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Old Feb 24th 2014, 05:35 AM   #1
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Big and small floating vessels

I have often wondered why a big tanker a) can stay stable in the water in spite of its flat bottom and b) why it floats so shallowly. A miniature model of said ship will go deeper and tend to roll much more easily. There seems to be a "scale factor" of some sort but I cannot get my head around it. Same shape, same element...
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Old Feb 24th 2014, 06:32 AM   #2
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I suspect that the center of gravity of your model ship is at a higher point than on a true oil tanker, and/or the center of buoyancy may be lower. When a ship starts to roll the center of buoyancy shifts more than the center of gravity, creating a righting torque. But if the center of buoyancy is too low, or the center of gravity too high, the ship would be unstable and would capsize.

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Old Feb 24th 2014, 09:14 AM   #3
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Scale Effects

There are many non-obvious scale effects that have to be considered when comparing the behaviour of models with the behaviour of full sized vehicles.

Try looking up "Froude Number"
http://en.wikipedia.org/wiki/Froude_number
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Old Feb 25th 2014, 06:39 AM   #4
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Scale effects

Thanks folks. I guess that is why small RC sailboats require a massive keel to sail properly. One problem I have is that my observations are also static, i.e. the phenomenons are apparant in still conditions. Remember when you used to play with small plastic boats in the bathtub? They sat deep in the water - and never straight.
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Old Feb 25th 2014, 10:21 AM   #5
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1.Small plastic boats sit deeper in the water compared to ships because their density is more as compared to that of a ship.
2.Floating bodies tilt so as to make the torque due to buoyant force 0. A ship is designed such that the summation of the buoyant forces passes through the centre of gravity(c.g) with minimum tilting.In case of your plastic ship,due to improper design,it needs to tilt more.
In case of the plastic ship,the centre of buoyancy(c.b) must be higher up compared to c.g and there must be a lot of horizontal distance(compared to breadth) between c.b and c.g .So,it would need to tilt more.
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Old Feb 26th 2014, 08:59 AM   #6
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Scale effects.

I have a problem with this. Small vessels do not necessarily have a higher density - or be of improper design - than large ones. An RC sailboat can be made extremely lightweight and it still needs lots of mass in the keel to sail properly. There seems to be an influence of scale when it comes to the relationship between CB an CG.
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Old Feb 26th 2014, 09:41 AM   #7
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1.First define what u mean by "sit deep"
If u mean that the (volume submerged divided by the total volume of the body)
is greater,then it follows that the density of the plastic ship is greater.
2.As for the problem of floating in a tilted manner,that is independent of density except for c.b position change.C.B,C.G relative position must be considered.And if ur sure that two ships having proportional dimensions,shape,positions of c.g,c.b still show different angles of tilting,then scale factor must play some role...
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Old Apr 28th 2014, 11:47 PM   #8
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Thanks folks. I guess that is why small RC sailboats require a massive keel to sail properly.
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Old Apr 21st 2019, 09:17 PM   #9
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Does the density of water make a difference? If the water is denser, the vessel might float higher in the water. Just wondering.
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Old Apr 21st 2019, 09:55 PM   #10
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If water is replaced with Hg, it's useless even if you try to make the center of gravity at the bottom of the boat. You must fly.
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