Ethan Allen capsize: Lake George

Discussion in 'Boat Design' started by Stephen Ditmore, Oct 3, 2005.

  1. sharpii2
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    sharpii2 Senior Member

    I think he meant MORE than 1/3rd the beam in ft in seconds.

    Probably a brain fart. I have them all the time.:eek:

    Also. Too much stability can be a bad thing too. If a vessel rolls to quickly, it can be very uncomfortable to be on. For example, have you ever seen a a sail boat with a keel leaving the harbor parallel to a swell without its sails up? It can behave like a mechanical bull. I saw a Flicka doing that one day and boy did it roll.

    The skipper restored order soon enough. He got the sails up.

    Bob
     
  2. Stephen Ditmore
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    Stephen Ditmore Senior Member

    You are completely correct, Trouty. I've edited my post to read as it should. I apologize for my embarassing error.

    Stephen
     
  3. trouty

    trouty Guest

    Struth

    :)

    You mean - theres actually some limited form of hope for me yet? ;)

    Ye gads - I scare me sometimes...in that - I love to read scientific stuff - which I usually have absolutely no idea what it means - and just try n read and understand it anyway, for the heck of it.

    Occasionally - (just very occasionally, mind you) - a small wee light comes on - (for a fleeting second) in a loooong dark tunnel...(usually a fast approaching freight train!).

    Just once - in a very brief, what appeared lucid, moment..I swear - I once distinctly heard a penny drop somewhere...

    The fact I'm actually getting a hang of some of the mumbo jumbo you guys talk - (sometimes) - actually scares me...

    It's just possible - I MAY be getting to know just enough to be dangerous.;)

    Luckily - I know these all too brief sentient moments, soon pass..

    In the meantime - I just keep taking the pills! :p

    One a these days - they are gonna let me outta my padded room...true!;) - but they said it wouldn't happen until I was 'stable' - whatever that means?
    (Yes - an intentional pun, no less!)...;)

    Ya see - I think I'm actually crazy rather than just plain stupid - but try telling my idiot shrink that!! :D

    Cheers!
     
  4. Stephen Ditmore
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    Stephen Ditmore Senior Member

    From Preliminary Design of Boats & Ships (Cyrus Hamlin, 1989), p. 70:
    Natural Period, T = K(r) * beam / GM^0.5
    GM = [K(r) * beam / T]^2

    where:
    GM = waterline beam, feet
    Beam = waterline beam, feet
    T = measured rolling period, seconds
    Kr = a constant, preferably derived..., otherwise use .42 for fine waterplanes, .44 for average, and .46 for full (metric .76, .80, .83 respectively).​
    Ibid, p. 71:
    A handy use of this relationship between GM and the rolling period was told to me by Jan-Olof Traung, for many years chief of the Fishing Vessel Section of the Food and Agriculture Division of the United Nations. We were in Indonesia discussing some of the perilously loaded small craft one sees in developing countries. In order to evaluate the safety of these craft quickly and on site, he used the criterion that if the rolling period of a boat was greater than the beam of a boat in yards (or meters), then the GM, and hence its stability, was questionable. For instance, if you time the rolling period of a boat with a beam of 18' (6m) as more than 6 seconds, it would be best to stay off it.​

    [End quote]

