Ethan Allen capsize follow-up

Discussion in 'Stability' started by Stephen Ditmore, Mar 17, 2007.

  1. Stephen Ditmore
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    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member

    This thread is intended as follow-on to:
    http://www.boatdesign.net/forums/showthread.php?t=8970
    and
    http://www.boatdesign.net/forums/showthread.php?t=15858
    concerning the capsize of the tour boat Ethan Allen on Lake George in New York.

    The NTSB accident report is now available at http://www.ntsb.gov/publictn/2006/MAR0603.htm. For the full text, click on "PDF Document" just below the heading.

    The NYS agency responsible for certifying state regulated passenger vessels is http://www.nysparks.com/boating/services.asp
    I believe the director of the Marine Division at the time of the accident was named Dominic JackAngelo.

    We have a new Governor of New York State. The new director of the Department of Parks, Recreation, and Historic Preservation is Carol Ash. Because there's an incoming administration, it's a time of transition, and I don't yet have the name of the current Director of the Marine Division.

    One issue I'd like to bring to the attention of the Marine Division is the following:
    A year and a half after the Ethan Allen accident, this 78 page study guide makes no mention of vessel stability: http://www.nysparks.com/boating/docs/vessel_lic_guide.pdf
    Stability is covered in this Canadian document, beginning on page 227: http://www.cdc.gov/niosh/docs/2006-114/pdfs/2006-114h.pdf (The authors are fellow members of SNAME ad hoc panel 15.)

    For over a year I've been serving on SNAME ad hoc panel 15: http://www.sname.org/committees/tech_ops/O44/passenger/activity-15.html
    One of our primary reponsibilities is to review this draft analysis done for the U.S. Coast Guard: http://www.uscg.mil/hq/g-m/mse/passweight/phase1-jan07.pdf
    (at http://www.uscg.mil/hq/g-m/mse/mse-home.htm).

    In my view, it's time to begin collecting data on passenger vessels regulated by the states and comparing it with the data in the above report on vessels regulated by the Coast Guard. In the coming weeks I hope to form a committee, possibly through SNAME and/or a college in the state of New York, to begin this task. Based on the above report, it's my suggestion that panels of naval architects review the stability tests and passenger capacities for all vessels where:
    • The rated capacity in number of individuals/(length*beam^2) > 0.0045 (units: # of individuals & feet)
    The Ethan Allen's capacity rating/(length*beam^2) was 0.009, twice the proposed screening value. I feel this is a good way to quickly identify the vessels in most urgent need of immediate retesting using the new 190 pound per person weight assumption.

    There's one other technical item I'd like to post, for anyone interested. 46 CFR 178.330 is the place in the United States Code of Federal Regulation where the "Simplified Stability Test" is described. This has two parts, one of which is a "Passenger Shift Criterion." The attached graph relates maximum passenger weight to the GM(req'd) by this criterion [i.e. the vertical distance from the highest center of gravity the rule says a boat may have when fully loaded to the full load transverse metacenter]. Units: pounds and feet.

    Cheers,
    Stephen
     

    Attached Files:

  2. TerryKing
    Joined: Feb 2007
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    TerryKing On The Water SOON

    Pain Up North...

    Thanks for your good work on this, and I'm very happy to see the amount of effort and competence that has gone into this investigation.

    As a Vermonter who has built and run power boats on the next lake North (Lake Champlain) for 30 years, I can tell you how gut-wrenching it was for many of us when we heard about this "accident". True accidents happen, but as you have uncovered, this appears to have been an incident waiting to happen.

    My Son and my Grandchildren are sailing this area, and my step-daughter's mate is a licensed Captain on Lake Champlain ferries. We all shudder when we think about this accident.

    I'm spending a lot more time reading about Stability, and testing, lately!
     
