by Jeff_H View Post
...
It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return, nor does it predict how likely a boat is to downflood at a comparatively small heel angle.

...
Respectfully,
Jeff
For knowing the amount of force needed to capsize a boat you need a RM curve (that is easy to get if the boat is new). Regarding the AVS I agree with Bob when he says that a difference between 120 or 125º is not that important but I would say that significantly below 120º I would start to think again (for an offshore boat). I mean there are some boats out there with AVS between 105 and 110º.

Sure, with 110º it should come back, but then there is the weight of the radar, the weight of the wet sails (worse if it has a in mast furler) and the water that can be caught in the sails and I know of some cases of boats that should have come up and stayed on a side for a long time.

I agree that the AVS is not the only point to consider. Probably more important is what some call final stability as a all, I mean the stability that goes between max RM till the AVS, particularly at 70, 80 and 90º. There are some boats (typically fast boats with big Max RM) that have a relatively small AVS ,that can be slightly below 120º, but have a big RM at 90º.

I would prefer one of those to one with a bigger AVS but much smaller values at 90º (there are the two types of curves). I would prefer a big RM at 90º because that's the one needed to quickly right the boat after a knock down. If the boat takes a lot of time to right itself up, it can be caught by another wave still laying down or at a great angle of heel where the boat would be much easily capsized. Many of the stories I have heard boat capsized boats have two do with a close two wave situation: the first one knock it down, the second rolls it.

The downloading angle value is needed to certificate the boat and the dealer can provide it to you (sometimes it comes on the stability curves). I agree that is an important factor in what regards seaworthiness.

Regards

Paulo
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Last edited by PCP; 10-11-2013 at 01:39 PM.
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Re: Stability question

Quote:
Originally Posted by PCP View Post
Your advise seems to be that older designers are better for offshore work. That does not make sense. yes it doesBoat evolution generally has provided safer faster and better boats even if each case is a case.not necessarily

And then there is that idea of "moderate designs"!!!! What the hell is that? You mean modern designs are immoderate as a all? frequently, yesThe older ones are more moderate and therefore better, generally speaking? correctOr are they just old and outdated and outperformed?no

Paulo
I believe his advice is correct, and agrees with the reasoned arguments in Coles' "Heavy Weather Sailing", which contains a great deal of analysis of boat designs and of storm survival. The authors also concluded that the avoidance of design extremes was key.

Let's not forget that the main influence in recent evolution of cruising boats has been the need for increased living accomodation. In particular, the trend has been towards larger beams, and greater freeboard. The former has negative effects on ultimate righting moment, and the latter has negative effects on the boat's resistance to a breaking wave on the beam.

Or if you want to talk about racing boats, the modern "sled" is a dreadful design when it comes to storm survival. The percentage of boats surviving major storms that have hit races has been steadily dropping over the decades, not increasing.

Given that one has a reasonably sound, moderate design, the key to survival seems to be avoidance of breaking waves on the beam. (that is, if you failed to stay out of the bad weather in the first place). That means that your boat control techniques in a storm are important. The book concluded that active techniques have been statistically more successful than passive.

If you want to take a look at an "immoderate" modern cruiser, there's a Hunter in the slip opposite mine. Compare and contrast to my Bristol.
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Re: Stability question


Boat "evolution" is to what SELLS.
The modern idea of a very wide boat carrying beam all the way aft to provide a huge cockpit and space for a double bunk under it combined with a shallow draft wing keel is VERY far from ideal for offshore work IMHO. This is entirely correct to for a builder's viewpoint. Since well over 90% of these boats are doing weekend cruises and the occasional week or two vacation trip, they are bundling for the market that exists.

No idea what you mean by "one" of the rudders? Do you perhaps think that the twin helm CatBenHuns have twin rudders as well? They do not as far as I have ever seen.

Having been on the modern Tartans, nothing makes me think they are better than a Tartan 37 for offshore. More room maybe, more modern looking, etc. etc. but not better ocean boats.

Quote:
Originally Posted by PCP View Post
You mean one of the rudders out while the other is full buried in the water?

Your advise seems to be that older designers are better for offshore work. That does not make sense. Boat evolution generally has provided safer faster and better boats even if each case is a case.

And then there is that idea of "moderate designs"!!!! What the hell is that? You mean modern designs are immoderate as a all? The older ones are more moderate and therefore better, generally speaking? Or are they just old and outdated and outperformed?

