Capsize Ratio's

[cal40john]

Originally Posted by Bash

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.

You've got the formula backwards. Under 2 is considered more suitable for offshore.

Who is Brewster? Do you mean Brewer? If so see my post #31. Brewer's website also lists the same CSF formula as US Sailing in addition to his tongue in cheek comfort ratio formula. US Sailing states they got it from Annapolis book of Seamanship, Annapolis says they got it from the source I referenced back in post #31.

From: Formulas

Adapted From: Rousmaniere, John The Annapolis Book of Seamanship Boat Selection. Chapter 1 p35 Simon & Schuster, New York, New York.

[stevensuf]

notpopeye the sea doesnt care much about capsize factors, simple fact is 18 foot of breaking wave will capsize any 30 foot boat regardless of its capsize factor or stability curve, it takes 24 foot of breaking wave to capsize a 40ft boat. nb this is breaking waves we are talking about.

On a side note, if i have my tender inflated and lashed on the fore deck how much does it increase the positive part of my stability curve?

[kefroeschner]

steventuf -- the inflated tender lashed to your foredeck will not probably not even get wet until you are at an angle of heel of nearly 90 degrees, ie. spreaders in the waves. I do not know your boat (Gib Sea 43) but if it is a good old classic design with positive righting moment maximium at 90 degrees and positive all the way to 180, then the dinghy on the foredeck won't do much from zero to 90 degrees but might help some in the really scary part - 90 to 180 degrees.

If your boat is one of the newer designs -- wide assed, plumb stemmed, 30% ballast to displacement ratio, relying on form stability with maximum righting moment at around 60 degrees falling rapidly to negative, like a dinghy or catamaran, then I think it would be even less efffective. It might hold the bow up whilst you are upside down, but won't do much to prevent a capsize, simply because by the time it gets immersed,roll angle of 90 degrees or so, it's all over in that kind of design.

And this all assumes that it did not just pop, like most inflatable toys, at the first sign of serious stress in the first place.

[Adelie]

Originally Posted by kefroeschner

I do not know your boat (Gib Sea 43) but if it is a good old classic design with positive righting moment maximium at 90 degrees and positive all the way to 180, then the dinghy on the foredeck won't do much from zero to 90 degrees but might help some in the really scary part - 90 to 180 degrees.

The only boats with positive righting moment all the way to 180deg were the so called 'Lead Mines' from the late 1800's which were a product of the racing rules of the times, specifically the Thames Measurement rule in England.

These days a sailboat with an angle of vanishing stability (AVS) of 160deg would be considered very good, perhaps exceptionally good. Generally if you have an AVS over 145 you are doing pretty well.

[Lagoon4us]

Originally Posted by kefroeschner

steventuf -- the inflated tender lashed to your foredeck will not probably not even get wet until you are at an angle of heel of nearly 90 degrees, ie. spreaders in the waves. I do not know your boat (Gib Sea 43) but if it is a good old classic design with positive righting moment maximium at 90 degrees and positive all the way to 180, then the dinghy on the foredeck won't do much from zero to 90 degrees but might help some in the really scary part - 90 to 180 degrees.

If your boat is one of the newer designs -- wide assed, plumb stemmed, 30% ballast to displacement ratio, relying on form stability with maximum righting moment at around 60 degrees falling rapidly to negative, like a dinghy or catamaran, then I think it would be even less efffective. It might hold the bow up whilst you are upside down, but won't do much to prevent a capsize, simply because by the time it gets immersed,roll angle of 90 degrees or so, it's all over in that kind of design.

And this all assumes that it did not just pop, like most inflatable toys, at the first sign of serious stress in the first place.

Would a Volvo 60 come under this description, they look very skifflike....?
Cheers

[cal40john]

Originally Posted by Lagoon4us

Would a Volvo 60 come under this description, they look very skifflike....?
Cheers

When these style boats started capsizing in the Southern Ocean and staying inverted until rescue came, complaints started happening. This resulted in capsize tests and proof that there was a method to get them back upright. The earliest I saw videos of were of an inflatable ball on the aft deck that inflated when the boat was over to start the righting process.

Then came the filling of the ballast tanks on one side to right them.

OceanPlanet inversion test - YouTube

Then when canting keels became the norm the keel would help right it.

IMOCA - PRB - Le 5ème PRB mis à leau à Port La Forêt - YouTube

[mitiempo]

125 degrees is considered good, if approaching 140 or more even better. But in flat water most boats are pretty stable upside down. The good news is that the conditions that will capsize a boat - breaking large waves - the boat cannot remain stable upside down.

The capsize formula, and even the ballast ratio do not in themselves mean much. Hull form and where the ballast is located is very important. Many boats today have the ballast in a bulb down low. One example is the Pogo 10.50 (and other similar boats). It has a ballast ratio of 30.56% and a capsize ratio of 2.57. Neither takes into account the fact that the ballast is in a bulb just over 9' deep. It has a very good AVS despite these numbers.