  3. Guillermo
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    Guillermo Ingeniero Naval

    Most interesting, Stephen, thanks.
    I would like to highlight from there these very concise, precise and similar-terms-world-wide apliccable paragraphs:

    Stability Concepts
    A vessel that is floating upright in still water will heel when an off-center force or heeling moment is applied. Stability is the tendency of the vessel to return to its original upright position when the force is removed. In still water, a vessel's stability is a function of its underwater hull form and the distribution of mass of the vessel. The properties of stability are usually expressed in terms such as the magnitude of a heeling moment necessary to heel the vessel to a certain angle, the angle a vessel may heel to before capsizing, the amount of reserve energy available to return the vessel to its upright position, and other parameters that can be calculated.

    Coast Guard regulations and international standards specify the amount of stability a vessel must possess, depending on the type of vessel and its service. The requirements are usually expressed in terms that are easily calculated, such as GM, range of positive stability, righting energy, or other recognized technical measures. The specific stability characteristics of an individual vessel are determined based on the design drawings of its hull form (lines plan) and an inclining experiment of the vessel while afloat to determine its actual weight (displacement) and center of mass (center of gravity).

    Although obtained under stationary conditions, the stability characteristics represent the vessel's ability to return to the upright position when the vessel is in service and is subject to external forces such as wind and waves, people moving about, and from inertial effects resulting from the accelerations and motions of the vessel as it moves around on the water's surface. Such dynamic influences on a vessel are random processes and can only be predicted statistically. Studies comparing the stability characteristics of vessels that capsized (inadequate stability) and vessels that did not capsize (adequate stability) form the basis of stability criteria that are in use today. To account for unknowns, uncertainties, and randomness of the physical environment and vessel response, large safety margins are built into the criteria. The stability criteria are generally recognized as providing an adequate level of safety for vessels that are operated prudently, which means not overloaded and not operating in dangerous conditions such as hurricanes.

    The stability analyses of most vessels involve substantial calculations that generally require the services of a naval architect. The calculations are based on an inclining experiment, in which very precise measurements are taken on board the vessel in order to determine its displacement and center of gravity. The inclining experiment and associated calculations and analysis cost several thousand to tens of thousands of dollars. For large oceangoing ships, the stability assessment is minor compared to the total cost, and is necessary in order to determine the maximum amount and stowage of cargo. In small passenger vessel design, stability is not optimized and usually exceeds the Coast Guard stability criteria. Because of the relatively high cost of an inclining experiment and full stability assessment, the Coast Guard permits an SST to be performed. The SST is quick, inexpensive, and more conservative than a full stability analysis. Being more conservative, the SST results in fewer passengers permitted than would be allowed based on a full stability assessment.

    Because the number of passengers permitted on a small passenger vessel is based on several criteria, such as deck area and seating capacity, stability is often not the governing criteria. In those cases, the SST is sufficient. However, the owner of a small passenger vessel might have space to carry more passengers than would be allowed by the SST, and so the cost of performing a full stability assessment with an inclining experiment is justified to show that additional passengers may be carried in compliance with the detailed stability regulations. The margin of safety built into the stability criteria is what provides the safety of the vessel against capsizing. The margin of safety is intended to accommodate all the things that happen with a vessel, such as rolling in waves, heeling due to wind, or listing as passengers move from one side to the other. The margin of safety is reduced if the vessel is operated in extremely high winds, surf, or is overloaded. The stability criteria are not intended for such conditions. In such extreme operating conditions, the margin of safety may be still adequate to keep the vessel upright, or it may not be adequate, resulting in a capsizing. In any case, if the intended margin of safety is not maintained, the vessel should not be considered seaworthy (safe), whether or not it capsizes.

