Box barge hull shape - which is better?

Discussion in 'Hydrodynamics and Aerodynamics' started by Justaguy, Jan 30, 2016.

  1. Justaguy
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    Justaguy Junior Member

    Hi,

    Wasn't sure if this belonged here or in Design. Probably six of one, half dozen of the other as it's related to both. As will be immediately apparent, I have very little knowledge of naval architecture (but I'm learning).

    My question relates to optimizing hull shape for a DIY plywood box barge. My first two questions are very basic on purpose. In the attached simple diagram, I've numbered three hull shapes (side views). Nothing is to scale. Just meant to communicate shapes. Assume vertical sides and a very long deadflat bottom regardless of end shape.

    1 - Box - the reason for improvement needs no explanation. It's just the baseline.

    2 - Box with 45 degree ends - a clear improvement on # 1.

    3 - Box with sharp curved ends - a potential, but not particularly clear improvement on # 2.

    At the moment, I'm assuming # 3 is the best one. However, that may be based on curved bows being my most common mental boat image vs. any scientific reason.

    Taking a stab at justifying my choice, I'd say # 3 has these advantages over #2:

    A. Improved structural strength (think "arch")
    B. Slight increase in hull volume
    C. Speed/drag differences - no earthly idea

    My questions:

    1. Do you agree that # 3 is the best, most optimized shape? Why/why not?

    2. Whichever is best, to what degree? In other words, am I deliberating over tiny fractions of one knot or minuscule fuel efficiencies, or would the difference be substantial?

    Of course, you have to generalize to answer these questions. I'd appreciate any helpful comments.

    Thanks.
     

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  2. NavalSArtichoke
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    NavalSArtichoke Senior Member

    This is the cheapest to build.

    I wouldn't put sharp pointy ends on a barge. You should have a vertical headlog at each end, approximately 12" to 24" deep, extending down from the deck. The rest of the ends can be sloped, however. The headlog should be reinforced so that you can push this barge, if that's how you intend to move it.

    The most expensive construction by far. You don't gain that much reduction in resistance at low towing speeds for the additional cost/problems of fabrication.
    Bote bows are curved, just not in this manner. Hull strength does not depend on the shape of the bow and stern; it comes from the amount of continuous longitudinal material between the bow and stern (side shell, bottom, deck, and longitudinal stiffening.)

    Optimized for what? Strength? Speed? Ease of construction? See discussion above.
    You haven't said much about what this barge is intended for, how it's going to be moved, what it's supposed to carry, how big it is, etc. How are we supposed to divine your intentions from such meagre information?
     
  3. gonzo
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    gonzo Senior Member

    Optimization needs to have a goal like NA says. You need to assign each parameter a value and then compare them. Usually a decision matrix is used for that.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Why do the ends have to be identical ? You want a good bow shape and a good stern shape, which doesn't necessarily equate to a mirror image.
     
  5. Justaguy
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    Justaguy Junior Member

    Purposeful simplicity

    Thanks for the info.

    Though failing, I tried to explain in my initial post that the meagre information was purposeful. I intended that those people with vastly more knowledge could use that knowledge to make useful generalizations vs. make the exercise more complicated, which would be opposite my goal at this stage.

    Optimized for what? Optimized overall in terms of performance (not ease of construction). If I had to describe the general scenario, I'd say, I'd like to identify that basic shape which would move most easily through the water, put up the least resistance, therefore reduce drag, therefore increase fuel efficiency, therefore optimize speed or at least ease of getting to its displacement hull speed. Assume it would be under its own power.

    If you'd like to help, it would be best not to think in terms of spreadsheets and calculators, but more in terms of maxims (or at least in maxims derived/condensed from more technical methods).
     
  6. Justaguy
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    Justaguy Junior Member

    Not necessary

    I know. They don't have to be. Again, I was shooting for simplicity in an attempt to avoid drowning in an endless pool of shades of gray. If it makes the question easier to answer, just chop the boat in half mentally and consider the remainder to be the bow.
     
  7. Justaguy
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    Justaguy Junior Member

    Yes, unless ...

    Thanks. Please see my replies to NA and Mr Efficiency.
     
  8. NavalSArtichoke
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    NavalSArtichoke Senior Member

    OK, here's one for ya: Maxims never designed a bote.

    If you don't know what you want to do with this barge, then folks on an anonymous forum certainly have no idea.
     
