# How exactly does a boat float? **URL:** \ **Category:** Factual Questions\ **Created:** [June 21, 2003, 12:30am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317 "2003-06-21T00:30:22Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [June 21, 2003, 11:40pm UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/21 "2003-06-21T23:40:29Z")
It doesn’t “push” it to the sides, it displaces it. To float the USS Nimitz in a swimming pool, you’d need a pool at least 38 feet deep, holding at least 24 million gallons of fresh water, and able to contain the weight and volume of at least 48 million gallons. ---
**Author:** ![tastycorn](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@tastycorn](https://boards.straightdope.com/u/tastycorn)\ **Post date:** [June 22, 2003, 1:04am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/22 "2003-06-22T01:04:43Z")
Dude, think about it this way. If you have a 24 million gallon pool (same shape as the carrier hull), filled to the brim, and drop that carrier in, 23 million gallons are going to overflow, right? Well, if they’re not in the pool, they’re not part of the system, and therefor don’t matter. They could be piped into the next ocean over and the carrier would still float. So, if they don’t matter once it’s floating, then they weren’t really needed in the first place, were they? **Mirage** Nice dude. 🙂 ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [June 22, 2003, 1:16am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/23 "2003-06-22T01:16:37Z")
But at that point the carrier isn’t floating, it’s sitting on the bottom of the pool. We’re talking high school physics here. ---
**Author:** ![tastycorn](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@tastycorn](https://boards.straightdope.com/u/tastycorn)\ **Post date:** [June 22, 2003, 1:20am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/24 "2003-06-22T01:20:34Z")
yoyodyne, did you get the point about the pool being the same shape as the carrier, only slightly larger? ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [June 22, 2003, 1:35am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/25 "2003-06-22T01:35:00Z")
Yes, and it doesn’t matter. If fact, if it’s only slightly larger it is impossible to make it float. The volume would have to be at least twice the below-waterline volume of the carrier, or something like 6 million cubic feet. Believe me, it won’t work the way you say. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [June 22, 2003, 1:40am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/26 "2003-06-22T01:40:54Z")
> [@](#): > > \*Originally posted by grienspace \* > \*\*Now most of us are aware of the factors of bouyancy and surface area doesn’r apply. > > I was going to rip him apart because he hasn’t owned up to a previous grevious error, but he is actually right in this case, although I nearly missed the context in which his assertion can be demonstrated. > \*\* What the fuck are you talking about? **yoyodyne** : Try the experiment with the glasses that **tastycorn** referred to above. ---
**Author:** ![tastycorn](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@tastycorn](https://boards.straightdope.com/u/tastycorn)\ **Post date:** [June 22, 2003, 1:52am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/27 "2003-06-22T01:52:40Z")
yoyodyne, if you can’t find two matching Burger King tumblers to try the experiment with, you could always just provide a cite for this thoery that you need a container twice the volume of the boat to make it float. I’m trying to imagine a boat and container just on the edge of this limit. You start to slowly toss pennies into the boat, ever so slowly increasing the displacement. Suddenly, you add one penny too many, the water says “fuck it”, shoots 100 feet into the air and the boat sinks like a rock. \omgwtf rofl lol!!!\ ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [June 22, 2003, 1:52am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/28 "2003-06-22T01:52:54Z")
Can I get a little help here? ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [June 22, 2003, 2:17am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/29 "2003-06-22T02:17:43Z")
In order for a boat to float in water, it has to displace its weight in water. The volume of that water is equal to the volume of the boat below the waterline when the boat is floating freely. The container hast to hold at a minimum that amount of water before you put the boat in. Once you put the boat in, it will have to displace that amount of water in order to float. That water has to remain in the container, or the boat touches bottom. Therefore, the minimum container volume is twice the ship displacement volume, or twice the volume below the waterline. WRT the glasses, if the inner glass is truly floating and not touching the sides, it is displacing its weight in water and the amount there was at least twice that much in the glass to start with. WRT your penny example, the water wouldn’t shoot and the boat wouldn’t sink, it would go from just barely off the bottom of the pool to touching the bottom of the pool. ---