[Eleven]

I can understand that this test is necessary for a racing formulae. Doing it without mast and sails raised may be a bit of a cheat, but it is also without wave action so it is reasonable.
The canting keel helping though? In action I don't know whether it's deployed to windward, for stability, or to the lee to present a greater area against leeway. In the first case it would assist the boat in rolling through 360deg, but this could be repeated several times, as has happened on the Fastnet Races. To the lee it would be trying to right the boat back the way it had come, against the immersed sails I'm not sure it would have much effect.
Modern Sea Worthy cats should, and some do, prepare for life inverted by storing stuff safely, making all tankage invertible, providing hatches below the normal water line for ready escape and re-entry. It's a survival pod, not a sailing vessel, but it will not sink.
I am waiting for these broad stern mono's to separate the rear to make a Trike Hull (I should register that name) with all the advantages of mono's and cats in one form. Tremendous floor area, good stability, great strength, huge cost! Oh. Well there's always the lottery.

[mitiempo]

Originally Posted by Eleven

I can understand that this test is necessary for a racing formulae. Doing it without mast and sails raised may be a bit of a cheat, but it is also without wave action so it is reasonable.

If a boat is capsized by a breaking wave it will right itself faster with the rig than without.

Most often the rig is lost in the initial roll though.

[cal40john]

The keel is canted to weather to move the lead bulb on the end. They also have two daggerboards that are tilted so the leeward one is vertical at the optimum heel angle.

What I meant in my original post was that the canting keel is deployed to its maximum angle after the boat is inverted to test that the keel will right the boat.

The keel is not necessarily always deployed to its limit. For the capsize test they start the keel at the worst case point of straight down (up). The keel must be canted by manual means during the test.

page 23
http://noticeboard.volvooceanrace.co...v2-amend09.pdf

Originally Posted by Eleven

I can understand that this test is necessary for a racing formulae. Doing it without mast and sails raised may be a bit of a cheat, but it is also without wave action so it is reasonable.
The canting keel helping though? In action I don't know whether it's deployed to windward, for stability, or to the lee to present a greater area against leeway. In the first case it would assist the boat in rolling through 360deg, but this could be repeated several times, as has happened on the Fastnet Races. To the lee it would be trying to right the boat back the way it had come, against the immersed sails I'm not sure it would have much effect.
Modern Sea Worthy cats should, and some do, prepare for life inverted by storing stuff safely, making all tankage invertible, providing hatches below the normal water line for ready escape and re-entry. It's a survival pod, not a sailing vessel, but it will not sink.
I am waiting for these broad stern mono's to separate the rear to make a Trike Hull (I should register that name) with all the advantages of mono's and cats in one form. Tremendous floor area, good stability, great strength, huge cost! Oh. Well there's always the lottery.

[kefroeschner]

Ok, let's not quibble over AVS of 160 vs. 180 degrees. My point is that boats with numbers like this are a far cry from the pointy scows or skiffs of today, like the Volvo 60 and the "bendy toys" that are following that design trend. As to AVS of 125 being "good," read the obits.

And as to the theory, oft posted, that if a breaking wave capsized you another will surely come along and right you again, this tacitly assumes that the wave which capsized you was not a freak or rogue, but rather just normal -- in which case you will be spending half your voyage upside down. On the other hand, if it was a rogue, then how long will you have to wait in your upside down boat until the next rogue comes along. For too many who have been there the answer was -- far too long. And as to the inflating balls, ballast tank filling, canting keel schemes to get things upright again -- do you really want to try getting all of this to function while you are in the briny with waves crashing over you, loose gear thrashing about, your mate(s) panicked ...?

Take a simple dinghy (El Toro, Optimist or similar) out somewhere safe and capsize it, then try to get things sorted out and upright all by yourself. Its a lot harder than it looks on paper.

[mitiempo]

I doubt any boat has an AVS of 180 and damn few at 160. Especially onler boats with the ballast in a longer keel.

[Sabbatical II]

The numbers do tell a story!

While many on this thread have said that "the numbers" mean very little, I tend to disagree. Each of the stability numbers can mean very little on their own, but if you look at the AVS, the capsize ratio and the ballast ratio together they usually tell a story. I note that a some modern fixed keel racing boats have about a 50% ballast ratio and very deep keels and therefore presumably a great capsize ratio, this doesn't make them a great cruising boat becasue the sail area to displacement ratio is far different to a cruising boat, as are other ergonomic features.

On the other end of the scale there are some production boats touted as "ideal" cruising boats out there with fat sterns and an AVS as low as 108 degrees (Hanse 43), some of the modern production cruisers with a ballast ratio as low as 28%. Not only that, the ballast is often cast iron which means that it's weight is not as low down as the draught may suggest. When they have to add fuel and water it is often in tanks that are above the waterline, so their real stability curve diminishes further.

In choosing the design for my current 42' boat (in build) the ratios certainly were considered. With an almost canoe stern at the waterline, an AVS of 140 degrees ballast ratio of 40%, a draught of 2.0m it should be a comfortable crusing boat. Not only that, the cabin sole is about a foot below the water line and 1100litres of water and 560 litres of fuel are below the cabin sole. There is a trade off of course in that the high volume production boats will leave me for dead when the wind is light.

[Adelie]

Originally Posted by mitiempo

I doubt any boat has an AVS of 180 and damn few at 160. Especially onler boats with the ballast in a longer keel.

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.

[barnakiel]

Originally Posted by Adelie

(...) The wider beam of newer boats may create a stiffer boat initially, but at the cost of increased inverted stability.

Except if one can afford a canting keel toy!

;-)
b.