Capsize ratio

[Jeff H]

One more point that I had wanted to make regarding the quote, "no boat with over a 3:1 length to beam ratio was rolled in Fastnet 79". I think that you are mistaken. During the Fastnet Storm almost all of the small boats were rolled to 180 degrees and many of those had beam to length ratios in excess of 3:1 including the Contessa 32 that was so widely studied and a Westsail 32 that was not racing but which was caught in the storm area. The majority of larger boats that were rolled also had length to beam ratios over 3:1.

(As I reread your paragraph, I think that perhaps you meant to say that the majority of boats that were rolled had length to beam ratios over 3:1, rather than under, meaning that the beamier boats in the fleet were more likely to be rolled.)

In any event, there is a reason that the beamier boats in the 79 Fastnet were more likely to be rolled and it had comparatively little to do with either thier displacement or their beam, and everthing to do with their high vertical centers of gravity. If you look at what was happening at the time in race boat design, the IOR rating rule had just gone through a couple revisions that overpenalized stability and over credited beam. As a result the IOR boats of that era were purposely designed with very low ballast ratios, high vertical centers of gravity and a very wide beam at the deck.

In the initial studies of the Fastnet, wide beam was dispropotionately seen a culprit because the most modern designs in the race were designed to beat the new rule and so were very beamy and comparatively light as compared to the boats that had preceeded by a decade or two. What was not immediately apparent was that they also had extremely high vertical centers of gravity.

Initially, as motion and stability studies began in the wake of the Fastnet, IOR typeforms were used as the model to study the impact of beam and lightweight on the likelihood of capsize. The result was that conclusions were drawn about the impact of light weight and beam that could not be universally applied to more moderately proportioned, non-IOR typeform, lightweight boats.

It was only as the problem was studied in more detail that it became apparent that neither wide beam at a single point in the hull nor light displacement was not the primary culprit in the capsizes of the Fastnet, but that the culprit was a collection of poor design decisions that were characteristic of the IOR typeform. Before being abandoned the IOR later changed a little to encourage greater seaworthiness. The IMS typeform that replaced the IOR, showed that comparatively light boats can offer all of the stability and motion comfort of earlier heavy cruisers. So much so that by the time of the Sidney Hobart disaster most of the boats that got into serious trouble were a mix of older (narrower heavier) designs like the Winston Churchill and IOR era designs, with the lighter, but more stable IMS era boats doing very well in the survival conditions faced during that race.

While you are right that a heavier boat may have a higher roll moment of inertia and may have substantially more area under the positive side of the stability curve than the negative side, that is not necessarily the case. Heavier boats often gain their weight by having heavier hulls and decks, heavier interior components, heavier engines and deck hardware, and greater tankage. Since these items are located comapratively close to the roll axis and the center of gravity of the boat, traditional heavier type boats generally do not have as high a roll moment of interia as a more modern lighter boat with a tall rig & deeper keel with a bulb. Similarly traditional heavier types often do not have as much area under their positive stability curve relative to their negative stability curved, because their heavier weight forces them to float deeper when inverted and thier weight is proportionately higher in the boat than a more modern lighter design.

Which comes back to the initial discussion about the CSF, I still say that there is nothing in that formula that really tells anyone anything useful about the relative stability of the boat in question. To poise my point more clearly, there is nothing in the CSF that would give a clue as to the relative stability of two otherwise identical displacement, length and beam boats even if one had something as a tons too heavy hull and interior and the other had a light interior and placed all of the saved weight in a lead bulb at the bottom of its keel.

Lunch done and I need to get back to work.

Respectfully,
Jeff

[kevlarpirate]

I have looked at the math, and physics and conclude the CSF formula to be reasonably well devised. I have compared several different production boats static stability curves to the formula and see a first order correlation, I therefore conclude it to be reasonably valid. The physics involved and the math can be defended. It is not meant to be exact and cannot be, that has been agreed. As stated before, it is a simple formula which yields a first order answer. There are of course exceptions when ballast bulbs substantially lower the cg. I would conclude that helps some. Also, a difference substantially less than 10 percent between two boats is likely inconclusive.

