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couldn't you build a heavier model to compensate? I mean most boats are hollow right? I guess that changes some of the internal distribution of weight but if i

by rtpg 12y ago

couldn't you build a heavier model to compensate? I mean most boats are hollow right? I guess that changes some of the internal distribution of weight but if its done in a uniform fashion things should work out ok right?

Pampi 12y ago
> couldn't you build a heavier model to compensate? I mean most boats are hollow right? Yes, it's possible. In fact, dimensional adjustments and compensations are standard practice in scale modeling.
Padding 12y ago
> but if its done in a uniform fashion things should work out ok right? I wouldn't be so certain of this. If the "uniform distribution" of the added weight results in the walls being (proportionately) thicker or denser than in the original, the model may be an invalid one.
baddox 12y ago
I don't think buoyancy would be much of an issue with scaling, because that's just related to density and shape, neither of which change when scaling the model (at least until you get small enough that surface tension becomes noticeable, which I presume doesn't happen for ship modeling). I suspect the bigger issue would be the structural integrity of the boat. A 10 foot steel ship is going to be a heck of a lot stronger than the same ship scaled up to 100 feet.
- lutusp 12y ago
  > I don't think buoyancy would be much of an issue with scaling, because that's just related to density and shape, neither of which change when scaling the model ... But that's false. The hull wetted area of the model changes as the square, while the volume and mass change as the cube, of the size change. If corrective measures aren't taken, the model will displace more water proportional to its wetted area as it becomes larger, as a result of which it will gradually sit lower in the water as its size increases. There are a number of ways to deal with these issues, but it's not true that one can scale a model without considering them in detail, and carefully ballasting the model to force it into an approximation of full-size reality.
  - baddox 12y ago
    Interesting. That doesn't match my initial intuition, but it sounds like you're right. Can you point to a source, or perhaps explain this a little more? I'm trying to imagine a pathological shape that would obviously float at one size but not when scaled up.
    - lutusp 12y ago
      > I'm trying to imagine a pathological shape that would obviously float at one size but not when scaled up. I wouldn't go that far. I know models sit at different heights for different scales, all else being equal, but I don't think you will ever see a non-pathological object sink at one scale but remain afloat at another. The reason I think this is true is that, if I take two objects having the same overall density and connect them together, this cannot change their position in the water, their buoyancy. If I think of the two objects as a single model, the same should be true. But the difference between connecting two models, and a proportional scaled-up model, is that the ratio of surface area to volume is different for the scaled-up model compared to the two independent models. So the comparison isn't perfect. (pause for thought ...) For a solid object sitting on a table, making the object larger and noting the previously described square-versus-cube rule, the table loading should increase for each square unit of table area as the model's size increases, by a unit rule, meaning if you double one of the three dimensions, the table loading (per unit of area) doubles also. But because a boat model sinks into the water as its mass increases, and because that sinking is across curved surfaces, it's more like a three-dimensional area increase than a two-dimensional one, so it can't be compared to an object with a flat bottom sitting on a table. The tl;dr: the more I think about this, the more I think I was wrong to say it the way I did -- and I say this because the shape of the boat hull means the wetted area can increase as fast as the boat's mass, i.e. as the cube of a dimensional change. Expressed another way, even though a larger boat model sits lower in the water, with some care the waterline position on the hull can be made to stay the same. My mistake -- sorry.
      - baddox 12y ago
        If the water line can be made to change when scaling the boat up, it should be trivial to make a pathological object that will sink when scaled. Just make the shape of a box without a lid, weighted so that the water line is just at the top of the walls. If scaling the shape raises the waterline, then water will flow into the box and it will sink. I still can't intuitively convince myself that the waterline would shift when the model scales.

by rtpg 12y ago

Pampi 12y ago

> couldn't you build a heavier model to compensate? I mean most boats are hollow right? Yes, it's possible. In fact, dimensional adjustments and compensations are standard practice in scale modeling.

Padding 12y ago

> but if its done in a uniform fashion things should work out ok right? I wouldn't be so certain of this. If the "uniform distribution" of the added weight results in the walls being (proportionately) thicker or denser than in the original, the model may be an invalid one.

baddox 12y ago

I don't think buoyancy would be much of an issue with scaling, because that's just related to density and shape, neither of which change when scaling the model (at least until you get small enough that surface tension becomes noticeable, which I presume doesn't happen for ship modeling). I suspect the bigger issue would be the structural integrity of the boat. A 10 foot steel ship is going to be a heck of a lot stronger than the same ship scaled up to 100 feet.

lutusp 12y ago
> I don't think buoyancy would be much of an issue with scaling, because that's just related to density and shape, neither of which change when scaling the model ... But that's false. The hull wetted area of the model changes as the square, while the volume and mass change as the cube, of the size change. If corrective measures aren't taken, the model will displace more water proportional to its wetted area as it becomes larger, as a result of which it will gradually sit lower in the water as its size increases. There are a number of ways to deal with these issues, but it's not true that one can scale a model without considering them in detail, and carefully ballasting the model to force it into an approximation of full-size reality.
- baddox 12y ago
  Interesting. That doesn't match my initial intuition, but it sounds like you're right. Can you point to a source, or perhaps explain this a little more? I'm trying to imagine a pathological shape that would obviously float at one size but not when scaled up.
  - lutusp 12y ago
    > I'm trying to imagine a pathological shape that would obviously float at one size but not when scaled up. I wouldn't go that far. I know models sit at different heights for different scales, all else being equal, but I don't think you will ever see a non-pathological object sink at one scale but remain afloat at another. The reason I think this is true is that, if I take two objects having the same overall density and connect them together, this cannot change their position in the water, their buoyancy. If I think of the two objects as a single model, the same should be true. But the difference between connecting two models, and a proportional scaled-up model, is that the ratio of surface area to volume is different for the scaled-up model compared to the two independent models. So the comparison isn't perfect. (pause for thought ...) For a solid object sitting on a table, making the object larger and noting the previously described square-versus-cube rule, the table loading should increase for each square unit of table area as the model's size increases, by a unit rule, meaning if you double one of the three dimensions, the table loading (per unit of area) doubles also. But because a boat model sinks into the water as its mass increases, and because that sinking is across curved surfaces, it's more like a three-dimensional area increase than a two-dimensional one, so it can't be compared to an object with a flat bottom sitting on a table. The tl;dr: the more I think about this, the more I think I was wrong to say it the way I did -- and I say this because the shape of the boat hull means the wetted area can increase as fast as the boat's mass, i.e. as the cube of a dimensional change. Expressed another way, even though a larger boat model sits lower in the water, with some care the waterline position on the hull can be made to stay the same. My mistake -- sorry.
    - baddox 12y ago
      If the water line can be made to change when scaling the boat up, it should be trivial to make a pathological object that will sink when scaled. Just make the shape of a box without a lid, weighted so that the water line is just at the top of the walls. If scaling the shape raises the waterline, then water will flow into the box and it will sink. I still can't intuitively convince myself that the waterline would shift when the model scales.