If I had the money I would buy a new boat if not the more recent my money could afford, provided it has the characteristics of a voyage boat or at least one that could be adapted for that.

Regards

Paulo
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Re: Stability question

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Originally Posted by MarkSF View Post
I believe his advice is correct, and agrees with the reasoned arguments in Coles' "Heavy Weather Sailing", which contains a great deal of analysis of boat designs and of storm survival. The authors also concluded that the avoidance of design extremes was key.
That is an almost 40 year old book that has a 35 old view on the subject.

Quote:
Originally Posted by MarkSF View Post

Let's not forget that the main influence in recent evolution of cruising boats has been the need for increased living accomodation. In particular, the trend has been towards larger beams, and greater freeboard. The former has negative effects on ultimate righting moment, and the latter has negative effects on the boat's resistance to a breaking wave on the beam.
You really believe that? I thought that they were influenced by Open class solo boats, Open 60 and class 40 because they were the more seaworthy and easy to sail with a short crew. Maybe the Open 60 are like that just to provide better accommodation to the solo sailor

Modern voyage boats, I mean those that are designed with voyage in mind are beamy boats. Do you think they are like that for increased living accommodation? Typically they are over 40ft and sailed by a couple, do you really think that the "accommodation" is the reason because they are beamy and have large transoms?

Many of those beamy boats have a better final stability and AVS than what you call more moderate designs due to a big draft and a torpedo keel.

Regards

Paulo
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Re: Stability question

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Originally Posted by bobperry View Post
Those numbers are fine and valid as references but there is a lot more to it than those static numbers. So long as you choose a "normal" boat with reasonable shape and avoid any of the extreme designs you should be fine. I think your safety in heavy weather depends a lot more on you than it does those numbers. Learn how and when to reduce sail. Would you pass on a boat that has LOPS (limit of positive stability) of 123 degrees compared to another boat with a LOPS of 126 degrees? I wouldn't. In extreme conditions I'm not sure that a LOPS of 120 degs makes any difference compared to a LOPS of 128 degs. Stick with boats that have a beam to length ratio (L/B) between 3.3 and 4.00. Avoid boats with an L/B near 3.00 or below. Pick a boat with a D/L around 220 to 270. Recognize that shoal draft will almost certainly raise your VCG and reduce righting moment (Rm)

It is very difficult to generalize about stability. I think there is a preoccupation with static stability numbers today. Probably because computers make it relatively easy to get those numbers. But they are Dependant for their accuracy on an accurate VCG and this has to come from a painstaking weight study and there is no computer short cut for that.






Years ago I worked for a very well known design office for a year. In that year we did one stability study and that was only to determine the Rm for rating purposes.
Bob why no (L/B) below 3???
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Re: Stability question

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Originally Posted by Coquina View Post

...

No idea what you mean by "one" of the rudders? Do you perhaps think that the twin helm CatBenHuns have twin rudders as well? They do not as far as I have ever seen.

....
Catalinas and Hunters have always some delay in what regards design, since they are not designed by any major NA but yes contemporary designed Benetaus have twin rudders when the size of the transom demands that. They have started on the Sense series:



and now they are expanding them to the other series. The last Oceanis, even if only a 38ft comes already with two rudders:

First review of the Oceanis 38 » BENETEAU



Regards

Paulo
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Re: Stability question

[QUOTE=PCP;1103002]That is an almost 40 year old book that has a 35 old view on the subject.

The book I read was a 2008 edition, and included an analysis of the 1998 Sydney to Hobart race

You really believe that? I thought that they were influenced by Open class solo boats, Open 60 and class 40 because they were the more seaworthy and easy to sail with a short crew. Maybe the Open 60 are like that just to provide better accommodation to the solo sailor

Yes I do.

Modern voyage boats, I mean those that are designed with voyage in mind are beamy boats. Do you think they are like that for increased living accommodation? Typically they are over 40ft and sailed by a couple, do you really think that the "accommodation" is the reason because they are beamy and have large transoms?

Ahh, so now you have separated boats into "cruisers" and "voyage boats". I thought we were talking about typical cruising designs? The ones I see all around me don't have bulb keels 10ft down. Yes, I firmly believe that the main priority of the typical Beneteau / Hunter / Catalina et al is cruising accomodation, not storm survivability.

Many of those beamy boats have a better final stability and AVS than what you call more moderate designs due to a big draft and a torpedo keel.

Regards

Paulo
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