To defend the formula :

Here are 2 real production boats, brands withheld.
Boat 1. LOA=35 B=12 WT=10500 Limit of static stability 105 deg., Area of positive vs neg stability 1.51 , CSF 2.2

Boat 2 LOA=41 B=11.2 WT=18000 Limit of static stability 132 deg Area of positive vs neg stability 7.5 , CSF 1.71

Conclusion: CSF formula worked, a difference of .5

Re; motion comfort, I made my own list of some 35 boats I have sailed and did an honest account of what I recall for motion comfort and not surprisingly, I see a first order correlation to the Brewer developed motion comfort formula. I therefore conclude
The formula reasonably valid.
For the above boats which I have sailed motion comfort boat 1= 19, Boat 2=33

Conclusion: Formula worked out of 35 boats I disagreed with about 5 answers

As a final note: The Fastnet and Sydney races did not yield enough data to make for more than just a few conclusions. The individual wave height, face angle, angle to boat were all too variable and dominant inputs to yield conclusions about variables like CG. This is not just my opinion. The few conclusions we can draw are that small, wide, light boats were vulnerable and that longer ,narrower and heavier boats were substantially less vulnerable to the wave actions. Another thing learned is that these wide ,low ballast boats stayed inverted for long periods. They are very happy to be upside down, Just look at the pos/neg ratio, easy! The physics of all of this is quite basic. When a boat is designed to create righting moment by increasing beam thus allowing removing ballast, the stability curve becomes sinusoidal in shape, and the limit of static stability comes down (in degrees) and more dramatically the pos/neg areas go toward unity. ALL bad things! The 35 foot boat above has only 4000 lbs of ballast (and by the way, was designed after the Fastnet and the dreaded IOR). I might also add that many sailors draw invalid conclusions. For instance Boat 1. above is noticeably stiffer than boat 2. And someone who doesn’t understand the physics would say boat 1 is safer offshore, very wrong. Just a peek at the stability curve would scare me off in a hurry. That stiffness rapidly diminishes after 60 degrees, whereas boat 2 keeps building righting moment up to near 90 deg.
Finally, I conclude that any withdrawing of this formula (Brewer) was political because of pressure from manufacturers and such, because lighter wide boats are more fun and sales are important to make an industry grow. The trend is for speed and surfing downwind. That is only my opinion.

[Jeff H]

Let me see if I can shed some light on the total uselessness of the capsize screen formula by using your own example, but first I want to touch on one side issue. In your example, you cite two LPS (limit of positive stability) values one at 132 degrees and the other at 105 degrees. I suspect that you are looking at LPS's calculated using two different methods. It is not unusual to see the IMS method of calculating the LPS which results from the IMS rating calculations. This method of calculation does not include any volumes above the hull to deck line and is measured with the tanks empty. For a typical cruiser-racer, the LPS calculated using the IMS metrhod tends to be 10 to 20 degrees lower than the actual stability calculated with the full volume of deck structures, cockpit, and tanks full or partially full. I am guessing that the LPS of 132 is actually a full volume calculation and the 105 is actually a IMS calculation.

But that is somewhat irrelevant to the point that I wanted to make. For the sake of discussion lets just focus on your example of Boat 2 with an LOA=41 B=11.2 WT=18000 Limit of static stability 132 deg Area of positive vs neg stability 7.5 , CSF 1.71. You have not said what that boat is but with that high a limit of positive stability I am assuming that you are looking at something like one of S&S's or Allan Gureney's late 1960's aluminum race boats, which typically has something approaching a 50% ballast/displ ratios, in cast lead and close to a 7 foot or more draft. Lets compare that to something a little bit more normal like the C&C Designed Newport 41. The Newport is just slightly wider, but would only have a ballast ratio around 43 percent and a draft of 6'-3" and an full volumeLPS somewhere down in the low 120 degree range. And just to add to the discussion, I would like to throw in a bit of an oddball whose statistics are probably pretty close to those that you have listed. This boat was loosely based on the Herreshoff Meadowlark except that it was roughly 41 feet, hard chined, had a centerboard rather than leeboards, had quite low freeboard, and was built in steel. She was lightly ballasted with concrete poured into her bilges. My sense is that she had at best roughly a 30% ballast/disp ratio and the center of gravity for that ballast was probably at most was around 1'-6" below the waterline. I would guess that her LPS stability was something less than 100 degrees (and with her heavy solid wood spars was perhaps as low as, or perhaps lower than 90 degrees). She carried an enormous sail plan in a L.F. Herreshoff style sharpie rig and frankly kept you jumping in gusty conditions.

