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Old 01-04-2012, 18:12   #61
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Re: Capsize Ratio's

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Originally Posted by Adelie View Post
Actually the older boats tended to have higher AVS's due to their narrower beams. A case in point would be the Contessa 32 at about 155deg AVS.

The wider beam of newer boats may create a stiffer boat initially, but at the cost of increased inverted stability.
Sure, a narrow boat like a Contessa will have a smaller inverted stability but also a smaller positive stability.

These two curves are not very dissimilar of the typical GZ curves of a narrow old design like a Contessa and a modern beamy boat like a 40class racer. Both are good curves.

You can see the massive bigger initial stability of the 40class boat and also the better AVS and smaller inverted stability of the Contessa but.............

you can see also that the wave energy required to capsize the 40class racer is much bigger and also that at 90º degree of heel its righting moment is much better.




Take your pick, both boats are very seaworthy but I would take the 40class boat over the Contessa not only on the acount of its superior power (initial stability) but also by its superior RM at 90º and much bigger positive stability
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Old 01-04-2012, 22:21   #62
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Re: Capsize Ratio's

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Sure, a narrow boat like a Contessa will have a smaller inverted stability but also a smaller positive stability.

These two curves are not very dissimilar of the typical GZ curves of a narrow old design like a Contessa and a modern beamy boat like a 40class racer. Both are good curves.

You can see the massive bigger initial stability of the 40class boat and also the better AVS and smaller inverted stability of the Contessa but.............

you can see also that the wave energy required to capsize the 40class racer is much bigger and also that at 90º degree of heel its righting moment is much better.



Take your pick, both boats are very seaworthy but I would take the 40class boat over the Contessa not only on the acount of its superior power (initial stability) but also by its superior RM at 90º and much bigger positive stability

But the stability curve is neither here nor there, righting moment is a static property and capsize a dynamic event, the real question is whether the 40class has proportionately more roll inertia than the Contessa. Since the 40class presents a lot more area to the capsizing wave than the Contessa does it have proportionately more inertia to resist the impulse of the breaking wave?.
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Old 02-04-2012, 09:28   #63
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Re: Capsize Ratio's

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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.
Of course you are right but what is really odd is that months after you have said that this thread goes on and on. You have said that on the 2th page of this thread that has now 5 pages

People just need to read what is said on the USA calculator:




All modern boats have bulbed keels and the tendency is for torpedo keels and more draft even in cruising boats (look at the new Hanse 415). The CG is lowered this way, instead of using much more weight carried higher with obvious effects on the stability curve giving an higher AVS and a lower inverted stability.

If you want to access the static stability curve of a sailboat, instead of looking the something that only takes in account some factors completely dismissing others get a stability curve. Any boat builder can get you one.

If you apply that formula to an Open 60, a very beamy boat, probably the safest and more seaworthy sailboat for its length with an huge stability the results would be:

2.94 and a note: May be vulnerable to capsize

These boats that are made to race solo in conditions were the waves can go over 40ft (and they have got them in some cases) have the highest stability requirements among racing boats and all these ones have higher stability requirements than cruising boats.

Regarding the stability requeirements of an Open 60, they have to be able to return from an inverted position in flat water by their own means, have an AVS superior to 127.5º and have a area under the positive part of the stability curve 5 times bigger than the area under the negative part.

That means that in a case of inversion a wave 5 times smaller than the one that has managed to invert the boat can re-right it.


And if compared the positive part of the curve of these type of boats (considering only the GZ ) these boats have an huge positive stability if compared with the Contessa type. The max value for GZ is about 3 times bigger.

No wonder that the designers of modern cruising boats have taken for inspiration of the hull the Open 60 type as reference and I am not talking only about performance cruisers like the Pogo 12.50 but abount mainstream cruisers like the Benetau Oceanis, the Sense, the Hanse among others.
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Old 02-04-2012, 10:08   #64
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Re: Capsize Ratio's

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But the stability curve is neither here nor there, righting moment is a static property and capsize a dynamic event, the real question is whether the 40class has proportionately more roll inertia than the Contessa. Since the 40class presents a lot more area to the capsizing wave than the Contessa does it have proportionately more inertia to resist the impulse of the breaking wave?.
Of course this has nothing to do with the validity of the Capsize ratio, a formula that has only 2 variables, weight and beam. It is incredible has someone with a minimum information can believe that weight and beam alone can be determinant to the lesser or bigger boat stability or its resistance to capsize in all types of sailing boats.

What tank testing proved (made 35 years ago with some types of hulls) is that any relatively small breaking wave can capsize a boat if all or almost all its energy is turned in a rolling movement.

What is really relevant is the bigger or smaller capacity that a boat will have to dissipate that energy any other way.

That energy can only be dissipated (if not by a rolling movement) by any other movement, rotating on a vertical axis or by a lateral sliding movement. In both cases a lighter boat with small inertia and a smaller keel and a narrow one (for rotating on an axis) are big advantages and will contribute to dissipate the wave impact energy any other way than by a rolling movement.

I don't understand what you mean with the sail area relevance. On conditions that can lead to a capsize a 40class boat will not be racing but on survival mode and for controlling the boat it will not need more sail than a Contessa 32 since both boats have about the same displacement.

