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Old 03-02-2007, 17:05   #31
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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?
Well, I've been on 'em in lots of different seas and that's not what I've eperienced. (Exept they -are- fun to sail) They do tend to have a bumpy ride to weather. But a bumpy ride is far better than fearing the lee shore..

I'd not trust any formula on stability that ignored draft.

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Old 05-02-2007, 16:28   #32
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Sorry for having to mention the boats, I knew this would happen, and I made a mistake. Jeff however has no problem with his negatives on good offshore boats like the Cal 39 and probably many others. Before being provoked, I referred to boat 1and 2 with anonymity.
I did not mean to say the J35 possesses “all” these bad qualities, J boats are strongly built. As for manners there are much worse offenders. My worst experience was a 37-foot lightweight, fat transom, noodle mast, fractional, (very different than a J35) in a 5-foot short rear quartering sea. I was being swung through over 45 degrees with tiller forces exceeding 60 lbs on the tip, with cavitations. I am a strong guy, but in ten minutes I had had it. BTW it was NO fun! This was in now way a big sea. I am in the camp of high degree static stability boats with high Pos/Neg ratios. Every owner makes his choice considering what he will want to do with the boat. My boat will point high enough and can do it in a bigger sea t(without pounding) than most 40 footers, but will be heeled more. That doesn’t bother me, To me it’s fun, but that’s me.
Performance boats with wide beams are that way so they can get righting moment from buoyancy in the beam and can reduce weight in the keel, displacing less water, which allows them to surf in lower wind speeds and closer to the wind, meaning fun. Boats like mine try to surf but can’t most of the time, (not fun). The cruiser wouldn’t care about this. The buyer must be aware that there is a consequence to this and that is born out in the static stability curve and other behaviors. Wide transoms, what they refer to as “powerful transoms” allow more control under downwind spinnaker (less roll) however they pay for it upwind and on a reach. You already know that. My point is, That’s fun, but everything a compromise. People should be careful as to what they call “better”

A sad thing here in this thread is the lack of understanding of the scientific process. The USYRU and SNAME had budget and brains to study this many years ago.
I would guess they had developed several models, which they refined, with help from very adequate data processing. (the physics and math don’t change)
In those models (mathematical) they discovered along with lots of empirical data, the parameters that were important and those that were second order. In the end, mathematical expressions can de developed, which may appear too simplistic but can be quite accurate at least within reasonable limits. I think in this case of the CSF those limits cover most all boats we live with. Many input parameters may be discounted through the development of the model because they don’t affect the outcome significantly or are just inconclusive when tested. With the CSF, I would bet that center of gravity is a second order effect and that moment of inertia is more important. Take the case of a boat loosing it’s mast and then being more susceptible to roll over. One may conclude intuitively that it would be less vulnerable to roll over, because the CG has been lowered a lot, however, being more of a dynamics than a statics problem, the moment of inertia (angular) being reduced from having lost the mast will be more dominant in determining the hulls behavior in the roll. Moment of inertia is based on distance “squared” from the axis of rotation. It is not linear relation like CG. It is an exponential relationship.

One should not think that a simple expression has no value just because it is simple.
This was not a back of the envelope estimate. The statement gives reasons for the layperson why only the beam and weight were used. It is assumed that all of these boats have keels and masts. The fact that one has a lower center of gravity is tempting to believe to be a dominant player but again it appears to not significantly affect the outcome. I think that mass and beam deliver the lion’s share of the message and are not that far from the truth.

I have also raced a C&C37R against Express37 and J-35 off Florida in wind and sea that would keep many sailors at the dock. The boats did well. The J35 won. All boats rated 72. My statements refer to dangerous conditions as in Fastnet. There are races still like Fastnet and Hobart, which because of their distances pretty much guarantee a storm will get you. In these races and for crossing oceans is where these stability issues should be very important. These involve extreme conditions.
Thank you, sorry for the mistake of mentioning names.

Any NA’s weigh in, If I’m wrong prove it with science. I said enough. I’m OUT ! A little math

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Old 05-02-2007, 16:40   #33

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Kevlar, before you head out, can you suggest any links or reading in basic naval architecture 101? I have BS in Physics and though rusty, would love to wade through some of the math you are speaking of in this last post. What are some good resources?
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Old 05-02-2007, 17:15   #34
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Sully - You and I have very simillar numbers. With my Endeavour 43, I have a 14.5' beam LOA being roughly 45 (I have no idea why LOA is important ... seems like LWL would be better - but I flunked diff e q - so :P ) anyway, my number came out 1.808 (my displacement in pounds is 33,000 - although I think it is more fully loaded). Does that mean my boat and yours would react the same? hummmmm dunno ... answers?
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Old 07-02-2007, 11:04   #35
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I thought that it might be helpful to hear an outside opinion on this topic so I contacted John Rousmaniere who was the author of a number of books dealing with yacht design for offshore use and offshore seamanship. We exchanged a number of emails and I have posted these below in the hopes that it would shed some light on this topic.


