what ship shape will hold the most weight

[tina123]

Hi:

My son is on the 4th grade and he is required to do a science fair project.

We need some help to understand the relationship between ship shape and buoyancy. The topic that my son selects is "what ship shape will hold the most weight". The suggested key words from the suggested topics are "buoyancy, water displacement, surface tension, density, volume,etc".

We found a similar topic from this website "How Much Weight Can Your Boat Float".

My son is still in the research stage. Most of the information are about the weight/volume/density and buoyancy. There are few sources talk about the relationship between "ship shape"/air/density and buoyancy. However, we could not find "direct" explanations about "why" "the ship shape" and air will affect the buoyancy.

I hope my question is clear enough since I don't have a science background and I am learning all those information with my son.

Thanks,
tina

[tina123]

Hi,

This post is based on my previous one "which ship shape hold the most weight".

I found the following tow phrases from two links said the different things about how "ship shape" affecting "buoyancy. Maybe I am the one misunderstanding the writings. Please advice?

Tina

http://www.sciencebuddies.org/science-f ... p020.shtml

With a steel-hulled ship, it is the shape of the ship's hull that matters. The hull encloses a volume of air, so that the total density, defined as: (mass of steel hull + mass of enclosed air) / volume, is less than that of water.

http://www.wisegeek.com/what-is-buoyancy.htm

If an object weighs more than the weight of the water it displaces, it will sink. If the object weighs less, it will float. This helps explain why a heavy ship can easily float in the water, while a much smaller and lighter brick will sink quickly. It isn't the size or shape of an object that primarily determines buoyancy, but the relation between an object's weight compared to the weight of the water the object displaces.

[agm]

Hi Tina,

I moved your thread to the Physical Science forum where more experts will see it -- Getting Ready for the Science Fair is more about communicating your results once the experiment is done.

I can see how reading these two explanations together could be confusing. Try looking at them again with different emphases:

With a steel-hulled ship, it is the shape of the ship's hull that matters. The hull encloses a volume of air, so that the total density, defined as: (mass of steel hull + mass of enclosed air) / volume, is less than that of water.

If an object weighs more than the weight of the water it displaces, it will sink. If the object weighs less, it will float. This helps explain why a heavy ship can easily float in the water, while a much smaller and lighter brick will sink quickly. It isn't the size or shape of an object that primarily determines buoyancy, but the relation between an object's weight compared to the weight of the water the object displaces.

Density is "the relationship between an object's weight and the volume it displaces", and total density (including both the hull and the air space inside) is the deciding factor: objects more dense than water will sink, while objects less dense will float. The two sources agree on this.

Reading this again:

With a steel-hulled ship, it is the shape of the ship's hull that matters. The hull encloses a volume of air, so that the total density, defined as: (mass of steel hull + mass of enclosed air) / volume, is less than that of water.

Okay, the person who wrote this was imagining that you're given a certain amount of steel to turn into a ship -- any ship you build will have a hull made of the same material with a predetermined thickness. You could build something nearly flat, say the shape of a book -- this wouldn't have much air inside, so that total density would only be a little less than that of steel; steel is a lot denser than water so this boat would sink. Instead, you could turn the same amount of steel into a shape that has a lot more area inside, like a cube or a sphere. Then the total density might be less than the density of water, enabling the object to float.

Now the second:

If an object weighs more than the weight of the water it displaces, it will sink. If the object weighs less, it will float. This helps explain why a heavy ship can easily float in the water, while a much smaller and lighter brick will sink quickly. It isn't the size or shape of an object that primarily determines buoyancy, but the relation between an object's weight compared to the weight of the water the object displaces.

This time, imagine that you have a brick or rock, something with no air trapped inside, that's the same shape as a boat. The total or average density of a brick of any size or shape is just the density of brick. However, the total density of a steel-hulled ship is less than the total density of steel -- even if the ship is the shape of a brick -- because the ship is hollow. The person who wrote this was comparing ships of the same shape, but with hulls of different thicknesses or perhaps different materials -- in the first source above, hull material and thickness was implicitly assumed to be constant.

Hope that helps to clarify things... this is a more complicated explanation than I'd try to give a fourth-grader directly, but he has you for help! This might be a good time to point out the important of control variables (things you keep constant so you can make a meaningful comparison), independent variables (the one variable you choose not to keep constant, so that you're only measuring its effect), and dependent variables (the variable whose change is measured):

http://www.sciencebuddies.org/science-f ... test.shtml
http://www.sciencebuddies.org/science-f ... bles.shtml

Amanda

[tina123]

Amanda:

Thank you for your help in moving the post to a better forum and also the detailed explanations.