Even if the static stability is not everything I think the fact that it will be needed about 2 times more wave energy to capsize the 40 class boat will also counts for something

By any means I am saying that the Contessa 32 is not a seaworthy boat, just saying the 40class boats, big beam, lousy capsizing ratio and all are too.

There is a circumnavigation race going one with them, they had got incredible bad conditions on the Southern Ocean and all survived, none has capsized. They had to go straight in a storm because they were racing and even given the frightening weather prevision only one boat decided not to continue racing. I guess that very few if any cruising boats will go to that lower latitudes and on the middle of a storm.
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Old 02-04-2012, 10:22   #65
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Re: Capsize Ratio's

Polux, are you Paulo from the sailnet forums?
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Old 02-04-2012, 10:35   #66
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Re: Capsize Ratio's

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Of course you are right but what is really odd is that months after you have said that this thread goes on and on. You have said that on the 2th page of this thread that has now 5 pages
Yeah. Funny. I wrote that post, went down to Baja for a couple of weeks of sea kayaking, and the thread is still going when I return. Only difference is that now my lips are sunburnt, and I've picked up a new recipe for ceviche.
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Old 02-04-2012, 10:37   #67
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Re: Capsize Ratio's

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Originally Posted by Polux View Post
Sure, a narrow boat like a Contessa will have a smaller inverted stability but also a smaller positive stability.

These two curves are not very dissimilar of the typical GZ curves of a narrow old design like a Contessa and a modern beamy boat like a 40class racer. Both are good curves.

You can see the massive bigger initial stability of the 40class boat and also the better AVS and smaller inverted stability of the Contessa but...................
..........
Take your pick, both boats are very seaworthy but I would take the 40class boat over the Contessa not only on the acount of its superior power (initial stability) but also by its superior RM at 90º and much bigger positive stability
Quite strange choice to compare these two boats.. Why not take a bit bigger "narrow" boat like Tuiga instead of Contessa. Tels you excatly the same.. Bigger is better
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Old 02-04-2012, 11:00   #68
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Re: Capsize Ratio's

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Quite strange choice to compare these two boats.. Why not take a bit bigger "narrow" boat like Tuiga instead of Contessa. Tels you excatly the same.. Bigger is better
It was not me that was comparing a Contessa with a 40 class boat but Adelle.

But it has some logic in what regards displacement that is not very different between the two boats. Anyway they are both very seaworthy boats, capsize ratio or not
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Old 02-04-2012, 11:18   #69
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Re: Capsize Ratio's

To get some logic into this it would be beneficial to read (or reread) Tony Marchaj's Seaworthiness the forgotten factor..
There's a "twist" in high initial stability... waves
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Old 02-04-2012, 11:28   #70
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Re: Capsize Ratio's

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To get some logic into this it would be beneficial to read (or reread) Tony Marchaj's Seaworthiness the forgotten factor..
There's a "twist" in high initial stability... waves
I am not talking about high initial stability but about high positive stability area and that is the one that prevents the boat to capsize. Other than that only the negative stability that is the one that prevents the boat to re-right itself when is inverted. Regarding the twist in high initial stability and waves I have no idea of what you are talking about and I know Marchaj's book.

What has that to do with the inability of the capsizing ratio formula to be of any help in accessing modern boat stability?
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Old 02-04-2012, 12:53   #71
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Re: Capsize Ratio's

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I am not talking about high initial stability but about high positive stability area and that is the one that prevents the boat to capsize. Other than that only the negative stability that is the one that prevents the boat to re-right itself when is inverted. Regarding the twist in high initial stability and waves I have no idea of what you are talking about and I know Marchaj's book.

What has that to do with the inability of the capsizing ratio formula to be of any help in accessing modern boat stability?
The simple fact that a wider, flatter bottomed boat is more likely to "tip with the wave" than a narrower boat with a v shaped hull.

Now you are going to say that the more modern boats are more stable than the older boats because of technology ....but the same technology could be used to make the narrower boat with a fuller keel more "stable"

Paulo, try to be more receptive of other peoples ideas..
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Old 02-04-2012, 13:33   #72
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Re: Capsize Ratio's

popcorn!

And what happens if we start at the other end, the outdated methods, you know ... we look at the boat first, we sail her some, then we look into the tables.

I think many of the boats that seemed very seaworthy to me indeed delivered 'good' CR numbers ... ?

b.
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Old 02-04-2012, 13:51   #73
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Re: Capsize Ratio's

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The simple fact that a wider, flatter bottomed boat is more likely to "tip with the wave" than a narrower boat with a v shaped hull.

.....
You mean trip in the wave? But the ones that trip are the ones with a big submerged area, like full keelers, normally modern beamy boats have a small surface fin with a torpedo keel, they trip less on the keel

Last post, if you guys did not understand that the capsize ratio gives only some useful information in what regards some specific type of old boats, it is not me that is going to insist
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Old 02-04-2012, 14:46   #74
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Re: Capsize Ratio's

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You mean trip in the wave? But the ones that trip are the ones with a big submerged area, like full keelers, normally modern beamy boats have a small surface fin with a torpedo keel, they trip less on the keel

Last post, if you guys did not understand that the capsize ratio gives only some useful information in what regards some specific type of old boats, it is not me that is going to insist
No, I mean tip not trip. We have been over this before ..is something being lost in the translation?

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Old 02-04-2012, 15:04   #75
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