Capsize Ratio's

[cal40john]

You're mixing up your formulas. Ted Brewer came up with the comfort ratio as tongue in cheek.

The simple capsize screening formula comes from the committee that studied the Fastnet disaster.

From the Kirkman and McCurdy chapter, Avoiding Capsize: Practical Measures in the book Desirable and Undesirable Characteristics of Offshore Yachts after the section describing a complex formula for capsize screening:

"We felt that it would be very desirable to get a general indication of a boat's survivability using only data that come readily to hand." (italics in original text.)

This section then describes the capsize screening formula.

From: Ted Brewer Yacht Design

COMFORT RATIO (CR): This is a ratio that I dreamed up, tongue-in-cheek, as a measure of motion comfort but it has been widely accepted and, indeed, does provide a reasonable comparison between yachts of similar type. It is based on the fact that the faster the motion the more upsetting it is to the average person. Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.

Originally Posted by Bash

No. It's total horsefeathers. Brewster came up with the formula "tongue in cheek," by his own admission. An attempt to sell narrow boats.

People selling narrow boats will sometimes use the ratio to attempt to convince the uninitiated that their boats are superior to boats with greater beam. Another reason to ignore yacht brokers when they get into their spiel. These are people who spend their lives either poisoning the well or fouling the nest.

Check your BS meter for proper calibration. If someone starts telling you that Vessel A is superior to Vessel B because of its capsize ratio, your meter should be pegged to the brown end of the spectrum.

[Bash]

Originally Posted by Hud3

Let's see, capsizing is rolling over on the long axis of the boat, pitch-polling is flipping stern over bow, so what do they call flipping the bow back over the stern?

I've done it in a sea kayak, but I'd be breaking the anti-obscenity rule were I to describe the maneuver as I did at the time.

It started with a reverse broach. Then, briefly, a tail-stand was involved. Then it got ugly.

[s/v Jedi]

Originally Posted by Bash

I've done it in a sea kayak, but I'd be breaking the anti-obscenity rule were I to describe the maneuver as I did at the time.

Did it with a dinghy in the surf off a beach in the Dominican Republic... killed my 15hp Mercury outboard. The lesson was to not run out of gas while maneuvering through the breaking waves... kills your timing

ciao!
Nick.

[barnakiel]

Originally Posted by Paul L

Not. (...)

;-)

Apologies! I thought it was clear that the second half of my post was a joke!

barnie

[Paul L]

Originally Posted by barnakiel

;-)

Apologies! I thought it was clear that the second half of my post was a joke!

barnie

Barnie
I know its a joke -- take the formula to its ridiculous extreme and it becomes funny . I just think people take these formulas to seriously. You have to look at the specific boat. I know it was created as a quick and dirty number based on easily available info (although the as-built, as sailed, displacement numbers are not that easy to get correctly). If you used this formula to analyze the 1998 Sydney Hobart Race disaster, I don't think it would give you much of anything meaningful.

[kefroeschner]

In my admittedly old-fashioned opinion, a proper sea-going vessel should have a righting moment vs angle curve which is POSITIVE always. (I would make exception for catamarans of good design, and really big.)

Modern designs with righting moments that decline rapidly after about 60 degrees are prone to capsize because the roll inertia in a wave can overcome the decreasing resistance to further roll. And with a curve that goes negative at around 120 degrees, they will only stand up again if you get lucky.

Just as we left California for Florida there was a story in the news about four bodies found floating near a capsized boat off southern California. I have not heard any details.

As to SCs and Js and the like being perfectly safe ...? Each to his own.

[barnakiel]

Originally Posted by kefroeschner

In my admittedly old-fashioned opinion, a proper sea-going vessel should have a righting moment vs angle curve which is POSITIVE always. (I would make exception for catamarans of good design, and really big.)

Modern designs with righting moments that decline rapidly after about 60 degrees are prone to capsize because the roll inertia in a wave can overcome the decreasing resistance to further roll. And with a curve that goes negative at around 120 degrees, they will only stand up again if you get lucky.

(...)

Well. There is a number of issues here:

- in fact, old fashioned ships (e.g. tall ships) may have had the curve positive to LESS than 90 degs ... (sic!),
- a big boat is less likely to be rolled over due to entailed required forces, hence it will have lower stability angles,
- if a boat encounters a wave big enough to roll her over, it is equally likely to encounter one to right her back,

Etc..

BTW You are not likely to find many sailing boats that have the curves always positive!

b.

[Eleven]

It's no good having a zero stability angle of 70 degrees if the ports are floppy plastic and the hatches don't dog down and keep the water outside.
Mono's go into wind in heavy weather because a wave from the rear will sink them if the companion way isn't securely closed.
Cats go downwind in heavy weather, escaping most breaking waves by speeds which give them extra stabilty. 15+ kts is a huge dynamic effect on the hulls. Sharp or vertical hulls can dig in, pitch poling. See the racing cats.
A cruising Prout was never been known to pitchpole or capsize. But they are slower than most modern cats. Do you want to get there quicker, or always get there?

