keel and flat bottom vs round bilge

Discussion in 'Boat Design' started by izaac, Jun 12, 2020.

  1. izaac
    Joined: Mar 2020
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    Location: brisbane Queensland australia

    izaac New Member

    i know a flat bottom boat (large boat 40ft plus) has more initial stability but will it roll more or less in a seaway than a round bilge boat of similar sizes (Same DL ratio beam ect)
     
  2. gonzo
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    gonzo Senior Member

    That is too simplistic. There are more parameters to consider. For example, prismatic coefficient and flare.
     
  3. DCockey
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    DCockey Senior Member

    Differences in CG height will also be a major factor.
     
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    I do not understand, I am not clear, how "prismatic coefficient and flare" can influence the stability and roll of the boat. It is possible that they influence, since everything in a hull influences its behavior afloat, but I do not see how these two parameters do it. Any explanation please?
     
  5. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    More.
     
  6. Will Gilmore
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    Will Gilmore Senior Member

    I don't know what prismatic coefficient is, but flair can act a bit like an outboard pontoon. As the boat heels, more hull, away from center, enters the water and shifts the center of buoyancy to that side.
    There are limits to this and the central attribute will be the height of the CG.

    As for flat bottoms, they are very stable, depending on location of center of gravity. Take a square block of wood, like a 2x4, compared to a round 4" dowel. That 2x4 will always settle on one 4" face or another. The dowel will roll all over the place.

    Once you put a weighted keel on one side, the story changes a bit. The keeled 2x4 will be a lot more stable than the keeled dowel, until the cg swings over and above a certain point. Then, the very stability of form that the flat 4" face has, will hold the board upside-down. Not so with the keeled dowel.

    Now, back to the uniform material form of a block of wood. In both cases, the cg is right in the center. A square block, like a 4x4, will also find a flat face to float on. Now, plane a flat face on your dowel. By removing material from one side of your round cross-section, you have shifted the cg of center. While it moves away from the center of the arc opposite the flat spot, it is actually closer towards that flat face, proportional to all the other surfaces. You will not only get the bobo doll effect of having that offset cg, but you will start to also get form stability from that flat face. Once you have cut the dowel in half, you have maximum form stability working perfectly with weight distribution so you get the best of both worlds. Add a keel to the center of that face and you have a vessel that is not only very very difficult to roll, but it will always roll back up, no matter the inverted angle.

    -Will ("artist, not an architect", Dragonfly)
     
  7. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    You are very right in what you say but it has nothing to do with what the OP has stated. The OP talks about the initial stability of both types of ships and, if by initial stability we understand the value of the initial GMT, the flare has absolutely no influence on it.
    Speaking of the "central attribute", since these are two equal and opposite forces, weight and buoyancy, the "central attribute" is the position of the center of gravity and is as attribute and as central as the position of the center of buoyancy (whatever "central attribute" means).
     
  8. gonzo
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    gonzo Senior Member

    Stability is the sum of all the moments that resist heeling. A boat with blunt ends will have higher moments. The overall shape needs to be considered, not just the bottom. The comparison of a 2x4 to a dowel would only be valid to a barge with square ends and no rocker. Basically no boats have that shape.
     
  9. TANSL
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    TANSL Senior Member

    Stability is much more than that. For example, what about the work done by the forces involved in the balance of the ship? Have you ever thought about that? This work, carried out when the boat heels, must reach certain values for the boat to be considered safe.
     
  10. Will Gilmore
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    Will Gilmore Senior Member

    In the case of vessels rolling in a seaway, it is often, though not the only case, that form stability contribute to the rolling as a wave picks up one side of the boat, by the buoyant forces, and not the other. As the wave rolls past, the boat with form stability will follow the surface angle of the water, "rolling". A skinny boat will react less than a wide boat until the width begins to equal the wave length. The skinnier boat with keel weight for stability, will stay upright, even when the water surface under the boat tilts. The shape of the bottom contributes, but in my limited sailing experience, a fat rounded bilged boat will roll more than a skinny flat bottomed boat. Trawlers roll, sharpies, not as much.

    -Will (Dragonfly)
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Well, first you need to define Initial Stability, so that you understand the terms and what they represent.

    When a body is floating in water, and it is in equilibrium, i.e it does not move, it floats upright. This means the centre of gravity (CoG) is in line with the centre of buoyancy (CoB). If there is a small disturbance to that body, the body will incline through a small angle. When it does this the CoB moves and is no longer in-line with the CoG.

    Assuming the CoG is above the CoB, the up thrust of buoyancy at the CoB will create a moment. The moment is defined by the distance, or lever, from the CoG to the vertical line of up thrust of the CoB, shown below:

    upload_2020-6-13_11-16-36.png

    This moment is a product of the lever, or the distance, from the CoG to vertical line from the CoB, and the buoyancy, or what is commonly called the displacement, W. The level is the GZ.

    Where this vertical line from the CoB intersects with the original vessel’s CL is a location denoted as M; the position of the metacentre. With the CoG being below M the body is said to have positive stability.

    Under small angles of inclination the body always remains stable and returns to the upright condition. This is what we define as Initial Stability or in simple words, when the vessel is upright or very close to it, when at rest. This is true for ANY boat... no matter its shape.

    Now you are talking about a dynamic behaviour, which different to initial stability but it is linked to its initial stability namely the hull shape. This goes into a very complex subject called seakeeping and is not a one liner explanation at all….

    BUT… as a first very quick simplistic guide (for now ignoring many other inputs for clarity) , take a flat bottom boat as you described:

    upload_2020-6-13_11-19-22.png

    Compared to a more conventional shaped one with round bilges:

    upload_2020-6-13_11-20-3.png

    There is a big difference in the pressure distribution and hence its damping.

    Increasing the damping reduces the amplitude, and if you reduce the amplitude you reduce the angle of the roll.

    Like all simple one liner Qs... there is far more nuances to it then a yes or no. It just leads to more questions and greater depth within the subject.
     
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  12. Ike
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    Ike Senior Member

    To put it simply a flat bottom boat of the same basic dimensions as a rounder bottom boat, in a sea way, will have very snap movements, quick but short roll and recovery, and pound a lot. In a really rough sea it will beat you up. A round bottom will roll more but it will be gentler, with a slower roll and recovery. It may roll a little farther than the flat bottom, but won't beat you up. Frankly in any kind of a sea way I would prefer to be on the round bottom boat. Initial stability may be better for the flat bottom but if it rolls too far it will capsize sooner than the round bottom in the same conditions. But as been said by others, a lot depends on the form of the hull, flare, CG, Beam length ratios and so on.
     
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