Mr. Rousmaniere

I noticed that you had posted a comment on a Sailnet discussion group and so hope that you don't mind this direct contact. I am a long time fan of your book, which I consider a seminal work for its day. If I remember correctly you were also involved in the development of the Capsize Screen Formula (CSF), which I consider an important first effort to provide an empirical formula to evaluate capsize safety. The CSF has become a frequent topic of discussion on the internet and, IMHO has taken on an importance in making individual boat selections that is beyond its intended purpose and frankly, in many, if not most individual cases can be very misleading in its results.

As I see it, perhaps erroneously, the CSF lacks any data on such factors as weight and buoyancy distribution, or even the vertical center of gravity and vertical center of buoyancy at various heel angles, or any data on dampening, which, at least in my mind, collectively would seem to be the major factors that control motion comfort or the likelihood of capsize. I also have come to believe that the formula overemphasizes the importance of heavy displacement and narrow beam, without looking at where that weight is placed or how that beam is distributed, but again I am an amateur and could be very wrong.

If you don’t mind, I would truly appreciate hearing your comments on the CSF nearly 30 years after its development.

Jeff Halpern

Thanks Jeff. You’re not the first to ask this question.

First, I didn't have anything to do with developing the screening test, though it is best described in a book that I edited and that people who have strong opinions on the test really should look at. It’s DESIRABLE & UNDESIRABLE CHARACTERISTICS OF OFFSHORE YACHTS (pp. 76-78), and it’s still in print.

Among other things, there you'll see that it was developed in the wake of the '79 Fastnet catastrophe to identify extreme IOR-type boats with dinghy-like hulls that may be unsuitable for taking offshore.

Now, too many people try to make way too much of it. Note that in ANNAPOLIS I carefully use the words “simple,” “estimate,” “and “guideline.” This is NOT a measure of stability range (the best gauge, which any reputable boatbuilder and designer will provide). It is NOT a test of ultimate seaworthiness. It IS a simple test that (to quote its creators of 20 years ago, Dick McCurdy and Karl Kirkman) offers "a general indication of a boat survivability using only data that comes readily to hand."

To put it another way, it's a way for someone considering the purchase of a boat to get an approximation of stability (resistance to capsize) from the basic dimensions, and then compare that boat IN THAT WAY to other boats. Call it a boat show test or a tire-kicker's test. Nothing like this test existed before it was developed, and I don't think another one has some along since.

Again, the ultimate test is stability range, which is based on many technical factors, and about which there’s lots of discussion in DESIRABLE.

What does the screening test take into account? First, beam. Wide beam means low range of stability. A wide boat will flip earlier and also stay turtled longer than a narrow boat. Second, displacement. Heavier boats have a greater moment of inertia than light ones, so are harder to flip. What the screening test does is provide a number that assists in making a comparison, which I think is extremely helpful.

I hope this is helpful, too. Thanks for writing. Maybe it should go up on the SN blog.


Thank you for your quick response. I did not realize that Karl and Ian were the authors of the Capsize Screen Formula. (I know Karl from his time here in Annapolis and Ian from the Chesapeake Sailing Yacht Symposiums.)

I don't know whether you have time to discuss this further. At the heart of it, with the extreme differences in boats, I have trouble saying that heavier displacement equates to more stability or that a wider max beam equates to greater likelihood of capsize.

Here an example that reflects my thinking on displacement, if we look at the 1960's era Bill Tripp designed Medalist and Galaxy. These boats had essentially the same hull design, but came in a number of configurations. A Medalist with teak decks and the original wooden spars would weigh roughly 800 lbs more than Medalist with aluminum spars and non-skid decks. The Galaxy was nearly a 1000 lb lighter boat (lighter interior, hull and rig) but the Galaxy had a slightly deeper bulb keel. The CSF would predict that the Teak decked Medalist would be the least likely to capsize and the Galaxy the least, but this is opposite of the reality.

Similarly with beam, within some degree of moderation, if we look at a narrower boat with its max beam carried further towards the end of the boat so that roughly 60 % of the length of the boat is within 80% of max beam (a boat like an Island Packet, Pacific Seacraft or some of the older CCA era designs) vs a perhaps 10% wider boat that has its max beam carried over a narrower zone, (perhaps with 80% beam carried over something well less than half of the length), the larger deck area would suggest that the narrower boat might be more prone to capsize.

It is for those reasons that I have concluded that the CSF potentially can provide dangerously misleading information. I'd love to hear your comments on this.

Thanks for getting back. These are important questions.

