New requierments for boats over 20ft.

Discussion in 'Stability' started by dougfrolich, Jul 16, 2012.

  1. tunnels

    tunnels Previous Member

    But i didn know ,no body told me !!!

    Because you are dealing with the idiot factor in this case the person/s could simply get off scott free !
    Go back a couple of steps and point the finger at the person that organised this outting as well they are equally to blame !! theres a collection of idiots all sitting for a length of time ,the clock was ticking for that one freak event and thats what nature is really good at catching you when you're least exspect it and are not prepaired . But when you step back a litttle further yet again and look at the chain of events theres more than one idiot person to share in the responcability of this accident . what about there parents of the person/s they should foot some blame for raising idiot children and so on and so on !!! :mad:
  2. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Thank you to Eric Sponberg & Eric Sorensen for undertaking to study this accident and publish on the issues raised. It sounds like you have a strong group of small craft designers organized to look into this, and I'm glad you're on the case.

    I was part of a similar group that looked at the capsizes of the Ethan Allen and the Lady D and recommended the Coast Guard increase the assumed weight of an individual to 190 pounds, among other things. I posted more about this here.

    It's clear that Eric & Eric's group is pursuing the information needed to properly evaluate the specifics of this accident. While I respect Tad's decision not to get out ahead of the available facts, I see nothing wrong with publishing related educational information.

    I do have a quibble of emphasis with one paragraph in Eric Sorensen's Soundings article. He states:
    An inshore planing hull such as the Silverton’s has little depth below the waterline
    and proceeds to explain, correctly, the distinction between initial and ultimate stability. But initial stability is important, and boats with a high ratio of waterline beam to hull draft have a high metacenter. Also, pounds per inch immersion correlates with waterplane area; so a wide, shallow hull has benefits.

    I was pretty much the small boat guy on the committee studying the Ethan Allen capsize, and one of the things I was looking for was a way to take a large quantity of very basic data about boats of differing size and design and organize it in a way that made sense. We had an opportunity to look at a database of inspected passenger vessels that included passenger and crew capacity ratings and which had passed and failed intact stability tests. I found organizing the data according to whether (#passengers+crew)/(L*B^2) exceeds 0.004 (0.0045 if waterline length and beam are used) was helpful over a wide size range. It became quite clear, at least in my view, that L*B^2 is a better predictor than:
    • Length alone
    • Beam alone
    • Displacement alone
    • Tonnage (volume) alone
    • L*B
    • L*B^3
    While not sufficient by itself as a basis for passenger capacity, correlation with L*B^2 is useful. I urge Eric & Eric's committee to calculate Kandi Won's L*B^2 and to compare passenger capacity to other vessels on this basis. Then other factors can be accounted for, especially those affecting VCG.

    Boats under 20 feet can continue to be rated according to 33 CFR Part 183 and ABYC H-5. But these standards are full load displacement based (simillar to a load line calculation for ships). For boats between 20 and 79 feet it's appropriate that capacity correlate with intact stability, and intact stability correlates better with L*B^2 than with displacement. One possibility for developing a rating system for medium sized recreational boats might be to require a higher standard of proof from manufacturers that claim a capacity over (#passengers+crew)/(L*B^2) = [some value]. I'd like to see Eric & Eric's committee explore it.

    FYI the number of people aboard the Ethan Allen when it capsized /(L*B^2) = 0.009.
  3. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Lee Dana, former chief engineer at Bertram, wrote an interesting piece about this sort of design issue in Professional BoatBuilder. Have you read it? Parting Shot, Issue 112.
  4. Frosty

    Frosty Previous Member

    There is also the sheep syndrome. "no one else seems concerned --must be me Baaaa I dont want to cause a problem"
  5. Tad
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    Tad Boat Designer

    Yes I have, and I agree totally with everything he said. The stability/seaworthiness issue has been glossed over by the American yacht industry forever.....thus they now reap the reward.......
  6. Eric Sponberg
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    Eric Sponberg Senior Member

    We certainly are considering simplified predictors of stability just as you suggest. The function for available deck area would be LxB, that for stability would be LxB^3, and we found from an advisor from England that he uses a function of LxB^2, exactly as you suggest. This is kind of between the two extremes.

    If and when we get more information about the specific design, we can do a more in-depth and accurate study. I have advocated as part of that study to evaluate a simple function such as # of Passengers = LxB/c or LxB^2/d against what the true stability is on that boat, plus whatever it may reveal on a population of boats. In these functions, in the former, the "c" would be a constant, and in the latter, it might be a number that varies linearly. If you make a regulation or guideline too complicated, requiring the use of a computer, for example, no one will use it. Also, you will never be able to rate existing boats, only newer boats where the lines can be analyzed for stability.