Okay, so those are our three boats to compare. Basically all are LOA=41 B=11.2 WT=18000. Therefore all would have a CSF of 1.71.

So, exactly which one of those three extremely different boats is the one that your subjective first order answer says should be a CSF of 1.71? Surely, it can't be all three. And if one of the three boats truly deserves a CSF of 1.71, what about the guy who is considering the steel Meadowlark with barely enough stability to make a decent marsh cruiser but thinks to himself this must be a very stabile boat because it has the identical capsize screen of 1.71? What does the CSF of 1.71 really tell him?

It tells him a first order piece of negative information; information so dangerously useless that if actually relied upon could prove fatal.

And as a final note, if you are correct that "The Fastnet and Sydney races did not yield enough data to make for more than just a few conclusions." then what was your point in saying, "As an anicdote (S.I.C.), no boat with ovet a 3:1 length to beam ratio was rolled in Fastnet 79."

And while you are basically correct that the races themselves provided little data, the100's of studies that have taken place over the nearly thirty years since the 79 Fastnet Disaster has provided us with a great deal of understanding of the factors that reduce the likelihood of a capsize, understanding that goes far beyond the Brewer's 1970's era simplisitic surrogate formula.

Respectfully,
Jeff

[mudnut]

lancercr,As you can see,Mathamatics can be debated.As you realize,at the moment,you are having a comfortable time with your sailing and it sounds like you might want to go a bit further.It must be hard with all this conflicting analysis information back and forth.Who and what,to believe,is up to you.But remember,Family safety allways comes first.Hope ya can make sense out of all these replies to make a sound judgement pertaining to family safty,I couldn't,Mudnut.

[rtbates]

Lots of folks dislike the capsize ratio numbers because they are in opposition to the desire of most owners. And that is to have a boat with as much interior space as possible. IE big and fat at the deck line. This leads to boats that when heeled tend to float up on that big fat side rather than putting the rail under and presenting the 'shoulder' to the sea. They also tend to be fairly stable when inverted on that big wide deck with all the mast and sails pointing down helping to hold her there. Old designers knew this very well and as such designed blue water sailboats with modest to narrow beams. Carl Alberg is one. I sail one of his designs, the Cape Dory 25D. Would you believe that in 30kts true 8-10 foot seas my 25D has a much better motion and is orders of magnitude less affected by the seas than a Hunter 34? I've been there on both and it's very true. The biggest difference is when sailing off the wind with a quartering sea striking the transom at about 45 degrees. The Hunter would get it's big fat hindend caught by a wave and it took some concentration by the helmsman and constant rowing of the wheel back and forth to keep her sailing in a straight line. My skinny little 25D on the other hand would hardly move as the wave passes under. It was my first real appreciation for why blue water sailboats benefit from having a full keel. The sea and 'being at the dock' have two diametrically opposed requirements and todays designers lean toward design that favor 'being at the dock'. Hence all the talk tending to diminish the importance of such factors as capsize ratio. You might want to read John Vigors book, "Seaworthy Offshore Sailboat: A Guide to Essential Features, Handling, And Gear" as well as "Twenty Small Sailboats to Take You Anywhere".

[ssullivan]

Randy... I agree with you completely. Everything you say makes sense from the perspective of having been out on both boats you use as examples.

