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Old 17-10-2022, 20:50   #61
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Re: Keel types

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Originally Posted by OneBoatman View Post
It seems like your OP has morphed as so many do. I find your question to be too vague. A keel or keel/centerboard as you've described can not be evaluated by anyone from afar. The length of your waterline, the boats displacement, beam, ballast, sailplan, rudder size, and I believe a few other design elements are used to determine a capsize ratio. If your architect has a handle on these things he would the person to ask which could get you to happy place. The 1.94 I believe falls as just slighter lower than many of today's production wider beamed boats which many agree not best suited for offshore. Most world cruisers (if that's the primary use), prefer for something a bit lower because when the first good blow hits you probably will feel more comfortable with one which helps keep your bottom mostly facing down and your mates less terrified. I would say target a capsize ratio closer to 1.8 with a comfort rating somewhere nearer the mid 30s. Maybe all that would be needed is less weight aloft and another 600-800 lbs in the keel, but you need professional help.
I must agree as a novice with a couple of years of internet research that wide beam boats are not best suited for offshore cruising. I wish it was a matter of just adding more ballast but when I put the numbers in an online stability calculator it takes an unreasonable amount of ballast to get the desired effect. So it seems unless one wants a fin keel that extends part way down the Marianis trench then decreasing the beam is necessary.
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Old 17-10-2022, 20:56   #62
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Re: Keel types

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You’re coming to the wrong place for information. If you can afford to have a custom designed 54 ft boat built for you , it’s a really really bad idea to base your design criteria on advice from random dudes on the internet. Go talk to your designer and builder.
LOL I find some of you pretty informative. I dream of the day I can sit down with a naval architect and drive him half mad with my inquiries and requests.
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Old 17-10-2022, 21:00   #63
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Re: Keel types

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You don't give your sailing experience/background.

I once read a sailboat review by a well know yachtsman that said those with no or limited experience boating should not design their first boat, custom or semicustom, for obvious reasons...your desired boat appears to fit that recommendation.

There are many sailboats out there already that achieve your desires without reinventing the wheel.
Actually I have a hard time finding yachts that meet half my desires. I allow myself no limitations.
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Old 17-10-2022, 21:17   #64
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Re: Keel types

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I once read a sailboat review by a well know yachtsman that said those with no or limited experience boating should not design their first boat,
I was told by a Naval Architect that you need to "Build your second boat first".

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Actually I have a hard time finding yachts that meet half my desires. I allow myself no limitations.
You will never satisfy all your desires in one boat.
The best you can hope for is a "good working relationship" with a multitude of various compromises.
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Old 17-10-2022, 21:20   #65
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Re: Keel types

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Originally Posted by barryglewis View Post
You can increase the vanishing stability angle by having ballast lower or by increasing the freeboard, that is by increasing the vertical separation between CG and inverted CB.
Also by shaping deck, cabin top, pilot houses etc so as to minimise Inverted shape stability, as the closer the inverted cross section is to semicircular the better
I was thinking about how a pilot house would effect the inverted stability a while ago. Thanks for the reminder.
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Old 17-10-2022, 21:32   #66
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Re: Keel types

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For those with an arch of some sort it makes sense to mount a spare antenna there (with wire and fitting back to VHF and ready.) I have one. Haven't needed it yet. I also have a Standard Horizon HX890NB, 6 watts, which floats, IP8, and has almost all the bells and whistles. It's in a waterproof case (though it is waterproof) and it seems to work well but I have only tested it over a couple miles. Haven't needed it yet.
But really aren't the new EPIRBS with AIS the best bet?
VHF works with line of sight. My guess less than 20 miles bobbing around in the water. So epirb for emergencies is my pick.
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Old 17-10-2022, 21:40   #67
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Re: Keel types

"I was told by a naval architect build your second boat first" Love that line.
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Old 17-10-2022, 21:40   #68
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Re: Keel types