**Author:** ![tastycorn](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@tastycorn](https://boards.straightdope.com/u/tastycorn)\ **Post date:** [June 22, 2003, 2:26am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/30 "2003-06-22T02:26:13Z")
Ok seriously. Go get a damn glass. Fill it with water. Put another inside. Watch it float. (you can use a finger to stabalize, if you need to, the important part is that the water is supporting the weight of the glass.) You can even push the floater down until it almost touches the bottom, which will let all unneeded water spill over the sides. Now, take the floating glass out of the container glass. Look at the amount of water left. Compare to the amount displaced. Hint: the water left sholud be a lot less than the glass displaced. Now, put the floater _back_ into the container glass. Will it float again? Let us know what you find. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [June 22, 2003, 2:41am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/31 "2003-06-22T02:41:22Z")
I just did the experiment for you, **yoyodyne**. Equipment: 2 plastic glasses, sugar, water. I put sugar in one of the glasses to make it heavier. The mass of the glass + sugar was 200 g. I filled the second glass with water, then put the first glass into the second. Water overflowed from the second glass. The first glass was most definitely floating inside the second. After removing the first glass, I weighed the water remaining in the second glass. There was 45 g left. Bottom line: 200 g “boat” floating in 45 g of water. ---
**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\ **Post date:** [June 22, 2003, 2:52am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/32 "2003-06-22T02:52:13Z")
It seems to me Yoyodyne is right. Imagine initially the boat is not floating and just resting in the “mold”. Now, if we begin to add water to it, how much do we have to add before it will float? As much water as is equal to its volume! Anything less would just fill in around it. ---
**Author:** ![tastycorn](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@tastycorn](https://boards.straightdope.com/u/tastycorn)\ **Post date:** [June 22, 2003, 2:53am UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/33 "2003-06-22T02:53:55Z")
:rolleyes: ---
**Author:** ![Omar](https://avatars.discourse-cdn.com/v4/letter/o/f0a364/32.png) [@Omar](https://boards.straightdope.com/u/Omar)\ **Post date:** [June 22, 2003, 8:55pm UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/34 "2003-06-22T20:55:50Z")
> [@](#): > > \*Originally posted by Desmostylus \* > \*\*I just did the experiment for you, **yoyodyne**. > > Equipment: 2 plastic glasses, sugar, water. > > I put sugar in one of the glasses to make it heavier. The mass of the glass + sugar was 200 g. > > I filled the second glass with water, then put the first glass into the second. Water overflowed from the second glass. > > The first glass was most definitely floating inside the second. > > After removing the first glass, I weighed the water remaining in the second glass. There was 45 g left. > > Bottom line: 200 g “boat” floating in 45 g of water. \*\* I could be entirely wrong on this but… If your second glass could not initially hold 45g of water, doesnt that mean that your “boat”, or glass of sugar, would not float? Wouldn’t it just sink to the bottom? It just seems to me that you need 45g of water to float your 200g boat. If your second glass, or “pool”, could only hold, say, 10g of water, then your 200g boat is sitting on the bottom. The pool needs to hold the proper amount of water. So if you take your 200,000,000 Ibs carrier, don’t you need a minimum amount of water to make it float? As well as the correct capacity pool to hold that minimum amount of water? Let’s say you drop your carrier into the mold, or pool, of 24 mil gallons of water, and 23 mil spill out over the side, leaving you with 1 mil gallons successfully floating the carrier. The carrier is floating. It only took 1 mil gallons of water. But that pool still has the capacity of 24 mil gallons of water. (Obviously, since we started off with an pool being filled with 24 mil gallons of water). Now what if the capacity of that pool were only 500,000 gallons of water. Half of a million. Only half of the minumum requirement needed to float the carrier. Wouldnt most, or all, of that water just spill out? Leaving the carrier sitting on the bottom? Again…I could be wrong… ---
**Author:** ![Mirage](https://avatars.discourse-cdn.com/v4/letter/m/eb9ed0/32.png) [@Mirage](https://boards.straightdope.com/u/Mirage)\ **Post date:** [June 22, 2003, 9:25pm UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/35 "2003-06-22T21:25:02Z")
Try this: Boat pushes down on the water and sinks until the water below it starts pushing back up on it. This water is pushing back up because it is being pushed on by the water to the side of it which is being pushed on by the water **above** it, splashing against the side of the boat. The key is water pressure, which is a function of depth, not the shape or size of the pool or ocean. The pressure of the water below the boat has to be enough to keep it aloft. A canoe might sink two feet into the water and a carrier might sink 200, but they both sink until their bottoms reach a depth pressurized enough to support their weight. Whether or not the boat is floating in an ocean, a pool, or a glass of coke it is the water pressure below it which is keeping it up. Displacement is just a fancy name for asking how much water could fill the gap occupied by the ship. It doesn’t need to actually displace that much water in order to float. ---