    Substituting K(r)=0.44 and T=B/3 into:
    GM = [K(r) * beam / T]^2
    GM = [0.44B / (B/3)]^2
    GM(req'd) = 1.74 feet​
    Compare to this energy criterion from §170.173:
    (4) An area under each righting arm curve of at least 10.3 foot-degrees (3.15 meter-degrees) up to an angle of heel of 30 degrees​
    Assume the area from 0-30 degrees is triangular.
    Area = 15GZ(30)
    GM = GZ(30) / sin(30)​
    Therefore:
    10.3 = 15GZ(30)
    10.3/15 = GZ(30) = 0.6867
    GM = 0.6867 / sin(30) = 2(0.6867)
    GM(req'd) = 1.373 feet​
    One could work backwards from this number to a modified roll period / beam ratio as follows:
    GM(req'd) = 1.373 feet
    GM = [K(r) * beam / T]^2
    1.373^0.5 = 0.44B / T
    1.172 / 0.44 = B / T
    1.172T = 0.44B
    T(max) = 0.44B / 1.172 = B / 2.66​
    To be conservative, one could use a slightly lower estimated K(r):
    GM(req'd) = 1.373 feet
    GM = [K(r) * beam / T]^2
    1.373^0.5 = 0.426B / T
    1.172 / 0.426 = B / T
    1.172T = 0.426B
    T(max) = 0.426B / 1.172 = B / 2.75​
    I conjecture that the area under the righting arm curve from 0-30 degrees is most likely to be triangular on flush deck vessels with beam / freeboard < 6. If such a vessel has no opening below the gunwale and operates on protected waters I would be comfortable with any as loaded roll period less than B / 2.75. For other vessels I'm inclined to endorse Jan-Olof Traung's guideline without modification.

    Here's why the experts are skeptical: Values for K(r) vary more than Cy Hamlin's values indicate because keels, skegs, rudders, bilge keels, stabilizer fins, other appendages, chines, and variations in hull shape can damp rolling. It's always better to use a K(r) that has been derived for an individual vessel or sistership at the time of an inclining experiment.

    Here's why I like it: It's a quick check that's a heck of a lot better than nothing!
     
  5. trouty

    trouty Guest

    I presume you mean loaded?

    When you quote:-

    I presume your talking about these perilously loaded 3rd world small craft in their perilously loaded condition?

    I.e. - when we are about to board som inter island ferry in indo / bali etc - we shpould do the quick calc and compare to roll period before we embark & they cast off?

    Sounds plausible to me!

    (course ya wouldn't catch me dead in one a them places for quids!).

    Cheers!
     
  6. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I'm saying that, yes. I'm also saying that if the captain of the Ethan Allen had done it with passengers aboard before casting off he would have known that the boat was unstable, as loaded. If rocking the boat with passengers aboard is disconcerting to them, other ways can be found to utilize roll period testing to perform frequent checks, which could be helpful in educating everyone concerned about the chatacteristics of individual boats. Captains of small craft do not carry stability books with information to compare their loading condition against, as ships do. I think roll period testing is a valuable tool for small craft in whatever part of the world.

    Since I've mentioned New York State Parks, Recreation, & Historic Preservation commissioner Bernadette Castro by name I should mention that she has replied to my letter assuring me (and other recipients of similar letters, I'm sure), that New York State is implementing the NTSB's 174 pound per passenger modification to the Coast Guard standards, and saying (in so many words) that New York State will be looking to the NTSB for further recommendations on corrective & preventative actions to be taken.
     
  7. trouty

    trouty Guest

    I agree

    with what you've said Stephen.

    I can't help wondering - if the test you describe couldn't be "abbreviated into a short "aid memoir" for captains to easily remember....just as they do with Navigation etc... (East is least, West is best, or...Red sky in Morning / sailors warning etc..you get the idea!).

    Something "witty" that skippers can pass on - to would be boat owner / operators - that they can easily recall - when wanting to test roll period/ passenger loading.

    Maybe thats something the board could all contribute too...

    A witty way to memorise / immortalise Stephens quick stability test...

    So - how bout it - anyone got any ideas?

    Somethin about "Rockin the boat" has to be part of it, no???? (And it should rhyme to make it easey to remember - so much the better of it's funny to boot!

    Cheers!
     
  8. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Beam in meters, the roll must be sweeter.... something like that? Might be best if it contained the notion that the roll must be FASTER, and that one measures a full period - OVER AND BACK.

    For some reason a roll period test is sometimes referred to as a "sally test." I used to do this to wooden Haitien freighters leaving the Miami River stacked with clothing & bicycles bound for Haiti. We used windage area to determine a GM(req'd) for each individual boat, including the pile of bicycles on the cabintop. We'd chock docklines on center bow & stern and remove the others. Then we'd float the boat just clear of the dock and slack the lines. I'd get the whole crew to jump onto the gunwale and then back onto the dock in unison to get the boat rolling. In order to get them to coordinate effectively I found I had to do it with them. I'd then time a group of 3 full rolls, over and back, and divide by 3. I'd repeat this 5 times, throw out the highest & lowest result, and average the middle three.
     
