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Old 30-03-2011, 16:12   #46
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Re: Minimum Draft for Blue Water?

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Originally Posted by KJB View Post
Example: Boat A and B are the same model, but boat A has a draft that is longer than boat B. Do both handle the open ocean equally well? I have come across this in my searches.

Any ideas?
For two boats of the same model, with different draft, you will also notice the designers (oftentimes) to use different amt of ballast. Some will also use different form of the keel/ballast.

Stability aside, these boats will behave differently - point differently, track differently and roll / pitch differently.

The two models may, as you put it, 'handle the ocean' equally well, or the shallower one may handle the ocean better, or else the deeper one may fare better. No hard rules, as there are other factors that interact with the draft to produce the final outcome.

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Old 30-03-2011, 16:20   #47
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Re: Minimum Draft for Blue Water?

estarzinger:
I am sorry to be slow... but help me understand what the "stability index" means. That the boat should right itself after a knockdown of "x degrees", or it a ratio of draft and ballast?
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Old 30-03-2011, 16:23   #48
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Re: Minimum Draft for Blue Water?

To answer the OP I don't think there is a "minimum draft for blue water." Depends on many design factors. But, I will agree with Mark Johnson that shallow draft can make coastal cruising and exploring much more fun and less stressful. Everyone is afraid of what will happen when you get caught out in a big storm offshore, but in reality you are much more likely to sink when your boat catches on fire or you sail up on a reef, and in both cases I would much rather have shallow draft. It also depends a lot on where you want to go. Very few of us spend any appreciable time well offshore, and when we do we always choose the best weather possible. Here along the East Coast I personally consider six feet to be the maximum draft I would want to put up with, and if you draw four feet or less you can probably double the number of places you can visit. On the West Coast you see lots of boats drawing seven or 8 feet of water, which would really limit you here on the East Coast.
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Old 30-03-2011, 17:12   #49
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Re: Minimum Draft for Blue Water?

Quote:
Originally Posted by callistov42 View Post
estarzinger:
I am sorry to be slow... but help me understand what the "stability index" means. That the boat should right itself after a knockdown of "x degrees", or it a ratio of draft and ballast?
"STIX is the product of a number of different safety/stability related features. These are: Length, Dynamic Stability, Inversion Recovery, Knockdown Recovery, Displacement Length, Beam Displacement, Wind Moment, Downflooding, and Reserve Buoyancy."

from:

http://ircrating.org/images/stories/pdf/STIX2009.pdf

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Old 30-03-2011, 22:46   #50
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Re: Minimum Draft for Blue Water?

Quote:
Originally Posted by callistov42 View Post
estarzinger:
I am sorry to be slow... but help me understand what the "stability index" means. That the boat should right itself after a knockdown of "x degrees", or it a ratio of draft and ballast?
Stability Index, STIX for short, is number calculated for each boat or class of boats that is indicative of its ability to resist capsize (roll over to an inverted position). The term started out as part of a European ISO. The term seems to be experiencing some creep and may refer to any calculated number indicative of capzsize resistance depending on the person using it.

Other similar numbers are the SSSN, the Capsize Screening Formula/Capsize Screen/Capsize Formula, and Capsize Length Formula.
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Old 30-03-2011, 22:49   #51
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Re: Minimum Draft for Blue Water?

OK, I've looked at the spreadsheets and see numbers for lots of boats. Now, how do we interpret these numbers? What is a good, a bad or an excellent value for each of them?

Perhaps it is there, but not seen on first viewing.

Thanks,

Jim
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Old 30-03-2011, 23:20   #52
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Re: Minimum Draft for Blue Water?

Capsize screening formula

From Wikipedia, the free encyclopedia


J"ump to: navigation, search
The capsize screening formula (CSF) is a somewhat controversial figure. It is defined for sailboats as:
CSF = Beam / (Displacement/64.2)1/3
It came into being after the 1979 Fastnet race in England where a storm shredded the race fleet. The Cruising Club of America (CCA) put together a technical committee that analyzed race boat data. They came up with this formula to compare boats based on readily available data. The CCA characterizes the formula as "rough".
A lower value is supposed to indicate a sailboat is less likely to capsize. A value of 2 is taken as a cutoff for acceptable to certain race committees. However this is an arbitrary cutoff based on the performance of boats in the 1979 Fastnet race. The CSF does not consider the hull shape or ballast location.
Any two sailboats will have the same CSF value if their displacement and beam are the same. As an example, one could have a light hull with 50% ballast in a bulb at the bottom of an eight foot fin keel, the other could have a heavy hull with 20% ballast in a 2-foot-deep (0.61 m) full-length keel. The stability characteristics of the two sailboats will be drastically different despite the identical CSF value."

This pretty much sums up my understanding of it - My translation = Stay under 2 and lower is better.
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Old 30-03-2011, 23:26   #53
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Re: Minimum Draft for Blue Water?

Quote:
Originally Posted by Jim Cate View Post
OK, I've looked at the spreadsheets and see numbers for lots of boats. Now, how do we interpret these numbers? What is a good, a bad or an excellent value for each of them?

Perhaps it is there, but not seen on first viewing.