**Author:** ![tastycorn](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@tastycorn](https://boards.straightdope.com/u/tastycorn)\ **Post date:** [June 22, 2003, 9:26pm UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/36 "2003-06-22T21:26:29Z")
Well, you are partially wrong, but mostly in misreading the post. > [@](#): > > Now what if the capacity of that pool were only 500,000 gallons of water. Half of a million. Only half of the minumum requirement needed to float the carrier. Wouldnt most, or all, of that water just spill out? Leaving the carrier sitting on the bottom? Uhhh, yeah. However, that’s not scenario I was laying out. The container I was speaking of was the same size and shape of the boat, just a tiny bit larger in all dimensions. So, you’re not trying to float a 23 million gallon boat in a 500,000 gallon container, you are floating a 23 million gallon boat in a 24 million gallon container. And you can do it without ever having more than a million gallons of water. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [June 22, 2003, 9:36pm UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/37 "2003-06-22T21:36:07Z")
**yoyodyne** , take it from a sailor: you are wrong. Not only that but you are just insisting on ignoring the evidence. Just do the experiment with glasses that has been mentioned ---
**Author:** ![Omar](https://avatars.discourse-cdn.com/v4/letter/o/f0a364/32.png) [@Omar](https://boards.straightdope.com/u/Omar)\ **Post date:** [June 22, 2003, 10:28pm UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/38 "2003-06-22T22:28:14Z")
> [@](#): > > \*Originally posted by tastycorn \* > \*\*Well, you are partially wrong, but mostly in misreading the post. > > Uhhh, yeah. However, that’s not scenario I was laying out. The container I was speaking of was the same size and shape of the boat, just a tiny bit larger in all dimensions. So, you’re not trying to float a 23 million gallon boat in a 500,000 gallon container, you are floating a 23 million gallon boat in a 24 million gallon container. And you can do it without ever having more than a million gallons of water. \*\* Ok, I see the point you are making. But let’s say your 24 mil carrier does float in 1 million gal of water. (For the record, I found the average weight of a carrier to be 200 million Ibs.) If your slightly larger container cannot hold that 1 million gallons of water, than your carrier is not going to float. I think that is what yoyodyne is trying to say. He can correct me if I am wrong. The real question is how much water does it take to float a 200 million Ibs carrier? I know it’s more than 20 gallons of water, as you mentioned earlier. Does anyone know this answer? Secondly, how much water does your “slightly larger” container hold? I believe that is being left out here. If someone can answer both those questions, then I think we are getting somewhere. If the slighty larger container cannot hold the proper amount of water, then the carrier will not float. Am I wrong here? I am by no means a math major. It just seems to make sense. ---
**Author:** ![tastycorn](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@tastycorn](https://boards.straightdope.com/u/tastycorn)\ **Post date:** [June 22, 2003, 10:44pm UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/39 "2003-06-22T22:44:34Z")
> [@](#): > > The real question is how much water does it take to float a 200 million Ibs carrier? I know it’s more than 20 gallons of water, as you mentioned earlier. Does anyone know this answer? > > Secondly, how much water does your “slightly larger” container hold? I believe that is being left out here. You are missing the point. Aside from having the water spread so thin that it no longer functions as a fluid (2 or 3 molecules thick?), there is no minimum amount of water necessary. It is wholly dependent on the container, not the water. yoyodyne was trying to say that you needed a container twice the volume of the carrier to make it float, which is just absurd. I stated very explicitly exactly how much _volume_ the boat displaced and exactly how much the pool contained. 23 million and 24 million respectively. Ok, what you do is get yourself a couple of glasses, see…:wally ---
**Author:** ![Omar](https://avatars.discourse-cdn.com/v4/letter/o/f0a364/32.png) [@Omar](https://boards.straightdope.com/u/Omar)\ **Post date:** [June 22, 2003, 11:06pm UTC](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317/40 "2003-06-22T23:06:05Z")
> [@](#): > > \*Originally posted by tastycorn \* > \*\*You are missing the point. Aside from having the water spread so thin that it no longer functions as a fluid (2 or 3 molecules thick?), there is no minimum amount of water necessary. It is wholly dependent on the container, not the water. yoyodyne was trying to say that you needed a container twice the volume of the carrier to make it float, which is just absurd. > > I stated very explicitly exactly how much _volume_ the boat displaced and exactly how much the pool contained. 23 million and 24 million respectively. > > Ok, what you do is get yourself a couple of glasses, see…:wally \*\* Well, according to this cite, you are pretty much wrong. [http://www.getsmarter.org/mstv/L3\_a.cfm](http://www.getsmarter.org/mstv/L3_a.cfm) In order for an object to float, it has to be lighter than an equal bulk or volume of water. So the amount of water does matter. [Previous page](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317.md?page=1) [Next page](https://boards.straightdope.com/t/how-exactly-does-a-boat-float/183317.md?page=3)