  9. cyclops
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    cyclops Senior Member

    I was on a old steam powered double deck boat in Canada. The capitain was very non- bending on no more than about 6 of us on the upper deck.-------------------- Hindsight, he under stood the limits. We crossed several wakes at full speed and various angles in 25 to 35 mph gusts with no more than a 5 to 10 degree list.------The boat was a restored original in private ownership.--------When the tour ended, everyone lined up on 1 side to look to shore. No real list then either.
     
  10. Stephen Ditmore
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    Stephen Ditmore Senior Member

    How 'bout:
    Beam in meters, the roll must be sweeter + Rock & roll, don't go if it's slow. (?)
     
  11. CDBarry
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    CDBarry Senior Member

    There used to be an IMO (maybe) standard for roll periods for fishing vessels. Try looking at NVIC 5-86 (or so) on the Coast Guard website. IOt was never widely used in the US as a primary requirement, but it might be interesting.
     
  12. Enrico
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    Enrico Sailor, Thinker

    3-D Rendition of Ethan Allen

    There are some new pictures posted on our web site which show a 3-D mock-up of the Ethan Allen. It is striking to me that it such a shallow draft, and with the modified canopy, it APPEARS top heavy to the naked eye. There is almsot nothing below the waterline. It look like a canoe.

    Would not a good captain/boat owner question this design on sight alone? Or at least the modification of the vessel which added so much more wieght up high.

    I have been on the water all my life and this vessel simply looks suspect. I can't imagine any math formula which would make this ship, with the modified canopy, stable even without passengers.

    I also can't imagine a captain not noticing that the ship had a slow righting moment (which the Ethan Allen must have had) and thus might have stability issues.
     
  13. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I don't think the 3d model at Enrico's site is an entirely accurate representation of the Dyer 40 hull. Also, if one could always judge these things by eye, there are a lot of cruise ships that look like they might flop over on their sides at any moment.

    I agree that there are reasons one might have been suspicious. Mine is feeling that the passenger rating is just plain high for that size boat. But hindsight is 20-20. The real question, as I see it, is what a professional should do if they are suspicious, and some valid, quantitative method that can be used on the spot, I think, could play a valuable role.

    It's still not clear to me whether New York ever performed a Simplified Stability Proof test on the Ethan Allen or a sistership, or how the passenger rating was arrived at. It seems to me this information should be available through a FOIL request (see http://www.pbs.org/now/politics/foiamap.html). I think a big part of the burden should be on the state to show that they performed their regulatory role properly.

    Enrico, do you have data from an actual test performed on the Ethen Allen or a sistership? I'd be particularly interested to know the freeboard and, if known, the full load displacement at the time of the test. How much freeboard was lost as how much weight was shifted how far with how much total weight on the deck?
     

  14. Enrico
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    Enrico Sailor, Thinker

    3-D Graphics Are Visually Suspect

    Stephen: I agree and have posted a comment on the issue here.

    Here is my comment and I would ask anyone else to weigh in on what other differences they can visuallly identify...

    A question has been raised at the BoatDesign.net BBS as to whether these 3D graphics are an exact replication of the Ethan Allen. I have to agree. If you look at the GUNWALE (the upper edge of a boat's sides), towards the bow, it certainly appears that there is more upward curve than on the actual pictures of the Ethan Allen. Also, the windows do look taller than in the photographs. The origin and purpose of the 3-D graphics is unclear. However, I think the graphic does offer some idea of how high the center of gravity must have been on this boat and how slow the righting moment must have been as it heeled (tilted) to either side.​

    Keep up the great work and discussion. I could not agree more with your thoughts about trying to find some easy vehicle for commercial boat owners (especially tour boats) to have an objective measure with which to easily gauge stability and roll moment. I would also be interested in compiling a 'red flag' list for boat operators where they might question a posted capacity limit. Must we rely exclusively on common sense?
     
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