I have a problem understanding all of this as it applies to my own boat, though. When I do the capsize screen it comes in at 1.8, which isn't stellar, but isn't bad either. However, the boat is *very* beamy in comparison to your design. We are 45'LOA with 13'8" beam. The boat has a reptuation for being an adequate (but not perfect) blue water boat that sacrifices some roll ability for increased speed and stiffness. (we average 6 knots overall in all weather) We are less tender than narrow body boats and keep upright well initially, but I fear your example of turtling would apply to this boat, despite the fact that the capsize screening ratio says we're fine. Does this disprove the screening ratio?

Also, much of our thought is done in a water tank with no waves. If there was a wave strong enough to put a boat to 180 degrees, surely there will be one along shortly that will nudge it from an inverted point of stability, no?

Just some questions. I agree with your post in its entirety. Just wish I could get a better handle on this stuff. We need more naval architects on this forum!

[Pblais]

Does this disprove the screening ratio?

I would argue it never was actually proved so there is nothing to disprove. Carl Alberg didn't set out to prove the capsize ratio he instead designed a good boat. The fact that it seems to support the rule does not prove the ratio valid nor does it lessen the observed performance of the boat design.

There is an assertion this very simple computation defines complex behavior properties of boats in the water. It does not mean that it really does. I think Jeff makes the better case that the number falls apart in too many cases.

Even if you still want to assert the capsize ratio has real meaning the actual ratio when used by even the most ardent proponents is really only a greater than 2 or less than 2 screen. Its only meant to be a is more or is less test. The decimal point means nothing.

[ssullivan]

Ahh.... maybe there is something to be learned from the actual name of this ratio - a screening ratio. Seems as though it might be used as a very preliminary "screen" before delving into the real numbers. (may have been said earlier in this thread, but seems more and more true)

[kevlarpirate]

Jeff,
Thank you for shedding some light on the subject, now here is some light of my own.

First of all you are wrong in your assumption that the calculations for boat 1 and 2 were under different formulas. They were the same. Both done by USYRU. very well respected, no intelligent dispute there. Both were worst-case numbers using no positive
buoyancy factors like coach roof. Both calculated identically using offsets.

2. You are wrong with your assumption of boat 1. Boat 1 is a Newport 41. And with a worst case of 132 deg. means that full volume will increase that number closer to 140 which means you were wrong about that also. And you were also off about the ballast ratio and the beam, which correctly are 11.2 feet and 45.6% (I own one and it was weighed)

3. Your example of the Meadowlark is still just an attempt to corrupt the formula as your 500 lb. sandbag on the mast tip. We are comparing apples with apples here and the formula works with (as I said) most production boats built, and most certainly applies to the above example: CSF 1.71 boat 1, 2.17 boat 2. Difference .46 (significant)

This brings up an interesting note: High sigma anecdotes, which you use, have little if any value in a technical discussion, which this is after all. I am not interested in quibbling about anecdotes and I don’t need boatbuilding lessons either. This issue involves physics and math and some technically educated people who developed it.
Please read:

CAPSIZE RISK = beam/(disp/(.9*64))^.333
An empirical factor derived by the USYRU after an analysis of the 1979 FASTNET Race. The study was funded by the Society of Navel Architects and Marine Engineers (SNAME). They concluded that boats with values greater than 2 should not compete in ocean races. Values less than 2 are "good". The formula penalizes boats with a large beam for their high inverted stability, and light-weight boats because of their violent response (low roll moment of inertia) to large waves, which are both very important during violent storms. It does not indicate or calculate static stability. Some modern coastal cruisers and many racing designs have problems meeting this criteria. An interesting note, the study concluded that static stability was relatively unimportant in predicting dynamic capsize. Beam and weight were much more important factors. Wide boats give waves a longer lever arm to initiate roll and light weight boats require less energy to roll over; both undesirable attributes in a cruising boat. The template value of 1.7 is very low.
Gee lets see; USYRU and The Society of Naval Architects and Marine Engineers according to you have developed a “totally useless” concept. Hmmm, seems a bit arrogant on your part. I guess we are to believe they are just a bunch of morons! And I guess the above statement is miss-information, but lets just pretend they knew what they were talking about.