Ok, can you remind me again why the Enduro's design attracted you in the first place? What is it you are shooting for again?
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Old 18-10-2022, 00:42   #69
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Re: Keel types

What attracted me to the Enduro54....It was her long blonde hair braided all the way to the arch in her back, an innocent smile that welcomed me, hips that swayed like old glory in the wind....oh yeah the boat, so sorry......the thick aluminum plating, lift up kickback and replaceable rudders, long shallow keel with swing centerboard, pilot house with watertight door, redundancy, on board maintenance, large forepeak for storage, short handed sailing, no large areas to get thrown across, comprehensive and ready to go sail plan, bulkheads fore and aft, water tight closures for all air vents, seacocks above water line(not sure how that works), emergency exit hatches, cockpit/lazzerete hatch above water line if rolled over, hybrid propulsion. What I don't like.... it's quite wide for it's length attributing to it's avs of 120. Might like a single helm in a more secure cockpit better.
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Old 18-10-2022, 13:08   #70
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Capsize ratio, AVS, Stability index

My 2 grains of salt :

A capsize ratio only using displacement and beam data is rather meaningless. Let's imagine for instance a boat with a light carbon hull and a ballast in a bulb at the bottom of a deep keel accounting for a very significant % of the total displacement, and on the other end an heavy steel hull with a shallow long keel with a ballast (therefore in a higher position relative to the waterline) accounting for a much smaller %; these 2 boats could have the same displacement and beam and hence this same
"capsize ratio" while having very different stability characteristics. Was mostly used in the 80's (after the 79 Fastnet) to compare similar designs of that time.
It's certainly not used by naval architects, see for instance "The design ratios" by Eric Sponberg.


An AVS figure is already more meaningful (you also need to know in which condition e.g. Min Operating, Arrival or Full load, which rig ...) but it only gives a narrow idea of the static stability. The AGZ (area below the curve between 0 and this vanishing stability angle) is already more interesting as it could translate to the amount of energy required
to capsize the boat, say by a rogue wave. See for instance : https://www.hallberg-rassy.com/filea...ilitycurve.jpg
with more or less the same AVS of 115 degrees, but the full load condition yields higher dynamic stability (higher AGZ) as one would expect. 115 may seem low for these excellent boats renowned for their comfort and seaworthiness, but bear in mind once you've gone over 90 degrees, it's likely to be ... wet ! don't need to go to 120 to tame the ocean.
Anyway the AVS is only a small part of the overall picture. E.g. 110 degrees min. is only required for the Imocas racing around the world including the southern oceans, showing that there is more in the stability equation than just the AVS. As a matter of fact a boat with an AVS of 115 could offer superior stability overall vs. a 130 deg. AVS boat with a flatter stability curve.

For this reason the ISO 12217-2 ISO norm defines a stability index, see page 23 of the 2013 version (available on the web) which includes, among other factors (see the overall formula #20 p26) :

a DYNAMIC STABILITY FACTOR (using this AGZ plus the boat length), and KNOCKDOWN and INVERSION RECOVERY factors, ... a much more comprehensive approach to a boat's stability.
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Old 18-10-2022, 19:00   #71
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Re: Capsize ratio, AVS, Stability index

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Originally Posted by Justinweb View Post
My 2 grains of salt :

A capsize ratio only using displacement and beam data is rather meaningless. Let's imagine for instance a boat with a light carbon hull and a ballast in a bulb at the bottom of a deep keel accounting for a very significant % of the total displacement, and on the other end an heavy steel hull with a shallow long keel with a ballast (therefore in a higher position relative to the waterline) accounting for a much smaller %; these 2 boats could have the same displacement and beam and hence this same
"capsize ratio" while having very different stability characteristics. Was mostly used in the 80's (after the 79 Fastnet) to compare similar designs of that time.
It's certainly not used by naval architects, see for instance "The design ratios" by Eric Sponberg.