Once again, the screening test is not a prediction! The idea behind the screening test was to give sailors a quick and easy approximation that should lead to just the sort of analysis that you employ on the two Tripp boats.

The only number that can be called a prediction of capsizability is range of positive stability, also called stability range and range of vanishing stability. That's the maximum angle of heel at which a boat wants to come upright. One degree beyond that angle of heel, and she'll capsize.

It's derived from a complicated calculation using maximum beam, displacement, center of gravity, and other characteristics. It's a fact that beamy boats have lower stability ranges, in the 100-degree area (a Bermuda 40 is a good example) than narrow ones (meter boats are in the 170-degree range. The difference is inherent in the two hull forms. Today 120-130 degrees is considered a safe number for an offshore boat.

But even if a boat is extremely wide, if it's heavy the weight provides inertia against the capsize action, which in a light boat can be extremely violent. There's a series of drawings in my book FASTNET, FORCE 10 that show just how quickly this happens.

All of this and much more are covered with clarity by Olin Stephens and Dick McCurdy/Karl Kirkman in the first four chapters in DESIRABLE & UNDESIRABLE CHARACTERISTICS OF OFFSHORE YACHTS. Take a look. Marchaj's SEAMANSHIP is a good source, too, though not so clear.

It was Dick McCurdy (now deceased) who developed the screening test with Karl Kirkman, not our friend Ian.
Best wishes,

John Rousmaniere

Thank you once again. I understand your point about extreme light weights, especially when considering boats with similar weight distributions, as was more typically the case when you wrote DESIRABLE & UNDESIRABLE CHARACTERISTICS OF OFFSHORE YACHTS. Today, lighter boats often have significantly lower centers of gravitie and less flare, and so are able to slide on their topsides rather than do the IOR era snap roll. The newer IMS derived lighter weight boats, with their deep bulb keels, also often have similar limits of positive stability to the narrower boats of yore.

Would you mind if I post our discussion on one of the sailing forums?


Not at all. Tell me when and where it goes up, will you?

Another point about IMS boats is that they're narrower than the IOR extremes. The capsize screen was used for at least a while in the IMS to penalize excessive beam in order to increase stability.

You follow this pretty thoroughly. Do you work in a design office?

John Rousmaniere

Here is a link to the site where I am posting our discussion:

"You follow this pretty thoroughly. Do you work in a design office?"

Thank you, No I don’t but that's very flattering. I am an architect (buildings) with my own office in Annapolis, Maryland. Growing up I was tutored in yacht design by a designer with S&S and have worked in a yacht design and naval architectural offices. I read a lot and attend various Yacht Design Symposiums as a way to keep current. I grew up sailing in the days when CCA boats were still hot racers and have spent much of my life cruising, racing and coaching, which has gotten me out on a very wide range of boats.

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Old 08-02-2007, 05:27   #36
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My correspondence with JeffH concerning the capsize screening test was conducted by E-mail before I knew about this thread. Now that I've read the thread (well, I've read some of it and scanned the longer parts -- this topic does stimulate opinions!), I see that many people appear to agree with me along the following lines:

What it is NOT: A prediction. A measure of seaworthiness. A measure of a boat's motion. A substitute for stability range (the best indicator of a boat's likelihood to capsize, available from the designer, builder, or rating certificate).

What it IS: A simple estimate of a boat's chances of capsizing in extreme offshore conditions. An estimate of the likelihood of a capsized boat's remaining turtled for an extended period of time.

Someone asked about further reading. I recommend the following. All are in print.

John Rousmaniere, ed., DESIRABLE & UNDESIRABLE CHARACTERISTICS OF OFFSHORE YACHTS. Chapters on the screening test, stability, rigs, and just about every other aspect of offshore design. Authors include many designers -- Olin and Rod Stephens, Mitch Neff, James McCurdy, Karl Kirkman, Bill Lapworth, Ted Hood, etc. -- plus chapters on ventilation, anchoring, emergencies, medicine, etc. A book I'm extremely proud to have been involved with.

C.A. Marchaj, SEAWORTHINESS: THE FORGOTTEN FACTOR (demanding but rewarding)

Ted Brewer, UNDERSTANDING BOAT DESIGN (the basics, clearly presented)

Olin Stephens, ALL THIS AND SAILING, TOO (his autobiography, with plenty on design characteristics)

Steve and Linda Dashew, SURVIVING THE STORM, SEAMANSHIP (lots of first-hand information and observations by experienced sailors and designers)

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Old 08-02-2007, 06:31   #37

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John, thank you very much for the post. The additional reading list is fantastic. Hopefully, you can stick around and correct us from time to time, as we often attempt to discuss various qualities of boats at sea, but much less often have the math to back it up.

That post should put this thread to rest.

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Old 08-02-2007, 06:42   #38
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Thanks for that!

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