Back to boat 1 and 2, Which by the way was a J-35, (#2) is a very revealing and reasonably accurate real world case where a buyer may be deciding to do some serious cruising. It so happens, the CSF 2.2 number has a very high correlation it’s static curve pos/neg. ratio, and also to the above statements by USYRU. Was that just by chance? A fluke? I doubt it. We were not dealing with bumblers there.

Lets explore a hypothetical case: (static only)
If both boats were hit broadside by a large breaking wave, and large enough to heel both boats to 135 deg. Then both boats would roll, however Boat 1. has a 450% better chance of re-righting itself as subsequent waves come along. (the ratio of areas under the negative side of the curve (energy required ), for both boat cases) (.134 Boat-1, vs. .593 boat 2). Other issues are come into play in a dynamic situation also. In fact, your 500 lb sandbag would actually greatly help the angular moment of inertia, resisting the roll, but not to overcomplicate this simple “static” problem I have described, and staying with this point, the high “inverted stability” as described by the, according to you, hopelessly misguided, in the above statement is a big problem. It likely ends in drowning, not just broken bones.

For those who need an explanation, when a boat is inverted (capsized), even though the center of gravity is now above the center of buoyancy, the boat is stable. This is because when tipped in a direction (to re-right it), the center of buoyancy moves out to the side, out from under the center of gravity. This resists in the form of an equal and opposite torque to the “tipping” input. Thus returning the boat to a 180 deg. Inverted position when the force is removed. Those tipping inputs were the next waves that came along. The wider the boat, the farther the CB can move off to the side and the more counter torque developed and the more stable the boat is upside down. These are the areas under the negative side of the curve we refer to and represents the energy needed to pass the zero point of the curve and re-right the boat. The skinnier boats have much smaller relative neg/pos number, and therefore have a much higher chance or re-righting. And by theory a narrower boats angular momentum allows rolling 360 without stopping at 180 deg. This is because the CB when passing through 180 cant move out far enough to generate enough counter torque to stop the roll. This general knowledge was learned from the Fastnet disaster. And by the way please don’t use an older wooden boat, which broke up and sank in Hobart as an example to rebut this. That is dis-information. Also this is simple physics you can do this experiment in your tub.

Now, as I said, the CSF math and physics produce closure in the real world for most cases. That is what science tries to do, and it has been done here with reasonable success. And that small calculated differences are negligible, but .5 counts.
Yes, it can be corrupted by extreme examples i.e. sandbags (and that’s questionable), and then boats full of concrete. What next, a Spanish galleon? People are out there trying to make intelligent decisions on what makes for a good offshore design. Thirty-five years ago most boats had Naval Architects behind their designs and were tank tested at places like the Stephens institute. Like the C&C team designed Newport 41, and Bruce King designed Ericson 46, (which I also own). Nowadays many boats are built around interiors or they end up as big sister surfing dinghies like the Melges 32. These boats are fun. They are fast and more fun than mine. But that is, for the most case where it ends. The J-35 is very fun to sail, yours probably is too. But in more stressing situations like going up in a big sea they have bad manners, like excessive weather helm, round ups, roll steering, burying their bows in a head sea, rudder lifting out of water and cavitating. They also get knocked around a lot, which wears out and injures crew. Need more?
Their behaviors only get worse when things get really bad like a Fastnet. That was why SNAME was formed. And with what effect? Many boats have just gotten worse because they are chasing interiors and off wind surfing. Considering the Newport 41 was designed almost 20 years prior to the J35. What does THAT tell us about boats getting better? Not so fast. Something else is going on here. Speed sells! And so does interior volume. I guess that is what better means today! Staying on point, this discussion is about capsize risk, That is what I thought we should mean as “better”, guess times have changed.