An AVS figure is already more meaningful (you also need to know in which condition e.g. Min Operating, Arrival or Full load, which rig ...) but it only gives a narrow idea of the static stability. The AGZ (area below the curve between 0 and this vanishing stability angle) is already more interesting as it could translate to the amount of energy required
to capsize the boat, say by a rogue wave. See for instance : https://www.hallberg-rassy.com/filea...ilitycurve.jpg
with more or less the same AVS of 115 degrees, but the full load condition yields higher dynamic stability (higher AGZ) as one would expect. 115 may seem low for these excellent boats renowned for their comfort and seaworthiness, but bear in mind once you've gone over 90 degrees, it's likely to be ... wet ! don't need to go to 120 to tame the ocean.
Anyway the AVS is only a small part of the overall picture. E.g. 110 degrees min. is only required for the Imocas racing around the world including the southern oceans, showing that there is more in the stability equation than just the AVS. As a matter of fact a boat with an AVS of 115 could offer superior stability overall vs. a 130 deg. AVS boat with a flatter stability curve.

For this reason the ISO 12217-2 ISO norm defines a stability index, see page 23 of the 2013 version (available on the web) which includes, among other factors (see the overall formula #20 p26) :

a DYNAMIC STABILITY FACTOR (using this AGZ plus the boat length), and KNOCKDOWN and INVERSION RECOVERY factors, ... a much more comprehensive approach to a boat's stability.
Following Fastnet-79 a bunch of research was done which showed that boats without masts were more likely to capsize even though the boat without the mast had more stability and a higher AVS... Counterintuitive.

What was ultimately realized was that stability provides mostly static resistance to heel even when the boat is moving and capsize is a dynamic phenomenon. Roll moment of inertia is what provides capsize resistance. And comparatively wide beam adds to the capsizing impulse by providing more area farther from the roll axis thus helping to drive a capsize.

The Fastnet research showed that a mast can provide nearly 50% of a boat's roll-moment of inertia. Heavy mast and deep bulb keel on a light narrow hull is one way to gain capsize resistance without gaining a lot of weight, though the increased capsized resistance won't be reflected by the CapsizeScreen. Also nobody designs boats like this, anybody ponying up for a very light hull is wanting to race so they want a light mast which allows them to carry more sail longer which provides somewhat greater speed on the whole.

AVS on the whole does a good job of predicting how long a vessel will remain inverted if turned turtle. I think that's in the Fastnet research but I may have read it elsewhere.

More specifically form or initial stability helps drive a capsize and gravimetric stability helps resist the capsize thru most of the roll.

Form stability is related to the relative beam of the boat which increases the lever arm a breaking wave is acting on and provides more area for the wave to act on and thus more capsizing impulse. Also, even before the wave breaks on the hull, the increasing slope of the wave is starting to roll the boat, more form stability means the boat will have rolled further before the impact. High freeboard also provides more surface area for a breaking wave to act on and greater capsize impulse.

Gravimetric stability related to a moving center of floatation and fixed center of mass will resist the capsizing moment all the way to the point AVS is reached.


NAs design boats for specific reasons, generally the CSF follows along with the designed purpose of the boat so NAs don't make it a design criteria.

STIX is a formula by committee. All the folks with input got their little pet ideas put in and weighted. I believe a lot of it is garbage because there is significant weight to max stability and total area under the stability curve. This favors boats with significant form stability, which as previously shown helps drive a capsize. Beamy boats with lots of habitable interior space that are easier to sell get a nice bill of health under STIX.

STIX has a number of good ideas in it, like down-flooding angle, but they need to be considered individually rather than rolled into a single number that may hide a glaring weakness in one area.
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Old 18-10-2022, 19:25   #72
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Re: Keel types

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Originally Posted by Mattdewizard View Post
I must agree as a novice with a couple of years of internet research that wide beam boats are not best suited for offshore cruising. I wish it was a matter of just adding more ballast but when I put the numbers in an online stability calculator it takes an unreasonable amount of ballast to get the desired effect. So it seems unless one wants a fin keel that extends part way down the Marianis trench then decreasing the beam is necessary.


