Capsize Index? - SailNet Community
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Capsize Index?

Hello,
Does anyone know where I can find a list with the capsize factors of modern sailboats.
I understand anything below 2.0 is a "Bluewater" vessel.This will help my search for the correct sailboat.

Thanks,
JerryO
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Old 08-06-2001
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Capsize Index?

RichH is offline
Old 08-07-2001
JeffH
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Capsize Index?

Simplified indexes, such as the so-called ''Stability Index'', really is of very little real use in determining whether a particular boat is indeed suitable as a ''bluewater'' vessel. For example, two of the most critical factors in determining how safe a boat will be offshore or its inherrent stability is the vertical heights of the center of gravity and center of buoyancy. No where do either of these appear in the ''capsize index''.

The EU developed a very sophisticated ''stability index'' (STIX). These series of formulas were intended to help determine the relative safety of a vessel in any given venue. But even the STIX formulas were merely ''sorogate formulas''. Sorrogate formulas are common in yacht design. These are formulas intended to predicty the behavior of a vessel using simple measurements. There are a helpful tool in comparing very similar types of boats. They do not produce specific accurate for an individual design.

To explain further, one of the key factors in the so-called ''Capsize Index'' is weight, but weight in and of itself has no bearing on a boat''s likelihood of capsizing. (Visuallize a equal weight boats but one boat with a 50% of its weight in its teak decks, overhead, ceilings and bulkheads vs another boat with 50% of its weight in a lead bulb deep below the boat.)

The Capsize index also has no way to compare different hull forms which of course affects stability as well as comfort at sea. So as you can see these simplified indexes really have little or no use in determining whether a particular boat will do offshore.

All of that said you can find a calculator for a number of surogate formula indexes at
Sail Calculator:

http://www.image-ination.com/sailcalc.html#bbb

The EU standards are available at:
http://europa.eu.int/comm/dg03/directs/dg3d/d5/reccraft.htm

Good luck
Jeff

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Capsize Index?

Thanks for all the input.There is alot of knowledge here.I am simply trying to shop for a boat that is 35k or less in price,able to handle bad weather(in case it comes up).As much as I like the J30 series I really don''t think they are suitable for my needs.(Limited cockpit space from pics I have seen).I would like to spend winter months cruising the Carribean.(6 months or so approx).I only hope to make the wise choice in purchasing my sailboat.

Thanks again for all the help,

JerryO
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Old 09-10-2001
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Capsize Index?

Ahoy, Jerry039. You didn''t indicate how you got the number you refered to, the 2.0. I assume it is from the Capsize Screening Formula developed by the U.S. Yacht Racing Union in conjunction with the Society of Naval Architects and Marine Engineers. This is a simpler way to estimate the positive stability range of a boat than the various complicated stability indexes that are usually done by architects and, on racing boats envolves measuring under the International Measurement System. Briefly, the formula that your 2.0 refers to is a two step formula. Take the total weight of your boat and divide by 64 (wt. of a cubic foot of sea water,62.2 for fresh water). This gives you the boat''s volume in cubic feet in sea water. Second, take the cube root of that number (almost any hand held calculator) and divide that into the beam of the boat. If the number is 2 or less, then the boat is pretty safe from capsize. There was an exhaustive study done on the Fastnet disaster and, believe it or not, the boat that came out of that as the benchmark boat was the Contessa 32. Find a picture or drawing of her and look at it. She has relatively low freeboard, does
not have a very high aspect ratio rig. Getting a boat with a shorter mast and longer
boom for the same given sail area lowers the center of effort, shortening the lever that produces heeling moment. She has adequate ballast to displacement ratio which is very important. In addition to these qualities, you want a swept back forefoot, not a straight entry, so you can deflect and or ride up on objects you might strike at sea. You want a moderately long keel for tracking and steering control in a sea, cut away aft, and you want a rudder and skeg so your rudder will not be so vulnerable. Thirty five feet is generally recommended as the minimum length for blue water cruising and I prefer a
boat with moderate beam to a fat one. All this having been said, JeffH is right about the location of weight,etc. This assumes reasonable construction. Finally, it is the skill of the crew that is the single most overriding factor. The smallest sailboat to cross the Atlantic was under five feet long.
People have sailed oceans in open boats, in production "lake boats" and just about every
thing else. You have to know your boat and you have to know what your routine is for a given set of conditions. Crewing on a blue water boat is a good way to get experience. In today''s market, if you search diligently, you can get a lot of boat for the 35K you have to spend. It will be older, but the late 60''s and early 70''s grp boats are bullit proof because they were over built out of anxiety generated by the lack of overall experience with fiberglass. Look for the qualities I mentioned, keep it at 2.0 or below, a minimum of 40% ballast to displacement ratio, and she will get you home
if you know how to help her. Finally, talk to
people who actually cruise and read Cruising World and other cruising magazines. Good luck. dhartdallas.
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