  9. Justaguy
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    Justaguy Junior Member

    I hesitated to reply to your first response because it seemed like you didn't really want to be helpful. You seemed more interested in showboating or pointing out what I didn't know. This second reply seems to confirm that.

    Maybe you're having a bad day, or maybe you're always like that. In any case, I can happily say that yours is my first unhelpful response on an otherwise helpful and understanding forum.
     
  10. NavalSArtichoke
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    NavalSArtichoke Senior Member

    It's not that, but if you have no idea what you're going to do with this barge, then we don't either.

    Botes aren't designed in a vacuum with a handful of maxims pulled out of a drawer. If someone wants to design a barge, then they first figure out what they want to carry with the barge, how much of it they want to carry, and how quickly they want to carry it.

    BTW, most barges are designed to be towed at relatively low speeds, say 5 knots of so maximum, so the impact of hull shape on resistance is pretty minimal. There are plenty of commercial barges which are just plain boxes, and quite a few which are raked at one or both ends.

    You apparently haven't decided if you want to push this barge, tow it on the hip, use a hawser, etc. That's not me having a bad day, but it is something you need to ponder, since this is going to be your barge.

    I think I gave you some useful replies to your initial post, but you didn't want to hear them. Fine. But don't be surprised or upset if things don't go as swimmingly as you hope.

    I've worked around barges most of my career as a naval architect, and I've seen what works and what doesn't.

    Good Luck!
     
  11. Justaguy
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    Justaguy Junior Member

    Wrong foot

    NavalSArtichoke,

    Somehow, we've gotten off on the wrong foot. It happens. For my part, I'll back up and assume that you generally want to help. Intended or otherwise, your tone distracted me. You certainly did provide some useful information in your first reply. I failed to acknowledge that. That was my mistake.

    I do know mostly what I want to do with this barge. For example, I mentioned that it would be self-powered. While that is only one piece of information, again, that is purposeful.

    What I don't want to do, in the beginning, is approach this like a NASA engineer. That is not due to a prejudice or lack of ability on my part. There is a very specific educational reason for that.

    Simply put -- if you're going to teach someone something, you have to meet them where they're at, not where you're at. Additionally, people new to a subject learn in a sequence, beginning with generalities, and moving to details. Why? Details tend to be rejected as irrelevant or incomprehensible until there is a relevant and comprehensible generality to hang them on.

    So, Take 2?
     
  12. gonzo
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    gonzo Senior Member

    Ok, the first generalities are the purpose of the barge and area of operation. That will give some constraints. For example, if it is going to be a dredging spoil barge, it needs flotation chambers and an opening bottom. An oil barge will have tanks with dividing bulkheads, etc.
     
  13. NavalSArtichoke
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    NavalSArtichoke Senior Member

    I must have missed that this vessel was intended to be self-powered.

    You don't want to design a barge then; you want to design a powered vessel which can carry as-yet unspecified cargo. This is an entirely different animal. Much more complex undertaking than just making a hull which floats and has some room to put stuff on it.
    I don't know what this means.

    NASA engineers do what they do the way they do it because the price of failure is very high.

    You should not approach this project in a less-than-professional fashion just because you want to design a boat rather than a rocket. Plenty of people get hurt or even killed with boats because they didn't take reasonable and prudent precautions.

    You haven't elucidated why you are not interested in assembling the necessary facts and figures which would be useful in designing this vessel. If you are not willing to do the basics, you are not designing anything, merely speculating.
    The first step in any boat design project is to lay out as completely and fully what you intend the vessel to do. You don't necessarily have to know how you're going to do it, but you must know what the end goal will be.

    If you want to carry 5 tons of cargo, that suggests a vessel of a certain size. If you want to carry 50 tons, then it's going to be a much bigger vessel than the 5-tonner.

    An anonymous forum is a terrible way to teach someone about a complex subject like boat-building. There are plenty of books written for the novice who knows nothing about boats. I urge you to select a couple of titles and familiarize yourself with the basics. There is even a section on this site which contains a Boat Design Book Store that you can browse for more info.
     
  14. Justaguy
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    Justaguy Junior Member

    Well, at least I tried to put my best foot forward.

    If there's anyone else who is able to comprehend what I'm asking, has the ability to respond at that level, and would like to offer a helpful reply, please do so.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    You were not specific enough about your ideas, I'm afraid.
     
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