In response to your question of which boat I would choose, I would respond that If anything is dangerous to a sailor it would be deluding themselves into believing they know what a safe design is, but are really only defending their own purchase and the emotion behind it, which I believe is exactly the case here. Your boat doesn’t sound like it faired well under this analysis, resulting in your attempt to denigrating the science and math behind the CSF. Take heed my friend, it is simple and reasonably sound.

These new lightweight boats are great for venues like Key West Race Week, but if the old SORC were still running, where you were pretty well guaranteed to get spanked hard on the St. Pete–Lauderdale and Nassau race, I would never be on one! Nor would I try to fit one out for a trans oceanic passage.

Now, if you want to talk science great, but please don’t bore me with more history and wacky anecdotes.
You talk of the hundreds of studies (hundreds??). With that many studies there should be a wealth of statistics, Where is it. Why do you have to illustrate with some oddball single issues.
The variables present ,(especially waves) in the bad races we talk about have left us with many questions and it is because small statistical samples don’t allow for accurate conclusions. This puts us back to the test tank and deducing physical behavior from controlled input parameters. Just like the empirical studies done by USYRU. That leads to mathematical expressions which describe physical behavior, all based on physics that makes sense. Please go back to the USYRU and read it carefully. The message is that wide, lightweight boats are suspect, and rightly so. The actual CSF number calculated between boats becomes significant the bigger that difference is. Little differences like a couple of tenths are likely insignificant, however when the differences become .4 and .5 and .6 it is time to look deeper, and guess what, very likely we see things like pos/neg. areas getting closer to unity as I stated in my last post. Any boat with over 2 for a CSF in my book better have some pretty convincing reasons to gain my confidence. Remember the number 2 was based on Fastnet data, which was a real world situation which any cruiser may encounter. And remember, we also have other data that dramatic CG changes were inconclusive. So who are you to be spouting off how useless this it? I, and many others have followed this issue for some time and I have never come across anything to warrant flushing it. It is a political hot potato with designers and manufacturers between a rock and a hard place and also some upset owners like yourself. Sorry, that’s life. My boats rate 1.71 and 1.66. Does that make me comfortable? Actually it does, some but I suspect there is more, especially size and mass. The rest may be second order stuff. Obviously a 16 ton boat will be much less fatiguing on me than a 9 ton boat, due to any waves being relatively smaller to the bigger boat. This I have proven already. Motion is less, pure and simple. Obviously the mass and the beam are first order parameters with any expression derived. Length of the boat is fairly linear with these and may become more important if boats became excessively long for their length. But even with that, now you have to start talking structural issues. So therefore that return may be diminished. With all of this, I say it is fool hearty to discount the USYRU work. It may be a little simplistic and but it was a good and maybe very good start. Using words like “totally useless” is laughable. You really have not shown any real data to rebut it, only wacky stuff that doesn’t exist, inconclusive data or is super high sigma.

[ssullivan]

Kevlarpirate:

Following your last post, it sounds quite reasonable. The trouble comes when I, as a simpleton owner without the knowledge you have of naval architecture come to try and understand the screening ratio. My boat comes out to 1.8. It's also very beamy. What does that mean? How do I weigh that number out against others? How am I to understand what some of the aspects of rolling would be in this boat?

I ask because I'm so curious. Nearly nothing is written on my boat. Only about 30 were ever made I read somewhere. It has a certain reputation, but I desire to put the numbers behind it. Are there any second order equations I should be looking at in addition to the screening ratio? Also, just how comfortable should I feel with a 1.8?

Thanks.

[kevlarpirate]

I see a 13'8" and a 13'3" beam and a weight of 26000 mentioned for your boat. My 46 Ericson is 13'3" and 32000 lbs. It would take a good size wave to upset either of these boats. I don't see your beam as excessive. I have sailed a Chance 46 with a 14'6" beam and I would say that is wide and it looked too wide. As long as your boat is structurally sound , equipped properly and sails well, I would not hesitate crossing an ocean. About this screening formula, 1.8
sounds fine. I wouldn't be concerned unless you hold that beam all the way to the transom. A top down looking view of the Ericson shows a very tight stern, almost symetrical, and is is amazing how balanced the boat is even when very overpowered, which I have deliberately done as a test. With your boat, you may not want to sail very agressively, just power down a bit. keep the heel down if a fat butt concerns you. By the way, I think all sailors are way beyond simpletons. Jet ski guys.. now there's a simpleton for you! good sailing.