I have a fair amount of offshore and passage making experience under both sail and power and the only one of the lot that had even a chance of self righting was my Dufour. The Hatteras had a narrow beam and fuel as ballast in the keel so no possibility of popping back up. I doubt that any motor cruiser or trawler would or could recover from a rollover with all the glass on the house.
Having strayed a bit from the yacht stability question....... my next boat will be an Ovni... for many reasons .....but not for its self righting ability.
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Old 18-10-2022, 19:48   #73
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Re: Keel types

Thank you for the grains.
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Old 18-10-2022, 23:36   #74
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Re: Capsize ratio, AVS, Stability index

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Originally Posted by Adelie View Post
Following Fastnet-79 a bunch of research was done which showed that boats without masts were more likely to capsize even though the boat without the mast had more stability and a higher AVS... Counterintuitive.



What was ultimately realized was that stability provides mostly static resistance to heel even when the boat is moving and capsize is a dynamic phenomenon. Roll moment of inertia is what provides capsize resistance. And comparatively wide beam adds to the capsizing impulse by providing more area farther from the roll axis thus helping to drive a capsize.



The Fastnet research showed that a mast can provide nearly 50% of a boat's roll-moment of inertia. Heavy mast and deep bulb keel on a light narrow hull is one way to gain capsize resistance without gaining a lot of weight, though the increased capsized resistance won't be reflected by the CapsizeScreen. Also nobody designs boats like this, anybody ponying up for a very light hull is wanting to race so they want a light mast which allows them to carry more sail longer which provides somewhat greater speed on the whole.



AVS on the whole does a good job of predicting how long a vessel will remain inverted if turned turtle. I think that's in the Fastnet research but I may have read it elsewhere.



More specifically form or initial stability helps drive a capsize and gravimetric stability helps resist the capsize thru most of the roll.



Form stability is related to the relative beam of the boat which increases the lever arm a breaking wave is acting on and provides more area for the wave to act on and thus more capsizing impulse. Also, even before the wave breaks on the hull, the increasing slope of the wave is starting to roll the boat, more form stability means the boat will have rolled further before the impact. High freeboard also provides more surface area for a breaking wave to act on and greater capsize impulse.



Gravimetric stability related to a moving center of floatation and fixed center of mass will resist the capsizing moment all the way to the point AVS is reached.





NAs design boats for specific reasons, generally the CSF follows along with the designed purpose of the boat so NAs don't make it a design criteria.



STIX is a formula by committee. All the folks with input got their little pet ideas put in and weighted. I believe a lot of it is garbage because there is significant weight to max stability and total area under the stability curve. This favors boats with significant form stability, which as previously shown helps drive a capsize. Beamy boats with lots of habitable interior space that are easier to sell get a nice bill of health under STIX.



STIX has a number of good ideas in it, like down-flooding angle, but they need to be considered individually rather than rolled into a single number that may hide a glaring weakness in one area.


All these “ single number “ metrics have limitations that includes AVS and STIX. Each must be evaluated in conjunction with other factors.

AVS can be more misleading then STIX , which was designed by a very well specified ISO technical working group. But equally both numbers can be misleading
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Old 19-10-2022, 00:41   #75
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Re: Keel types

Another point perhaps.
Of two boats, the one with the "wine glass" shape will exhibit a different motion from the onset of a large beam sea than the boat with a "champagne glass" hull shape.
The champagne glass has a greater waterplane area for its weight and so is lifted and then pushed over to leeward.
The wineglass boat, with its lessor waterplane area does not present as much resistance to lift, and its response is to roll into the wave as the water passes beneath.
This vid of a PJ/Swan 43 is a good illustration.
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