Now, I understand there are some "implied assumptions" behind those two different writings.

Since my son's topic is about "what ship shape will hold the most weight", he plans to build different ship shapes, such as a very flat one like book, a one like bowl that has a circle base, etc. Therefore, I think he should use the same amount of the same type material (with predetermined thickness????) as a constant variables like the first writings.

If we use the same amount of clay to build different ship shapes and calculate the volume of each boat hull using either "rule method" or "dry rice method" (suggested by the article from science buddies), we can guess which ship shape will hold the most weight by the outcome of the "calculated volume"????

If all the ship shapes all have the area to contain the "air" and we know the "calculated volume" will determine the most weight (???), then the question is "what ship shape" can hold the most volume of air? For example, a flat book shape? a bowl shape? a triangle shape?, or a "traditional ship shape"?

I guess a traditional ship shape will hold the most volume. Then the question is "why" this particular ship shape can hold the most volume of air? Is there any formula to support and guide this type of "boat design"

Does it bring us into "boat design" area?

I am not sure whether my son needs to get this far for his science fair project. I ask all those questions to satisfy my own curiosity and hope to steer him to a right level of understanding (for his age or for his own level - it can be higher or lowe than the 4th grade????)

Thanks,
Tina

[tina123]

I have one more thought coming.....

If we make all different ship shapes (using the same amount of the same material) that all hold the same volume of dry rice, will all those different ship shapes hold the same amount of pennies before it can sink? If yes, that means the ship shape itself does not affect the buoyancy. But the ship shape that can hold the most volume will be able to hold the most weight.

So, if we add "volume" as another "constant variable", it will change the game. right?

Am I on the right track or it is a crazy thought?

Tina

[Craig_Bridge]

Ignoring the word "ship" which implies that you want a shape that will efficently transport materials by minimizing the energy to move it and to protect the materials from weather while transporting for some maximum wave height, the problem changes...

What shape maximizes the displacement volume and minimizes for a given surface area while maintaining structural integrity? This becomes a calculus problem which is well beyond this grade level. If I remember correctly, the answer comes out to be a hemisphere which also turns out to be one of the stronger shapes for a uniform thickness material in resisting uniform pressure. This design fails miserably in steerability and protection of goods.

This is one of those Engineering problems where all of the specifications for everything that the design has to do matter and each design will trade off how well it behaves with respect to one criterion versus other criterion. Different designs will do different things better. This is why river barges, oil tankers, cargo container ships, great lake ore boats, great lake sea going grain boats, boats that have to pass through various canals all have different shapes and or length to width ratios.

[tina123]

Hi, Greg:

Thank you for the quick response. I was out of town for a while and did not get a chance to respond your post.

Thank you for your guidance about the ship/boat shape. I definitely will not go any further to that direction because it is way beyond our capability.

I guess we stay with the "boat article" from the science buddies. It has enough information to digest.

More questions to come.

Tina

[tina123]

http://www.sciencebuddies.org/science-f ... p020.shtml

What determines whether an object floats or sinks? It's the density (mass per unit volume) of the object compared to the density of the liquid. With a steel-hulled ship, it is the shape of the ship's hull that matters. The hull encloses a volume of air, so that the total density, defined as: (mass of steel hull + mass of enclosed air) / volume, is less than that of water.

Hi:

I have some questions about the above statements (and formula) from the link. I am having my son to experience with a clay ball and a clay boat with a same amount of the clay. He understand that the air play an important role to keep the clay boat float.

I have a question about the "volume". I found a definition about volume from the internet

Volume: how much "space" a three-dimensional object takes up

Does it mean that the above article (with link) calculates the "volume" using either "ruler" or "dry rice" method for the "space" area of a boat. I am guessing it calculates the volume of the "air" that a boat can hold, right?

Does a ball clay have any "space" to hold the air? Based on the clay ball that we make, I could not find any "space" in a clay ball. So, does it mean that a clay ball does not have "volume". So, the total density of the clay ball based on this formula (mass of steel hull + mass of enclosed air) / volume should be more than that of water because "mass of enclosed air = 0 and volume = 0????

Will the "mass of steel (clay) hull" = "mass of clay ball" since the boat and ball are coming from the same amount of clay.

I am wondering whether this quesion should have its own new subject, such as " total denity of a clay ball and a clay boat"?????

Tina