[ssullivan]

Jet ski guys... Ha ha ha! Thanks, Kevlarpirate. I appreciate the 2nd opinion there. We do not carry the beam aft. In fact, the transom measures only 7.5 feet wide. We are beamy only amidships. Again, thank you very much for posting. I appreciate the extra analysis. It's hard to come by second opinions in this area. I had done the capsize screen ratio prior to purchase, and it was 1.8, so... I figured it was ok. Glad it still seems that way.

[kevlarpirate]

Very Good, I don't know however, if you will be able to find much more about your boat. Much is learned by sailing her on heavy days close to home.
If the boat has some bad manners, it will only get worse in a bigger sea.
But if you find a weakness, a modification may help. On my Newport, a better and bigger rudder was a big help.
The ability of a crew is important, but many cruisers are under crewed, if not single handed or with one partner. When I sailed SORC (that is, the real SORC through 1975) we had bullet proof crews, I was never concerned about the ability to manage the boat in bad stuff. As we go to shorter handed sailing and passagemaking however ,we need to rely on the boat more as a safety net , and size matters.

But in my haste the other night to get to the RX to buy aspirin, sedatives, nerve relaxers and various other pain killers, after having to make such a tedious and long post, I used an improper term when stating a linear relation between length and weight. What I meant to suggest is that the CSF may not work if the boat were very long and light meaning that it WOULD be more linear of a relation. If a boat were 80 feet and a 15 foot beam and only weighed 25000 lbs it would have a CSF above 2, however I would guess it could fare well in a nasty sea. That is as long as it held together. In todays world length goes with the cube of weight generally, therefore with boats like ours I think the CSF is very valuable.

[rtbates]

Originally Posted by ssullivan

Randy... I agree with you completely. Everything you say makes sense from the perspective of having been out on both boats you use as examples.

I have a problem understanding all of this as it applies to my own boat, though. When I do the capsize screen it comes in at 1.8, which isn't stellar, but isn't bad either. However, the boat is *very* beamy in comparison to your design. We are 45'LOA with 13'8" beam. The boat has a reptuation for being an adequate (but not perfect) blue water boat that sacrifices some roll ability for increased speed and stiffness. (we average 6 knots overall in all weather) We are less tender than narrow body boats and keep upright well initially, but I fear your example of turtling would apply to this boat, despite the fact that the capsize screening ratio says we're fine. Does this disprove the screening ratio?

Also, much of our thought is done in a water tank with no waves. If there was a wave strong enough to put a boat to 180 degrees, surely there will be one along shortly that will nudge it from an inverted point of stability, no?

Just some questions. I agree with your post in its entirety. Just wish I could get a better handle on this stuff. We need more naval architects on this forum!

Sean:

Paul nailed it. It's only a ballpark figure that needs to be used along with the vessel's other characteristics to be meaningful. My biggest observation was the benefit that exposing a narrow hull, especially the stern, to following seas had on the vessel's tendency or lack there of to want to pivot the stern around the keel. Granted I'm not comparing apples to apples because besides having a narrow hull I also have a full keel. Another feature that has been lost on modern boats. Take a full keeler, narrow hull and a fin keeler, beamy hull out in 10 foot swells and sail with the swells striking the stern at 45 degrees or so and the difference is night and day. Want to see a few Cape Dory hull? Go here. you'll find Seraph in there as well as a CD30 and 33, 36 & a 40. Seraph is the baby 25D at the top.
http://www.todddunnmicroyachts.com/C...ry_Yachts.html

[GordMay]

FWIW:
”Well Heeled” ~ from European Boatbuilder
Andrew Blyth & Tom Nighy review the stability & buoyancy requirements of sailing boats.
Goto: http://www.ibinews.com/ibinews/ebb/tech_8_pt1.html