    The purposes of such an evaluation would be exactly as you say, to easily rate boats, both by boat manufacturers and by users. Such a system simply does not exist at the moment.

    I think your study would be most helpful and enlightening if this work continues forward. I'll be sure to contact you if it does.

  7. Lister

    Lister Previous Member

    This so right! Thank you Frosty.
  8. Ike
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    Ike Senior Member

    Many years ago the author Robert A. Heinlein said in one of his novels;

    "Never underestimate the power of human stupidity."

    Both he and Frosty are right.
  9. RigPig
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    RigPig Junior Member

    I am taking a state boater safety education course and they state with great authority that "the approximate capacity for a vessel is beam x LOA / 15 = the capacity in 150lb persons." Using this calculation this vessel would be rated for 29 persons...I think this rule might be in serious error for vessels over 20 ft.
  10. Stephen Ditmore
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    Stephen Ditmore Senior Member

    What state? Did they cite a source?

    In consultation with SNAME ad hoc panel 15 the USCG has raised the weight of a person for regulatory purposes to 185 pounds.
    See for a partial explanation.

    While I think your rule-of-thumb sounds better than nothing, I don't think it will scale right for larger & smaller boats - in particular narrower vs beamier boats. So I think you're right that it's utility is probably limited to a certain size range. For any boat where passengers stand on a cockpit sole or deck that is well above the inside bottom of the hull, my preference would be to use 0.004*L*B^2, which in this case would be 0.004*34*12.5*12.5 = 21.
    Last edited: Sep 17, 2012
  11. tunnels

    tunnels Previous Member

    How can you regulate against stupidity ??? rule as many thumbs as you can find it still comes down to common sense just becasue you can get 10 people in you car does it mean its safe ??
    just because theres enough room in a boat for 40 people does that mean thats what you can carry ???
    Reguardless of the guy loosing his daughter he deserves a good swift term in prison for being a idiot !!! the word NO is one of the shortes words in the english language but the hardest to say so it seems !! :mad:
  12. Saildude
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    Saildude Junior Member

    I think the rule of thumb is for boats under 20 ft if my memory is correct (I teach boating safety)

    when ask about the rule of thumb I usually state with authority that all rules of thumb have problems and that I personally don't trust them very far - yes there are some small boats that have a high capacity plate listing that somehow the manufactures manage to get on their vessels - they get a bit extra attention when loaded to the capacity plate those boats get a bit more attention from law enforcement boarding parties as the boats really don't look safe -
  13. Ike
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    Ike Senior Member

    Having spent a good 25 years of my life enforcing USCG regulations for capacity on recreational boats, I can state with authority that the person who told you that is wrong. L X B / 15 is an old rule of thumb that goes way back to the 30's and 40's and applies only to boats under 20 feet, and then only to monohull power boats. It is based on volume. But on boats over 20 feet the volume of the boat increases geometrically, and so rapidly that it no longer applies. The capacity of larger boats is almost always based on stability. The following is all for the USA. For Canada, and Europe the rules are different.

    The USCG does not use that rule of thumb. It uses the displacement weight (the weight it would take to sink the boat) divides it by five for outboards, by seven for inboards, and subtracts such things as engines, batteries, controls, to get a weight for persons Wp. That weight is punched into a formula Wp + 32 /141 This is only for monohull power boats under twenty feet in length , and rowboats. For canoes. kayaks, inflatables, multihulls and so on, and all recreational boats twenty feet or more in length there is no Federal Rule. There are ABYC standards, and ISO standards and a raft of other standards such as ABS, Lloyds, MCA, Det Norske Veritas, the list goes on.

    For commercial vessels carrying six or less passengers the formula is the same.

    For commercial vessels carrying more than six the number of allowed passengers is based on stability. An inclining test is performed. The amount of weight in the test (assuming a successful test) is then divided by 184. (it used to be 160) to determine the number of passengers.

    See Safe Loading Capacity
    See Capacity Vs Seating
    See: How Many People Is Too Many

    See also Boating and Boatbuilding Myths: there are three items that deal with this
    2 people like this.
  14. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Excellent info from Ike. Thanks and I agree.

    Having said that, the slightly more sophisticated rule-of-thumb I advocate, 0.004 x L x B^2, nicely defines where the line is that defines "stupidity". It does leave more to say, though, about weight on the upper deck or flying bridge affecting center of gravity...

  15. mydauphin
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    mydauphin Senior Member

    Darn it has maths, physics and laws, may be I should hire a professional to figure it out and not guess at it. What a concept.

    I believe to concept of passengers is obsolete. Those 350 and 400 pound passengers really throw the chart out of wack when you do it by passengers. The boat should have a Gross and net weight rating and that is it. Whatever you put in goes against the difference, the sticker should have weight of fuel and water if fuel and then captain can weight the passengers. And it should be next to the steering wheel.
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