[kefroeschner]

Barnakiel:

You are quite correct about the old ships -- some even capsized on launching. Was it the 'Wasa?"

As to finding a sailboat with a fully positive righting curve, you are quite correct again. You have to look for the older designs -- Morgan, Sparkman & Stephens, Irwin, etc. Almost anything prior to the CCA, RORC, IOR and whatever goofy measurement rules they have these days. First clue is ballast to displacement ratio. Anything less than 50% is asking for it. My IOD is 7100 lb, 4100 of it lead about 6'deep and she will never, ever capsize. Might sink, but that's any other issue. Our Morgan 45 OR is 25,000lb, 12,500 of it lead 7 feet deep, plus 300 gallons of water in the bilge so, again, capsize is just not on my list of worries.

And, Eleven, I quite agree it's no use having zero stability -- at ANY angle of heel. But it is more or less unavoidable at 0 and 180. Anything that will turn turtle and stay that way for more than 15 seconds is a dinghy (Windward Passage was the first big one as I recall) and ought to stay very close to rescue.

Cats are another story and deal with the issue by not sinking. I have a lot of respect for that approach and almost built a 69' cat modeled on the Tongan double canoes so beautifully illustrated in Cook's journals by his Secretary whose name I can not recall. I had lifting (horizontal) hydrofoils on the bows in the design as a possible way to keep her head up, but the hull design with LONG overhangs may not have needed them.

And your comments about out-running the waves rather than having them in the cockpit trying your usually inadequate c'way sounds exactly like what I read in Arthur Piver's books, oh. so many years ago. He's still right. But I must say that most of the cats being turned out today would do best at a pier in some backwater along the ICW.

Nice discussion. Thanks for responding. Any news out there on the four drowned in S Cal? Could have been a fishing boat for all I know.

[kefroeschner]

One more thing, Barnakiel, If a boat encounters a wave big enough to roll her, she will likely encounter another to roll her back ...? Yep, quite true, but ... when? In the Fastnet, most of the boats that were rolled or capsized came ashore, most of them right side up even -- that's why the current crop of idiots think that if the area of the negative part of the curve is less than the area of the positive part, then you are ok, because, statistically speaking, things will get better.

However, in the Fastnet, before that happened, people got real dead.

Roussomaniere's book is quite chilling. And even though he takes pains to exonerate the sailors, boat and designs -- focussing on the weather as some sort of supernatural event -- I think the issue bears more examination. The recent 'Wingnuts' tragedy as well as many others, such as the recent Sydney Hobart suggest to me that there is something seriously, fatally, wrong with the current trends in yacht design.

OK, I'll shut up.

[Eleven]

Quite right kefro eschner. It's something that's thrown at Cats often, they are very stable in two plane, rightside up, and rightside down.
The current racing mono's are also VERY WIDE. (Not that they are copying cats, they're just envious) They certainly have a pretty stable inverted position too, and with rig, sails and gear underneath the wave line they don't move much. The sails provide a sea anchor dampening out the surface wave effects. There is also the issue of keels falling off, which guarantees they won't turn rightside uppards.
Escaping from a rectangular box, water filled and vigourously shaken is not going to be easy. In fact, on the last Fastnet I believe a cat rescued crew from an inverted mono.
The other factor with cats is that with increasing wind they run faster than the approaching storm so can avoid the bad segment etc. Displacement cruisers find this much harder. 'Maxing Out' has some great video of heavy weather cat sailing.

[childofgodus]

stormy weather

[Notpopeye]

Originally Posted by stevensuf

... and show me a 30ft boat that will resist capsize better than a 40ft boat...

Ok, and Alberg 30 is less likely to capsize than a Beneteau 40. feel free to input numbers into the formula.
Sailboat Design and Stability

[Bash]

Originally Posted by Notpopeye

Ok, and Alberg 30 is less likely to capsize than a Beneteau 40. feel free to input numbers into the formula.
Sailboat Design and Stability

That's crazy. An Alberg 30 only has an LWL of 21'. It's capsize screening ratio comes out to 1.68, as opposed to the Beneteau's 2.01. US Sailing, which developed the capsize screening ratio (different than Brewster's capsize ratio) suggests that any boat with a CSR under 2.0 is not suitable for offshore work.

I don't care how lovely the Alberg's full keel looks when it's out of the water; a 21' LOA isn't much in big seas.

[mitiempo]

Bash

I think you have it backwards. Segment from US Sailing below. Full info here: Capsize Formula

I think that of 2 boats with the same overall length the one with a smaller surface area under the water is less likely to trip if hit broadside by a breaking wave - a Beneteau 30 less likely to trip than an Alberg 30 for example. And a boat like the Ovni from Alubat with the centerboard up is even less likely to trip than other boats the same length. It offers very little resistance to being pushed sideways unlike a boat with a large keel area.

From US Sailing: