(Stupid) Pointy Scow

Discussion in 'Hydrodynamics and Aerodynamics' started by misanthropicexplore, Apr 10, 2018.

  1. misanthropicexplore
    Joined: Apr 2018
    Posts: 72
    Likes: 6, Points: 8
    Location: Upper middle Missouri River

    misanthropicexplore Junior Member

    Hello. It's customary to introduce yourself with your first post, so: First things first, I'm literally never going to design an awesome boat. Yrvind, Wharram, or Leydens I ain't. I If I'm lucky, I might be able to buy a good used boat someone else designed. If I win the lottery or something (I'd have to play first) I might go as far as sketching up my ideas for someone who knows what the hell they are doing so they can translate them. And I'm not a student doing naval architecture or marine engineering. I'm just a guy that thinks boat design is really cool and wants to learn more. I have a decent (hobbyist) background in aerodynamics from building my own RC models, but hydrodynamics seem to be another beast all together, mostly because of wave formation? A lot of my posts are going to begin with "So I recognize that this is a stupid idea, but I would like to have a better understanding of WHY it's a stupid idea" because that's how I learn. Onward...

    Stupid idea number one:

    We have two boxes that are 1' tall, 4' wide, 8' long. We want to push both through the water. Because I'm trying to understand first principles here, and not practicality, we have a magic motivator that moves the boat with changing it's angle to the water, both boats have exactly the same mass, the water is still and mirror smooth other than the waves caused by the boats, etc. The difference between the two boats is that one is just a box, and the other adds to that box a 1' high fence of some minimal thickness that makes a 22' long fine entry/exist shaped area in front of and behind the box. The boat's displacement puts the hull down 4", so the depth of water fenced off i 4" deep. So both boxes have a plumb, square bow and stern, but one has fence (no floor, no deck, just a wall) that give it a 12:1 slenderness ratio (total length of 52') and one is just a 8' long box. We want both hulls to go 3 knots (80% of standard "hullspeed" for an 8' waterline length).

    Now, it would seem to me that one with the extensions should have more drag because it has (1.) more wetted surface (2.) two extra "eddy makers". It would seem also that the water in the fenced off area would be moving with the box, making a greater mass, and requiring more energy to accelerate. Yet, doesn't have minimal wave making compared to the plain 8' box? Aren't the huge eddies off the bow of the plain box going to make a lot of drag?

    At the same time, if made a 52' x 4' shape and enclosed the bottom, we don't have those square bow and transom dragging eddies anymore...but assuming we still have a 4" depth throughout, we would still have the mammoth displacement of the fine end fenced box, AND vastly more wetted area. So has our energy to get it to 3 knots less than the much lower displacement box? Less than the higher drag fenced box?

    I'm guessing the answer is going to be something like "per lb of displacement, the one that takes the least energy to maintain 3 knots is example X, but per inch of draft the one that takes the least is Y, because in one case effect A is the predominate, but in the other case effect B is."

    But I really don't have any idea.
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Misanthrope, your thread has been idle for several days. One of the reasons for absence of response is that there is a possibility that you are merely messing with us. Troll like. That is not an accusation, merely a possibility. If you are serious about this inquiry, some drawings would help.

    I take it that you have three conditions. One is a simple box. According to your dimensions the box would displace 665 pounds in fresh water. It is blunt nosed and blunt tailed. One would need considerable energy to move that thing at 3 knots. It will have about 40 square feet of wetted surface. as well. That is not going to be an ideal boat. If you add 22 foot wedges at front and back, you will have added about 58 more square feet of wet surface. Those fins are going to make a lot of eddies and you will almost surely need to add a bunch more energy to move the beasty thing. You still have 4 inches of draft so you will have to remove some of the load from the box to compensate for the weight of the fins. If you put a bottom on the fore and aft, triangular parts, the wetted surface will become huge, maybe about 88 more than the 58 you already have. Now the displacement will be something on the order of 2500 pounds if the 4 inch draft is maintained.

    This is merely an exercise in basic arithmetic. The problem is hydrodynamically indeterminate with such simple math. . If I was forced to paddle or row one of those things, I'd probably choose the basic 4 x 8 box...... and a PFD.

    Lets see what the other guys think......
     
  3. misanthropicexplore
    Joined: Apr 2018
    Posts: 72
    Likes: 6, Points: 8
    Location: Upper middle Missouri River

    misanthropicexplore Junior Member

    Yeah, I know it's stupid question, but I feel like it's a stupid question that I should immediately have the answer for and I don't: in a very small displacement barge at 50-80% of hull speed, what is the biggest source of drag: skin friction, or wave formation. If it's skin friction anything increasing the skin would slow you down, no matter how "boat shaped" it made it. If it's wave formation, than making it more boat shaped would make a difference. I don't have anyway to get pictures online from my home computer. I'll post some pictures next time I'm at my friend's place and can scan in a sketch.
     
  4. misanthropicexplore
    Joined: Apr 2018
    Posts: 72
    Likes: 6, Points: 8
    Location: Upper middle Missouri River

    misanthropicexplore Junior Member

    More than anything I'm trying to understand why barges and tankers move some of the greatest loads per horsepower, but have some of the least "boaty" shapes. I'm asking the question to understand that better. I know part of the answer is economy of construction. I know part of the answer is < ratio of speed to hull speed. But frequently barges look like they have HUGE bow waves, and big tankers have minimal bow waves, despite having very round, blunt bows. So I feel like there is something about the relationship of wetted area, prismatic coefficient, hull shape, driven speed to hull speed, that I am NOT understanding.
     

  5. alan craig
    Joined: Jul 2012
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    Location: s.e. england

    alan craig Senior Member

    Here is my guess. If you scale a hull by 200% (twice as long, wide and deep) you have 2^3 x displacement but only 2^2 x wetted area, so the bigger version needs four times as much power to overcome surface drag to carry eight times as much cargo. Bigger ships don't tend to travel anywhere near "hull speed" so maybe this also becomes less of an issue.
     
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