Thanks,

Jim
STIX ranges from 5 to 50 and perhaps higher and lower. Higher is better. The IRC which handicaps boats for the Royal Ocean Racing Club requires a STIX of 32 for essentially unlimited ocean racing and a STIX-23 for shorter distance open ocean racing say 250-600miles. What investigation I have done indicates the number is calculated from static stability numbers for the boat, specifically the righting moment curve, an inclining test and weight measurements.

SSS Numerals seem to range a bit higher. Numbers required for racing are similarly a bit higher. SSSN seems to be phasing out in preference to STIX. I have no idea how the number was calculated or what went into the number.

Capsize Screening Formula (CSF) number range from say 1 to 3, though most numbers I have seen range 1.5-2.5. Lower is better. The original cutoff for offshore suitability was 2.0. If you want to take the Westsail 32 as a standard, its CSF is 1.69. The number is calculated from displacement and beam and is gives a very rough idea of capsize resistance.

Capsize Length Formula gives results in feet length with the original cutoff for offshore at 30'. Longer is better. The value is calculated from length, beam, CG and roll moment of inertia.

CFS is a very simplified formula that approximates the Capsize Length Formula.

IMS has a stability index where the higher number seem to be better and the Westsail 32 has an index of 138. I have no knowlege of how this number is calculated.

The Capsize Length Formula is the most compelling system to me. Capsize is a dynamic event for which one must take into account roll moment of inertia. I really need to do some research into STIX and the IMS Stability index to develope a better informed opinion of their utility.
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Old 31-03-2011, 08:04   #54
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Re: Minimum Draft for Blue Water?

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(...) STIX ranges from 5 to 50 and perhaps (...) Higher is better. (...)
Only for same / similar boats!

The IRC page (link given above) quotes boat length and other relationships involved.

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Old 31-03-2011, 10:15   #55
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Re: Minimum Draft for Blue Water?

OK..maybe I am just a little slow (I am a sailor "going no where slowly at great expense" after all) but on the spread sheet that estarzinger gave us the link to, the last column "stab index" the values are running between 100 (J30 has 99.5) and 150 (a Farr 40 has a value of 144. My big old heavy Vagabond 42 has a value of 119.2. This just doesn't seem to correlate to me.
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Old 31-03-2011, 11:10   #56
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Re: Minimum Draft for Blue Water?

V42:
The Farr has a much lower VCG than you have so it has more righting moment.
This is typical of modern deep draft, fin and bulb boats.
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Old 31-03-2011, 13:50   #57
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Re: Minimum Draft for Blue Water?

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This just doesn't seem to correlate to me.
Which part actually? Vagabond vs. J? Vagabond vs. F40?

Farr 40 has 45% of ballast hanging in a lead bulb attached to the lower end of a 8.5' iron fin. Vagabond 42 has 32% ballast attached to a grp stub some 5.5' deep.

To me, the Farr - Vagabond correlate as the formula would expect. Farr gets the higher number Vagabond the lower one.

J30 is a different size altogether so comparing STIX numbers does not say a thing vs. the other designs.

I might be wrong but that's what I understood from the IRC site.

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Old 31-03-2011, 14:48   #58
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Re: Minimum Draft for Blue Water?

Barns right. Consider on your Vagabond your VCG is probably around 4" above the DWL while on the Farr 40 the VCG is probably around 12" BELOW the DWL. You probably draw 5'10" and the Farr draws probaly around 8'. It really is not complicated.
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Old 31-03-2011, 22:39   #59
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Re: Minimum Draft for Blue Water?

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Originally Posted by barnakiel View Post
Only for same / similar boats!

The IRC page (link given above) quotes boat length and other relationships involved.

b.
Not just for the same or similar boats. ISO places no restrictions that I can find on comparing boats with these numbers. As used by the IRC (Safety and Stability Indices (SSI) | special-regulations See Tables 3 and 4) restrictions are based strictly on the STIX number without reference to other factors.

The 5-50 range I indicated was quoted from a IRC page I can't find again. It indicated most boats are in this range.
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Old 31-03-2011, 23:04   #60
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Re: Minimum Draft for Blue Water?

Quote:
Originally Posted by callistov42 View Post
OK..maybe I am just a little slow (I am a sailor "going no where slowly at great expense" after all) but on the spread sheet that estarzinger gave us the link to, the last column "stab index" the values are running between 100 (J30 has 99.5) and 150 (a Farr 40 has a value of 144. My big old heavy Vagabond 42 has a value of 119.2. This just doesn't seem to correlate to me.
The IMS Stability Index is for stability, most people conflate this with Capsize Resistance which is incorrect.

The Farr was designed for very high stability (wide beam, very deep fin, bulb keel giving much lower CG, ballast ratio approaching 50%, minimum weight rig) in keeping with it's need to stand up to its much larger sail area, 1100sf vs 70 for the Vagabond. The increased stability is reflected in the higher IMS stability index.

That, however, does not translate into higher capsize resistance. While stability is somewhat related to capsize resistance, of vastly greater importance is roll moment of inertia. The beefier rig and rigging of your Vagabond don't help your stability compared to the Farr, but do wonders for your ability to stand up to a breaking wave that could capsize you.
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