    Because of the nature of stability, and the randomness and variabilities associated with it, safety is not absolute. Not meeting stability standards does not mean a vessel will capsize; it only means the margin of safety is lower than what the regulations require. A vessel can operate for years in an overloaded condition that does not meet the stability standards; because the margin of safety is less than it should be, the probability of capsizing is higher, but it could still be remote. It would take other forces, such as high winds or large waves, to cause a vessel to capsize. The more a vessel is overloaded, the less the margin of safety for stability and the higher the probability of capsizing. There is no obvious way to tell if a vessel fails to meet the stability criteria, other than through a stability test. This is why a stability test is required for all small passenger vessels. If something changes about the vessel, such as a structural modification that might affect the vessel's stability, another stability test and assessment should be conducted. After the stability assessment is completed, the results of the assessment and any limitations, such as number of passengers or limiting operating conditions, will be placed on the vessel's stability letter or COI.
     
  4. TerryKing
    Joined: Feb 2007
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    TerryKing On The Water SOON

    Stability Information

    Guillermo, that is a wonderfully-done summary! Please consider adding it to the boatdesign.net Wiki, or asking Jeff (or me :) ) to add it.
     
  5. Stephen Ditmore
    Joined: Jun 2001
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    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member

    I certainly agree, Guillermo. Thanks for the contribution!

    The problem we're struggling with is that, as you pointed out, the smaller tour boats (generally those under 65 feet with capacities of 49 passengers or less) do not get the same sort of full scrutiny and testing as the larger vessels, and do not, as a rule, carry stability letters with detailed, usable information. There is also a separation of powers (federal vs. state jurisdiction) issue here in the U.S., the result of which is that vessels that operate on lakes fully within a single state are not Coast Guard inspected passenger vessels, but are licensed by the states. You need look no further than the NTSB executive summary to see the result.

    SNAME Panel O-49, under the leadership of the same two naval architects who wrote the stability material for Canadian fishing boat captains, is currently working on calibration/validation standards for inclinometers. They plan to conduct a demonstration of a faster, and therefore less expensive inclining experiment procedure next year. This would hopefully allow for more frequent testing of the smaller vessels. I also hear that one of the universities in Vancouver, BC, Canada is working on a method to take a quick set of approximate vessel lines to determine key parameters.

    If you take a look at Figure 4e of the draft study for the U.S. Coast Guard you'll see that the small boats are carrying a much higher number of passengers/(L*B^2), on average, than the larger vessels, and this makes them more likely to fail a new stability test if the assumed weight of an individual passenger is increased. In addition, a small group is statistically more likely to deviate from an average than a large group. This problem may be greater still if the group is self-selecting, not random, or if it favors a specific demographic. Federal Aviation Administration standards for small aircraft weight estimation correct for this, but U.S. Coast Guard regulations do not.

    On the basis of Figure 4e, I question whether the SST results in a more conservative rating, and an adequate margin of safety, in all cases. Having said that, it's far better than nothing, and the Ethan Allen's sister ship failed a SST, miserably, when the NTSB performed one post-accident, even using the old 140 lb per person weight estimate (which is supposed to be for mixed adult-child groups, and which is being abandoned).

    Interimly, how do you feel about using roll period tests on the smaller vessels as a quick check? I've noticed you frequently use roll period as part of your stability method. But if one doesn't have a derived roll constant from a previous inclining, one is left to estimate (though I have seen a formula for calculating it in the fishing vessel literature) ...
     

  6. Guillermo
    Joined: Mar 2005
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    Location: Pontevedra, Spain

    Guillermo Ingeniero Naval

    Terry,
    I'll try to add the text to the Wiki.

    Stephen,
    rolling period is a quick way to assess stability, commonly used for fishing vessels for which you have conducted a rolling test and have enough stability related information to derive the roll constant (There is available also a table to roughly estimate such constant for certain types of vessels). In Spain is mandatory to add to the stability booklets of all fishing vessels over 20 gross tons, a guide for the captains to use roll period as a means of estimating their boat stability. Nowadays there are also some computerized systems which constantly measure aboard the roll period, and tell the captain (alarms) if the boat is in a dangerous load condition. See http://www.shipmotion.se/SMCfish/ (*). Those systems have been developed based in John Womack's work.

    Cheers.


    (*) SMC is working on a substantial and more accurate improvement of the SMCfish, for larger fishing vessels.
     
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